Structure and elasticity of composite nanoparticle/polymer nanoshells (hybridosomes®).

Our group recently introduced a new process to synthesize nanoparticle shells of about 100 nm, named "hybridosomes®". Here, the structure and mechanical properties of hybridosomes® made from iron oxide nanoparticles and poly(acrylic acid) are characterized using TEM, AFM and an osmotic compression technique. For the latter, the size distribution of the hybridosomes is monitored by nanoparticle tracking analysis (NTA) in the presence of poly(ethylene glycol)s of different molecular weights. It is found that the size of the hybridosomes® can be tuned from ca. 80 nm to over 110 nm by adjusting the amount of nanoparticles and that their shell consists of a single layer of nanoparticles, with a porous structure. The size of the pores is estimated from osmotic compression experiments at ca. 4000 g mol-1. The mechanical properties are measured both at the ensemble level using size measurements under osmotic pressure and at the single nanoparticle level by atomic force microscopy nanoindentation. Both osmotic and AFM experiments are analyzed in the framework of the continuum elastic theory of thin shells and yield a value of Young's modulus of the order of MPa.

Photocontrol of luminescent inorganic nanocrystals via an organic molecular switch.

A photo-controlled and quasi-reversible switch of the luminescence of hexadecylamine-coated ZnO nanocrystals (ZnO@HDA Ncs) is operated via a molecular photoswitch (dithienylethene, DTE). The interaction between the DTE switch and the ZnO@HDA Ncs is thoroughly investigated using NMR spectroscopy techniques, including DOSY and NOESY, showing that the DTE switch is weakly adsorbed at the surface of the Ncs through the formation of hydrogen bonds with HDA. Steady state and time-resolved luminescence quenching experiments show a complex behavior, related to the spatial distribution of the emitting defects in the Ncs. Analysis of the data using models previously developed for Ncs supports static quenching. Both isomeric forms (open or closed) of the DTE switch quench the emission of Ncs, the efficiency being more than ten times higher for the closed isomer. The mechanism of quenching is discussed and we show that quenching occurs mainly through resonant energy transfer for the closed isomer and through electron transfer for the open one. The HDA layer mediates the quenching efficiency as only defects located near the surface are quenched.

Specific inhibition of ectodomain shedding of glycoprotein Ibα by targeting its juxtamembrane shedding cleavage site.

BACKGROUND: Ectodomain shedding of glycoprotein Ibα (GPIbα), a proteolytic event in which metalloprotease ADAM17 cleaves the Gly464-Val465 bond and releases glycocalicin to the plasma, is considered a critical step in mediating clearance of stored platelets. Supporting evidence has largely come from studies using ADAM17 inhibitors. However, the definitive proof is lacking due to the broad substrate specificity of ADAM17. AIM: To achieve substrate-specific inhibition of GPIbα shedding. METHODS: Development of monoclonal antibodies that directly bind the sequence around the GPIbα shedding cleavage site and inhibit GPIbα shedding by blocking ADAM17 access to the cleavage site. RESULTS: Six anti-GPIbα monoclonal antibodies with varying binding affinities were obtained. The prototypic clone, designated 5G6, and its monomeric Fab fragment bind specifically purified GPIb-IX complex, human platelets, and transgenic murine platelets expressing human GPIbα. The clone 5G6 showed similar inhibitory potency as a widely used shedding inhibitor GM6001 in both constitutive and induced GPIbα shedding in human platelets. It does not recognize mouse GPIbα or inhibit shedding of other platelet receptors. Finally, 5G6 binding displays no detectable effect on platelet activation and aggregation. CONCLUSIONS: The clone 5G6 specifically inhibits GPIbα shedding with no detectable effect on platelet functions. The method of substrate-specific shedding inhibition by macromolecular binding of the shedding cleavage site can be applicable to many other transmembrane receptors undergoing ectodomain shedding.

Microfluidic focal thrombosis model for measuring murine platelet deposition and stability: PAR4 signaling enhances shear-resistance of platelet aggregates.

BACKGROUND: Flow chambers allow the ex vivo study of platelet response to defined surfaces at controlled wall shear stresses. However, most assays require 1-10 mL of blood and are poorly suited for murine whole blood experiments. OBJECTIVE: To measure murine platelet deposition and stability in response to focal zones of prothrombotic stimuli using 100 microL of whole blood and controlled flow exposure. METHODS: Microfluidic methods were used for patterning acid-soluble collagen in 100 microm x 100 microm patches and creating flow channels with a volume of 150 nL. Within 1 min of collection into PPACK and fluorescent anti-mouse CD41 mAb, whole blood from normal mice or from mice deficient in the integrin alpha(2) subunit was perfused for 5 min over the patterned collagen. Platelet accumulation was measured at venous and arterial wall shear rates. After 5 min, thrombus stability was measured with a 'shear step-up' to 8000 s(-1). RESULTS: Wild-type murine platelets adhered and aggregated on collagen in a biphasic shear-dependent manner with increased deposition from 100 to 400 s(-1), but decreased deposition at 1000 s(-1). Adhesion to patterned collagen was severely diminished for platelets lacking a functional alpha(2)beta(1) integrin. Those integrin alpha(2)-deficient platelets that did adhere were removed from the surface when challenged to shear step-up. PAR4 agonist (AYPGKF) treatment of the thrombus at 5 min enhanced aggregate stability during the shear step-up. CONCLUSIONS: PAR4 signaling enhances aggregate stability by mechanisms independent of other thrombin-dependent pathways such as fibrin formation.

Nucleotide sequence and predicted functions of the entire Sinorhizobium meliloti pSymA megaplasmid.

The symbiotic nitrogen-fixing soil bacterium Sinorhizobium meliloti contains three replicons: pSymA, pSymB, and the chromosome. We report here the complete 1,354,226-nt sequence of pSymA. In addition to a large fraction of the genes known to be specifically involved in symbiosis, pSymA contains genes likely to be involved in nitrogen and carbon metabolism, transport, stress, and resistance responses, and other functions that give S. meliloti an advantage in its specialized niche.

The composite genome of the legume symbiont Sinorhizobium meliloti.

The scarcity of usable nitrogen frequently limits plant growth. A tight metabolic association with rhizobial bacteria allows legumes to obtain nitrogen compounds by bacterial reduction of dinitrogen (N2) to ammonium (NH4+). We present here the annotated DNA sequence of the alpha-proteobacterium Sinorhizobium meliloti, the symbiont of alfalfa. The tripartite 6.7-megabase (Mb) genome comprises a 3.65-Mb chromosome, and 1.35-Mb pSymA and 1.68-Mb pSymB megaplasmids. Genome sequence analysis indicates that all three elements contribute, in varying degrees, to symbiosis and reveals how this genome may have emerged during evolution. The genome sequence will be useful in understanding the dynamics of interkingdom associations and of life in soil environments.

Expression of the platelet receptor GPVI confers signaling via the Fc receptor gamma -chain in response to the snake venom convulxin but not to collagen.

The mechanism of signal transduction underlying the activation of platelets by collagen has been actively investigated for over 30 years, but the receptors involved remain incompletely understood. Studies of human platelets, which are unresponsive to collagen, mouse knockout models, and platelet biochemical studies support the hypothesis that the recently cloned platelet surface protein GPVI functions as a signaling receptor for collagen. To directly test this hypothesis, we have expressed wild-type and mutant forms of GPVI in RBL-2H3 cells, which express the Fcepsilon receptor gamma-chain (Fc Rgamma), the putative signaling co-receptor for GPVI in platelets, but lack GPVI itself. Expression of GPVI in RBL-2H3 cells confers strong adhesive and signaling responses to convulxin (a snake venom protein that directly binds GPVI) and weak responsiveness to collagen-related peptide but no responsiveness to collagen. To elucidate the mechanism of GPVI intracellular signaling, mutations were introduced in the receptor's transmembrane domain and C-terminal tail. Unlike reported studies of other Fc Rgamma partners, these studies reveal that both the GPVI transmembrane arginine and intracellular C-tail are necessary for coupling to Fc Rgamma and for signal transduction. To our knowledge, these studies are the first to demonstrate a direct signaling role for GPVI and the first to directly test the role of GPVI as a collagen receptor. Our results suggest that GPVI may be necessary but not sufficient for collagen signaling and that a distinct ligand-binding collagen receptor such as the alpha(2)beta(1) integrin is likely to play a necessary role for collagen signaling as well as adhesion in platelets.

Delayed onset of inflammation in protease-activated receptor-2-deficient mice.

Endothelial surface expression of P-selectin and subsequent leukocyte rolling in venules can be induced by mast cell-derived histamine and binding of thrombin to protease-activated receptor-1 (PAR1). We hypothesized that activation of endothelial PAR2 by mast cell tryptase or other proteases also contributes to inflammatory responses. Leukocyte rolling flux and rolling velocity were assessed by intravital microscopy of the cremaster muscles of wild-type mice following perivenular micropipette injections of a control (LSIGRL) or PAR2-activating (SLIGRL) oligopeptide. Injection of SLIGRL increased mean rolling leukocyte flux fraction from 34 +/- 11 to 71 +/- 24% (p < 0.05) and decreased mean rolling velocity from 63 +/- 29 to 32 +/- 2 micrometer/s (p < 0.05). No significant changes occurred with control peptide injection. To further evaluate the role of PAR2 in inflammatory responses, PAR2-deficient mice were generated by gene targeting and homologous recombination. Perivenular injections of SLIGRL resulted in only a small increase in rolling leukocyte flux fraction (from 21 +/- 8 to 30 +/- 2%) and no change in rolling velocity. Leukocyte rolling after surgical trauma was assessed in 9 PAR2-deficient and 12 wild-type mice. Early (0-15 min) after surgical trauma, the mean leukocyte rolling flux fraction was lower (10 +/- 3 vs 30 +/- 6%, p < 0.05) and mean rolling velocity was higher (67 +/- 46 vs 52 +/- 36 micrometer/s, p < 0.01) in PAR2-deficient compared with control mice. The defect in leukocyte rolling in PAR2-deficient mice did not persist past 30 min following surgical trauma. These results indicate that activation of PAR2 produces microvascular inflammation by rapid induction of P-selectin-mediated leukocyte rolling. In the absence of PAR2, the onset of inflammation is delayed.

New substrates for the dicarboxylate transport system of Sinorhizobium meliloti.

The dicarboxylate transport (Dct) system of Sinorhizobium meliloti, which is essential for a functional nitrogen-fixing symbiosis, has been thought to transport only dicarboxylic acids. We show here that the permease component of the Dct system, DctA, can transport orotate, a monocarboxylic acid, with an apparent K(m) of 1.7 mM and a V(max) of 163 nmol min(-1) per mg of protein in induced cells. DctA was not induced by the presence of orotate. The transport of orotate was inhibited by several compounds, including succinamic acid and succinamide, which are not dicarboxylic acids. The dicarboxylic acid maleate (cis-butenedioic acid) was not an inhibitor of orotate transport, which suggests that it was not recognized by DctA. However, maleate was an excellent inducer of DctA expression. Our evaluation of 17 compounds as inducers and inhibitors of transport suggests that substrates recognized by S. meliloti DctA must have appropriately spaced carbonyl groups and an extended conformation, while good inducers are more likely to have a curved conformation.

Glycoprotein V-deficient platelets have undiminished thrombin responsiveness and Do not exhibit a Bernard-Soulier phenotype.

Adhesion of platelets to extracellular matrix via von Willebrand factor (vWF) and activation of platelets by thrombin are critical steps in hemostasis. Glycoprotein (GP) V is a component of the GPIb-V-IX complex, the platelet receptor for vWF. GPV is also cleaved by thrombin. Deficiency of GPIb or GPIX results in Bernard-Soulier syndrome (BSS), a bleeding disorder in which platelets are giant and have multiple functional defects. Whether GPV-deficiency might also cause BSS is unknown as are the roles of GPV in platelet-vWF interaction and thrombin signaling. We report that GPV-deficient mice developed normally, had no evidence of spontaneous bleeding, and had tail bleeding times that were not prolonged compared with wild-type mice. GPV-deficient platelets were normal in size and structure as assessed by flow cytometry and electron microscopy. GPV-deficient and wild-type platelets were indistinguishable in botrocetin-mediated platelet agglutination and in their ability to adhere to mouse vWF A1 domain. Platelet aggregation and ATP secretion in response to low and high concentrations of thrombin were not decreased in GPV-deficient platelets compared with wild-type. Our results show that (1) GPV is not necessary for GPIb expression and function in platelets and that GPV deficiency is not likely to be a cause of human BSS and (2) GPV is not necessary for robust thrombin signaling. Whether redundancy accounts for the lack of phenotype of GPV-deficiency or whether GPV serves subtle or as yet unprobed functions in platelets or other cells remains to be determined.

Citrate synthase mutants of Sinorhizobium meliloti are ineffective and have altered cell surface polysaccharides.

The gltA gene, encoding Sinorhizobium meliloti 104A14 citrate synthase, was isolated by complementing an Escherichia coli gltA mutant. The S. meliloti gltA gene was mutated by inserting a kanamycin resistance gene and then using homologous recombination to replace the wild-type gltA with the gltA::kan allele. The resulting strain, CSDX1, was a glutamate auxotroph, and enzyme assays confirmed the absence of a requirement for glutamate. CSDX1 did not grow on succinate, malate, aspartate, pyruvate, or glucose. CSDX1 produced an unusual blue fluorescence on medium containing Calcofluor, which is different from the green fluorescence found with 104A14. High concentrations of arabinose (0.4%) or succinate (0. 2%) restored the green fluorescence to CSDX1. High-performance liquid chromatography analyses showed that CSDX1 produced partially succinylated succinoglycan. CSDX1 was able to form nodules on alfalfa, but these nodules were not able to fix nitrogen. The symbiotic defect of a citrate synthase mutant could thus be due to disruption of the infection process or to the lack of energy generated by the tricarboxylic acid cycle.

Distribution of two isoforms of NADP-dependent isocitrate dehydrogenase in soybean (Glycine max).

Two different cDNAs that encode NADP-specific isocitrate dehydrogenase (NADP-IDH) isozymes of soybean (Glycine max) were characterized. The nucleotide sequences of the coding regions of these cDNAs have 74% identity to each other and give predicted amino acid sequences that have 83% identity to each other. Using PCR techniques, their coding regions were subcloned into a protein overexpression vector, pQE32, to yield pIDH4 and pIDH1, respectively. Both IDH4 and IDH1 enzymes were expressed in Escherichia coli as catalytically active His6 tagged proteins, purified to homogeneity by affinity chromatography on nickel chelate resin and rabbit polyclonal antibodies to each were generated. Surprisingly, antiserum to IDH4 did not react with IDH1 protein and IDH1 antiserum reacted only very weakly with IDH4 protein. IDH4 antibody reacts with a protein of expected molecular weight in cotyledon, young leaf, young root, mature root and nodules but the reaction with mature leaf tissue was low compared to other tissues. Western blot results show that IDH1 was not expressed in young roots but a protein that reacts with the IDH1 antibody was highly expressed in leaves, showing that there was tissue-specific accumulation of NADP-IDH isozymes in soybean.

Protease-activated receptors 1 and 4 mediate activation of human platelets by thrombin.

Because of the role of thrombin and platelets in myocardial infarction and other pathological processes, identifying and blocking the receptors by which thrombin activates platelets has been an important goal. Three protease-activated receptors (PARs) for thrombin -- PAR1, PAR3, and PAR4 -- are now known. PAR1 functions in human platelets, and the recent observation that a PAR4-activating peptide activates human platelets suggests that PAR4 also acts in these cells. Whether PAR1 and PAR4 account for activation of human platelets by thrombin, or whether PAR3 or still other receptors contribute, is unknown. We have examined the roles of PAR1, PAR3, and PAR4 in platelets. PAR1 and PAR4 mRNA and protein were detected in human platelets. Activation of either receptor was sufficient to trigger platelet secretion and aggregation. Inhibition of PAR1 alone by antagonist, blocking antibody, or desensitization blocked platelet activation by 1 nM thrombin but only modestly attenuated platelet activation by 30 nM thrombin. Inhibition of PAR4 alone using a blocking antibody had little effect at either thrombin concentration. Strikingly, simultaneous inhibition of both PAR1 and PAR4 virtually ablated platelet secretion and aggregation, even at 30 nM thrombin. These observations suggest that PAR1 and PAR4 account for most, if not all, thrombin signaling in platelets and that antagonists that block these receptors might be useful antithrombotic agents.

Generation of up-regulated allosteric variants of potato ADP-glucose pyrophosphorylase by reversion genetics.

Mutagenesis of the large subunit (LS) of the potato ADP-glucose pyrophosphorylase generated an enzyme, P52L, that was insensitive to 3-phosphoglycerate (3-PGA). To identify additional residues involved in 3-PGA interaction, we subjected P52L LS DNA to a second round of mutagenesis and identified second-site revertants by their ability to restore glycogen accumulation as assessed by iodine (I2) staining. Enzymes from class I revertants with normal I2-staining had an 11- to 49-fold greater affinity for the activator 3-PGA compared with the P52L mutant and a decreased sensitivity to the inhibitor orthophosphate. Sequence analysis of these class I revertants identified a P66L mutation in R4, an E38K mutation in R20, and a G101N mutation in R10 and R32. These mutations appear to restore 3-PGA binding by counteracting the effect of the P52L mutation because introducing E38K or G101N into the wild-type LS led to enzyme variants with higher affinity for the activator 3-PGA and increased resistance to the inhibitor orthophosphate. The generation of these revertant enzymes provides additional structure-function information on the allosteric regulation of higher plant ADP-glucose pyrophosphorylases and validates a strategy for developing novel variants of the enzyme that may be useful in manipulating starch biosynthesis in higher plants.

Gene and locus structure and chromosomal localization of the protease-activated receptor gene family.

Protease-activate receptors (PARs) mediate activation of platelets and other cells by thrombin and other proteases. Such protease-triggered signaling events are thought to be critical for hemostasis, thrombosis, and other normal and pathological processes. We report here the structure of the mouse and human PAR3 genes as well as the organization of a PAR gene cluster encompassing the genes encoding PARs 1, 2, and 3. We also report the structure of the mouse and human PAR4 genes, which map to distinct chromosomal locations and encode a new thrombin receptor. PARs 1-4 are all encoded by genes with the same two exon structure. In each case, exon 1 encodes a signal peptide, and exon 2 encodes the mature receptor protein. These are separated by an intron of variable size. The genes encoding PARs 1-3 all map to chromosome 13D2 in mouse and chromosome 5q13 in human. In mouse, all three genes are located within 80 kilobases of each other. The PAR1 gene is located centrally and is flanked upstream by the PAR3 gene and downstream by the PAR2 gene in both species. The proximity of the PAR1 and PAR3 genes suggests the possibility that these genes might share regulatory elements. A comparison of the structures of the PAR amino acid sequences, gene structures, locus organization, and chromosomal locations suggests a working model for PAR gene evolution.

A dual thrombin receptor system for platelet activation.

Platelet-dependent arterial thrombosis triggers most heart attacks and strokes. Because the coagulation protease thrombin is the most potent activator of platelets, identification of the platelet receptors for thrombin is critical for understanding thrombosis and haemostasis. Protease-activated receptor-1 (PAR1) is important for activation of human platelets by thrombin, but plays no apparent role in mouse platelet activation. PAR3 is a thrombin receptor that is expressed in mouse megakaryocytes. Here we report that thrombin responses in platelets from PAR3-deficient mice were markedly delayed and diminished but not absent. We have also identified PAR4, a new thrombin-activated receptor. PAR4 messenger RNA was detected in mouse megakaryocytes and a PAR4-activating peptide caused secretion and aggregation of PAR3-deficient mouse platelets. Thus PAR3 is necessary for normal thrombin responses in mouse platelets, but a second PAR4-mediated mechanism for thrombin signalling exists. Studies with PAR-activating peptides suggest that PAR4 also functions in human platelets, which implies that an analogous dual-receptor system also operates in humans. The identification of a two-receptor system for platelet activation by thrombin has important implications for the development of antithrombotic therapies.

P Metabolism in the Bean-Rhizobium tropici Symbiosis.

Nodulated legumes require more P than legumes growing on mineral nitrogen, but little is known about the basis for the higher P requirement. Experiments were conducted to determine how Rhizobium tropici responds to P limitation and to understand how P is partitioned between the symbionts under conditions of adequate or limiting P. Free-living R. tropici responds to P stress by increasing P transport capacity and inducing both an acid and an alkaline phosphatase. This P-stress response occurs when the medium P concentration decreases below 1 [mu]M. Both P-stress-inducible phosphatases are found in bacteroids taken from plants growing with adequate P, suggesting that P levels in the symbiosome space is low enough to induce the expression of these enzymes. Bacteroid alkaline phosphatase-specific activity was highest during vegetative growth of the bean plant, but decreased approximately 75% during the host reproductive stages. In hydroponic experiments 32P-tracer studies showed that in vivo rates of P accumulation were significantly higher in bacteroids from P-limited plants compared with those from plants that had been supplied with adequate P. In contrast, label accumulation in leaves was greatest in plants grown with adequate P.

Protease-activated receptor 3 is a second thrombin receptor in humans.

Thrombin is a coagulation protease that activates platelets, leukocytes, endothelial and mesenchymal cells at sites of vascular injury, acting partly through an unusual proteolytically activated G-protein-coupled receptor. Knockout of the gene encoding this receptor provided definitive evidence for a second thrombin receptor in mouse platelets and for tissue-specific roles for different thrombin receptors. We now report the cloning and characterization of a new human thrombin receptor, designated protease-activated receptor 3 (PAR3). PAR3 can mediate thrombin-triggered phosphoinositide hydrolysis and is expressed in a variety of tissues, including human bone marrow and mouse megakaryocytes, making it a candidate for the sought-after second platelet thrombin receptor. PAR3 provides a new tool for understanding thrombin signalling and a possible target for therapeutics designed selectively to block thrombotic, inflammatory and proliferative responses to thrombin.