Kosmotropic anions promote conversion of recombinant prion protein into a PrPSc-like misfolded form

Prions are self-propagating proteins involved in transmissible spongiform encephalopaties in mammals. An aberrant conformation with amyloid-like features of a cell surface protein, termed prion protein (PrP), is thought to be the essential component of the infectious particle, though accessory co-factor molecules such as lipids and nucleotides may be involved. The cellular co-factors and environmental conditions implicated in PrP misfolding are not completely understood. To address this issue, several studies have been done inducing misfolding of recombinant PrP (recPrP) into classical amyloid structures using partially denaturing conditions. In this work, we report that misfolding of recPrP into PrP(Sc)-like aggregates can be induced by simply incubating the protein in the presence of kosmotropic salts at concentrations that are known to retain or increase the stability of the protein. We used a simple experimental reaction (protein, buffer and salts) submitted to agitation/incubation cycles at physiological temperature and pH. The formation of protease resistant-recPrP was time and salt-concentration dependent and required the presence of kosmotropic anions such as F(-) or SO(4)(-2). The molecular weights of the protease resistant recPrP fragments are reminiscent of those found in degradation assays of bona fide PrP(Sc). The aggregates also exhibited PrP(Sc)-like ultrastructural features including rod-shape morphology under electron microscope, high beta-sheet content and thioflavin-T positive signal. The formation of recPrP aggregates with PrP(Sc) biochemical features under conditions closer to physiological in the absence of organic co-factor molecules provides a simple setup that may prove helpful to understand the molecular mechanism of PrP misfolding.     

Spontaneous conversion of PrPC to PrPSc.

Octa-repeats of prion proteins (PrP) contain histidine and tryptophan residues which are known to function as ligands for transition metals. It is proposed that the spontaneous conversion of the PrPC (cellular) isoform into PrPSc (scrapie) isoform may be triggered by the coordination of these metals.     

Acidic pH induces fusion of cells infected with baculovirus to form syncytia.

The enveloped baculovirus insect cell system has been used extensively for expression of recombinant proteins, including viral fusion proteins. We tested wild-type baculovirus for endogenous fusion protein activity. Syncytia formation, dye transfer, and capacitance changes were observed after incubating infected Spodoptera frugiperda cells in acidic media, consistent with fusion protein activity. Only a short acidic pulse of 10 s is needed to trigger syncytia formation. Identical results were obtained with recombinant baculovirus. This new system is convenient for studying pH activated cell-cell fusion. However, using this enveloped virus to study the mechanism of recombinant fusion proteins requires caution.     

Preparative conversion of native human antithrombin to the latent form

Human native antithrombin (AT) can be converted to a partially denaturated form of AT, known as latent AT (L-AT). This latent form of AT has been shown to exhibit strong antiangiogenic activity and also to suppress tumor growth in mice models. In the present work, a method is presented which induces the conversion of native AT to L-AT, using incubation at 60 degrees C, for 16 h, with 0.9 M ammonium sulfate, in 5mM Hepes buffer, pH 7.4, giving a recovery of more than 70%. L-AT was determined by integration of the low heparin affinity peak when analyzed by the affinity chromatography method. Native polyacrylamide gel electrophoresis was used to show that the preparation contained no aggregates. Hydrophobic interaction chromatography was also used for the separation of AT and L-AT.     

Acidic pH as a physiological regulator of human cathepsin L activity.

Human cysteine protease cathepsin L was inactivated at acid pH by a first-order process. The inactivation rate decreased with increasing concentrations of a small synthetic substrate, suggesting that substrates stabilize the active conformation. The substrate-independent inactivation rate constant increased with organic solvent content of the buffer, consistent with internal hydrophobic interactions, disrupted by the organic solvent, also stabilizing the enzyme. Circular dichroism showed that the inactivation is accompanied by large structural changes, a decrease in alpha-helix content being especially pronounced. The high activation energy of the reaction at pH 3.0 (200 kJ.mol-1) supported such a major conformational change occurring. The acid inactivation of cathepsin L was irreversible, consistent with the propeptide being needed for proper folding of the enzyme. Aspartic protease cathepsin D was shown to cleave denatured, but not active cathepsin L, suggesting a potential mechanism for in-vivo regulation and turnover of cathepsin L inside lysosomes.     

Sphingomyelinase activity at pH 7.4 in human brain and a comparison to activity at pH 5.0.

A hitherto undescribed sphingomyelinase (sph'ase 7.4) of human brain has been studied in crude and partially purified (3- to 4- fold) extracts of grey matter, and compared to the known sphingomyelinase with an acid pH optimum (sph'ase 5.0). Its specificity for sphingomyelin as substrate is similar to that of sph'ase 5.0, but it differs from sph'ase 5.0 in its pH optimum (7.4 vs 5.0) and in a requirement for Mg2+ for optimal activity. Other properties of sph'ase 7.4 that distinguish it from sph'ase 5.0 include (a) its lack of appreciable solubilization during dialysis of crude homogenates (b) a more marked concentrations in grey matter than in white matter (9- to 13- fold vs 1.5- to 2-fold for sph'ase 5.0); (c) inhibition by Ca2+ and Cd2+ ions, and by EDTA; (D) stimulation by dithiothreitol, and inhibition by cysteine, N-ethylmaleimide, and p-hydroxymercuribenzoate; (e) lack of inhibition by nucleotides (AMP.ADP, and ATP) and by NAD plus NADH; and (f) relative instability to storage or manipulation between -20degrees C and 40degrees C. These differences indicate the SPH'ASE 7.4 is a different enzyme protein from sph'ase 5.0. Unlike sph'ase 5.0, which is widely distributed in mammalian tissues, sph'ase 7.4 occurs predominantly in grey matter and little activity was observed is spleen, liver, or leukocytes. The high levels of this enzyme in brain suggest a role related to the specific functions of this organ or to the need for a more stringent control of sphingomyelin catabolism in brain as compared to other organs.     

Rat brain and kidney metalloendopeptidase: Enkephalin heptapeptide conversion to form a cardioactive neuropeptide,Phe-Met-Arg-Phe-Amide

Rat brain or kidney metalloendopeptidase purified from particulates cleaved Met-enkephalin-Arg⁶-Phe⁷ and its amide at the Gly³-Phe⁴ bond to release Phe-Met-Arg-Phe or the tetrapeptide amide. The latter, a neuropeptide with cardioactive properties, was relatively stable upon further incubation. The metallo-nature of the enzyme was established by inhibition with chelating agents (EDTA, o-phenanthroline) and its endopeptidase nature by cleavage at the Gly³-Phe⁴ bond of pentapeptide enkephalins or precursors such as the heptapeptide, or analogs bearing N- or C-terminal protective groups. Presence of C-terminal amides decreased the rate of hydrolysis. Thiorphan, (DL-3-mercapto-2-benzylpropanoyl)-glycine, competitively inhibited cleavage at the Gly³-Phe⁴ bond of enkephalin (Ki 10 nM). The thiorphan sensitive metalloendopeptidase provides a pathway for conversion of an enkephalin precursor to form a non-opioid peptide of biological interest.     

Isolation and characterization of a tumor-derived human protein related to chromogranin A and its in vitro conversion to human pancreastatin-48

A protein with pancreastatin-like immunoreactivity has been isolated and purified from liver metastasis of a patient with insulinoma. NH2-terminal residue analysis, in conjunction with the use of antibodies that are specific for the C-terminal amide peptide of porcine pancreastatin, identified this protein as a 186-amino-acid protein corresponding to human chromogranin A-116-301 (the fragment corresponding to the positions from 116 to 301 of human chromogranin A). Digestion of this protein with trypsin yielded a 48-amino-acid peptide with the retention of full pancreastatin activity. Serum from patient with insulinoma contains a peptide specie(s) that comigrates with the 48-amino-acid pancreastatin, suggesting that this peptide might be a physiologically important circulation form of pancreastatin in humans. A sensitive radioimmunoassay was established using antibody developed against a synthetic 29-amino-acid peptide amide of pancreastatin. Immunocytochemical staining revealed that a major population of human pancreatic islet cells were immunoreactive to the antiserum but with varying intensity of staining. Pancreastatin-like immunoreactivity was not observed in exocrine acinar cells.     

Anaplasma phagocytophilum-infected neutrophils enhance transmigration of Borrelia burgdorferi across the human blood brain barrier in vitro

The manifestations of Lyme disease, caused by Ixodes spp. tick-transmitted Borrelia burgdorferi, range from skin infection to bloodstream invasion into the heart, joints and nervous system. The febrile infection human granulocytic anaplasmosis is caused by a neutrophilic rickettsia called Anaplasma phagocytophilum, also transmitted by Ixodes ticks. Previous studies suggest that co-infection with A. phagocytophilum contributes to increased spirochetal loads and severity of Lyme disease. However, a common link between these tick-transmitted pathogens is dissemination into blood or tissues through blood vessels. Preliminary studies show that B. burgdorferi binds and passes through endothelial barriers in part mediated by host matrix metalloproteases. Since neutrophils infected by A. phagocytophilum are activated to release bioactive metalloproteases and chemokines, we examined the enhanced B. burgdorferi transmigration through vascular barriers with co-infection in vitro. To test whether endothelial transmigration is enhanced with co-infection, B. burgdorferi and A. phagocytophilum-infected neutrophils were co-incubated with EA.hy926 cells (HUVEC-derived) and human brain microvascular endothelial cells in Transwell cultures. Transmigration of B. burgdorferi through endothelial cell barriers was determined and endothelial barrier integrity was measured by transendothelial electrical resistivity. More B. burgdorferi crossed both human BMEC and EA.hy926 cells in the presence of A. phagocytophilum-infected neutrophils than with uninfected neutrophils without affecting endothelial cell integrity. Such a mechanism may contribute to increased blood and tissue spirochete loads.     

In vitro self-propagation of recombinant PrPSc-like conformation generated in the yeast cytoplasm

Self-propagation is characteristic property for a prion conformation. Previous studies revealed that prion protein expressed in the cytoplasm gained a PrP(Sc)-like conformation. However, it remains unclear whether the PrP(Sc)-like conformation has the self-propagating property. We found that PrP partially purified from yeast cytoplasm formed amyloid fiber like structures, and we found that the PrP(Sc)-like conformation is able to convert normal PrP(C) in the brain homogenate to a proteinase K-resistant conformation. These results suggest that yeast cytoplasm expressed recombinant PrP(Sc)-like conformation has the characteristic self-propagating property of a prion, which may have implications in the pathogenesis of sporadic and inherited prion diseases.     

Fusion of Human Sperm to Prostasomes at Acidic pH

Prostasomes are membranous vesicles (150–200 nm diameter) present in human semen. They are secreted by the prostate and contain large amounts of cholesterol, sphingomyelin and Ca²⁺. In addition, some of their proteins are enzymes. Prostasomes enhance the motility of ejaculated spermatozoa and are involved in a number of additional biological functions. The possibility that they may fuse to sperm has never been proved. In this work, we studied the fusion of sperm to prostasomes by using various methods (relief of octadecyl Rhodamine B fluorescence self-quenching, fluorescence microscopy and flow cytometry) and we found that it occurs at acidic pH (4–5), but not at pH 7.5 pH-dependent fusion relies on the integrity of one or more proteins and is different from the Ca²⁺-stimulated fusion between rat liver liposomes and spermatozoa that does not require any protein and occurs at neutral pH. We think that the H⁺-dependent fusion of prostasomes to sperm may have physiological importance by modifying the lipid and protein pattern of sperm membranes. Received: 19 June 1996/Revised: 4 September 1996     

Induction of ligand-specific PrPC signaling in human neuronal cells

Cellular prion protein (PrP^C) has attracted considerable attention for its role in transmissible spongiform encephalopathies (TSEs). In spite of being a point of intense research effort critical questions still remain regarding the physiological function of PrP^C and how these functions may change with the conversion of the protein into the infectious and pathological conformation (PrP^Sc). While emerging evidence suggests PrP^C/Sc are involved in signal transduction there is little consensus on the signaling pathways associated with the normal and diseased states. The purported involvement of PrP^C in signal transduction, and the association of TSEs with neural pathology, makes kinome analysis of human neurons an interesting and appropriate model to characterize patterns of signal transduction following activation of PrP^C by two commonly employed experimental ligands; antibody-induced dimerization by 6H4 and the amino acids 106-126 PrP peptide fragment (PrP 106–126). Analysis of the induced kinome responses reveals distinct patterns of signaling activity following each treatment. Specifically, stimulation of human neurons with the 6H4 antibody results in alterations in mitogen activated protein kinase (MAPK) signaling pathways while the 106-126 peptide activates growth factor related signaling pathways including vascular endothelial growth factor (VEGF) signaling and the phosphoinositide-3 kinase (PI3K) pathway. These pathways were validated through independent functional assays. Collectively these results indicate that stimulation of PrP^C with distinct ligands, even within the same cell type, results in unique patterns of signaling. While this investigation highlights the apparent functional versatility of PrP^C as a signaling molecule and may offer insight into cellular mechanisms of TSE pathology it also emphasizes the potential dangers associated with attributing activation of specific intracellular events to particular receptors through artificial models of receptor activation.     

Induction of ligand-specific PrPC signaling in human neuronal cells

Cellular prion protein (PrP^C) has attracted considerable attention for its role in transmissible spongiform encephalopathies (TSEs). In spite of being a point of intense research effort critical questions still remain regarding the physiological function of PrP^C and how these functions may change with the conversion of the protein into the infectious and pathological conformation (PrP^Sc). While emerging evidence suggests PrP^C/Sc are involved in signal transduction there is little consensus on the signaling pathways associated with the normal and diseased states. The purported involvement of PrP^C in signal transduction, and the association of TSEs with neural pathology, makes kinome analysis of human neurons an interesting and appropriate model to characterize patterns of signal transduction following activation of PrP^C by two commonly employed experimental ligands; antibody-induced dimerization by 6H4 and the amino acids 106-126 PrP peptide fragment (PrP 106–126). Analysis of the induced kinome responses reveals distinct patterns of signaling activity following each treatment. Specifically, stimulation of human neurons with the 6H4 antibody results in alterations in mitogen activated protein kinase (MAPK) signaling pathways while the 106-126 peptide activates growth factor related signaling pathways including vascular endothelial growth factor (VEGF) signaling and the phosphoinositide-3 kinase (PI3K) pathway. These pathways were validated through independent functional assays. Collectively these results indicate that stimulation of PrP^C with distinct ligands, even within the same cell type, results in unique patterns of signaling. While this investigation highlights the apparent functional versatility of PrP^C as a signaling molecule and may offer insight into cellular mechanisms of TSE pathology it also emphasizes the potential dangers associated with attributing activation of specific intracellular events to particular receptors through artificial models of receptor activation.     

Mycorrhizal Response to Experimental pH and P Manipulation in Acidic Hardwood Forests

Many temperate forests of the Northeastern United States and Europe have received significant anthropogenic acid and nitrogen (N) deposition over the last century. Although temperate hardwood forests are generally thought to be N-limited, anthropogenic deposition increases the possibility of phosphorus (P) limiting productivity in these forest ecosystems. Moreover, inorganic P availability is largely controlled by soil pH and biogeochemical theory suggests that forests with acidic soils (i.e., <pH 5) are particularly vulnerable to P limitation. Results from previous studies in these systems are mixed with evidence both for and against P limitation. We hypothesized that shifts in mycorrhizal colonization and community structure help temperate forest ecosystems overcome an underlying P limitation by accessing mineral and organic P sources that are otherwise unavailable for direct plant uptake. We examined arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) communities and soil microbial activity in an ecosystem-level experiment where soil pH and P availability were manipulated in mixed deciduous forests across eastern Ohio, USA. One year after treatment initiation, AM root biomass was positively correlated with the most available P pool, resin P, while AM colonization was negatively correlated. In total, 15,876 EcM root tips were identified and assigned to 26 genera and 219 operational taxonomic units (97% similarity). Ectomycorrhizal richness and root tip abundance were negatively correlated with the moderately available P pools, while the relative percent of tips colonized by Ascomycetes was positively correlated with soil pH. Canonical correspondence analysis revealed regional, but not treatment, differences in AM communities, while EcM communities had both treatment and regional differences. Our findings highlight the complex interactions between mycorrhizae and the soil environment and further underscore the fact that mycorrhizal communities do not merely reflect the host plant community.     

Acidic pH modulates the interaction between human granulocyte-macrophage colony-stimulating factor and glycosaminoglycans.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) controls growth and differentiation of hematopoietic cells. Previous reports have indicated that the mitogenic activity of GM-CSF may be modulated by the glycosidic moiety of proteoglycans associated with the membrane of stromal cells. In this work, we have performed in vitro studies of the interaction between GM-CSF and glycosaminoglycans. The addition of heparin promoted a marked blue shift in the fluorescence emission spectrum of GM-CSF as well as a 30-fold increase in the intensity of light scattering, which indicates formation of large molecular weight complexes between the two molecules. Interestingly, heparin-induced changes in the spectral properties of GM-CSF were only observed at acidic pH. The dependence on acidic pH, together with a strict dependence on glycosaminoglycan sulfation and the fact that high ionic strength destabilized the interaction, indicates that the association between GM-CSF and glycosaminoglycans is mediated by electrostatic interactions. These interactions probably involve sulfate groups in the glycosaminoglycans and positively charged histidine residues in GM-CSF. We propose that negatively charged glycolipids present on the plasma membrane of the hematopoietic and/or the stromal cell could promote an acidic microenvironment capable of triggering interaction between GM-CSF and membrane-bound proteoglycans in vivo.     

The PrPC C1 fragment derived from the ovine A136R154R171 PRNP allele is highly abundant in sheep brain and inhibits fibrillisation of full-length PrPC protein in vitro

Expression of the cellular prion protein (PrP^C) is crucial for the development of prion diseases. Resistance to prion diseases can result from reduced availability of the prion protein or from amino acid changes in the prion protein sequence. We propose here that increased production of a natural PrP α-cleavage fragment, C1, is also associated with resistance to disease. We show, in brain tissue, that ARR homozygous sheep, associated with resistance to disease, produced PrP^C comprised of 25% more C1 fragment than PrP^C from the disease-susceptible ARQ homozygous and highly susceptible VRQ homozygous animals. Only the C1 fragment derived from the ARR allele inhibits in-vitro fibrillisation of other allelic PrP^C variants. We propose that the increased α-cleavage of ovine ARR PrP^C contributes to a dominant negative effect of this polymorphism on disease susceptibility. Furthermore, the significant reduction in PrP^C β-cleavage product C2 in sheep of the ARR/ARR genotype compared to ARQ/ARQ and VRQ/VRQ genotypes, may add to the complexity of genetic determinants of prion disease susceptibility.     

In vitro conversion of a methionine to a glutamine-acceptor tRNA

A derivative of Escherichia coli tRNAfMet containing an altered anticodon sequence, CUA, has been enzymatically synthesized in vitro. The variant tRNA was prepared by excision of the normal anticodon, CAU, in a limited digestion of intact tRNAfMet with RNase A, followed by insertion of the CUA sequence into the anticodon loop with T4 RNA ligase and polynucleotide kinase. The altered methionine tRNA showed a large enhancement in the rate of aminoacylation by glutaminyl-tRNA synthetase and a large decrease in the rate of aminoacylation by methionyl-tRNA synthetase. Measurement of kinetic parameters for the charging reaction by the cognate and noncognate enzymes revealed that the modified tRNA is a better acceptor for glutamine than for methionine. The rate of mischarging is similar to that previously reported for a tryptophan amber suppressor tRNA containing the anticodon CUA, su+7 tRNATrp, which is aminoacylated with glutamine both in vivo and in vitro [Yaniv, M., Folk, W. R., Berg, P., & Soll, L. (1974) J. Mol. Biol. 86, 245-260; Yarus, M., Knowlton, R. E., & Soll, L. (1977) in Nucleic Acid-Protein Recognition (Vogel, H., Ed.) pp 391-408, Academic Press, New York]. The present results provide additional evidence that the specificity of aminoacylation by glutaminyl-tRNA synthetase is sensitive to small changes in the nucleotide sequence of noncognate tRNAs and that uridine in the middle position of the anticodon is involved in the recognition of tRNA substrates by this enzyme.