Human furin is a calcium-dependent serine endoprotease that recognizes the sequence Arg-X-X-Arg and efficiently cleaves anthrax toxin protective antigen

The cytoplasmic domain of anthrax toxin receptor 1 affects binding of the protective antigen

The protective antigen (PA) component of anthrax toxin binds the I domain of the receptor ANTXR1. Integrin I domains convert between open and closed conformations that bind ligand with high and low affinities, respectively; this process is regulated by signaling from the cytoplasmic domains. To assess whether intracellular signals might influence the interaction between ANTXR1 and PA, we compared two splice variants of ANTXR1 that differ only in their cytoplasmic domains. We found that cells expressing ANTXR1 splice variant 1 (ANTXR1-sv1) bound markedly less PA than did cells expressing a similar level of the shorter splice variant ANTXR1-sv2. ANTXR1-sv1 but not ANTXR1-sv2 associated with the actin cytoskeleton, although disruption of the cytoskeleton did not affect binding of ANTXR-sv1 to PA. Introduction of a cytoplasmic domain missense mutation found in the related receptor ANTXR2 in a patient with juvenile hyaline fibromatosis impaired actin association and increased binding of PA to ANTXR1-sv1. These results suggest that ANTXR1 has two affinity states that may be modulated by cytoplasmic signals.

Activity of the Bacillus anthracis 20 kDa protective antigen component

Background

Anthrax is caused by Bacillus anthracis that produce two exotoxins, lethal toxin and edema toxin. The lethal toxin is composed of the lethal factor (LF) complexed with the cell binding protective antigen (PA₈₃, 83 kDa). Likewise, the edema factor (EF) binds to the PA₈₃ to form the edema toxin. Once PA83 is bound to the host cell surface, a furin-like protease cleaves the full-length, inactive protein into 63 kDa and 20 kDa antigens (PA₆₃ and PA₂₀). PA₆₃ forms a heptamer and is internalized via receptor mediated endocytosis forming a protease-stable pore, which allows EF and LF to enter the cell and exert their toxic effects.

Both proteolytically cleaved protective antigens (PA₆₃ and PA₂₀ fragments) are found in the blood of infected animals. The 63 kDa protective antigen PA₆₃ fragment has been thoroughly studied while little is known about the PA₂₀.

Methods

In this study we examined the role of PA₂₀ using high throughput gene expression analysis of human peripheral blood mononuclear cells (PBMC) exposed to the PA₂₀. We constructed a PA mutant in which a Factor Xa proteolytic recognition site was genetically engineered into the protective antigen PA₈₃ to obtain PA₂₀ using limited digestion of this recombinant PA₈₃ with trypsin.

Results

Global gene expression response studies indicated modulation of various immune functions and showed gene patterns indicative of apoptosis via the Fas pathway in a subset of the lymphoid cells. This finding was extended to include observations of increased Caspase-3 enzymatic activity and the identification of increases in the population of apoptotic, but not necrotic cells, based on differential staining methods. We identified a list of ~40 inflammatory mediators and heat-shock proteins that were altered similarly upon exposure of PBMC to either rPA₂₀ or B. anthracis spores/vegetative cells.

Conclusion

This study shows that the PA₂₀ has an effect on human peripheral blood leukocytes and can induce apoptosis in the absence of other PA components.

Comparative analysis of two attacin genes from Hyphantria cunea

A full-length clone corresponding to attacin was isolated from a cDNA library made from fat body of immunized Hyphantria cunea larvae. This newly isolated attacin B shows characteristics different from those previously reported for attacin A. The two attacin cDNAs encode precursor proteins of 233 and 248 amino acid residues, respectively. The two attacins show 45.9% identity at the amino acid level, and 35.2% identity at the nucleotide level. Attacins A and B of H. cunea show significant identities with the attacins of Lepidoptera. Attacin B is a typical glycine-rich protein, while attacin A is leucine-rich. Attacin B is expressed from last instar larvae to adult, while attacin A showed stage-specific expression during the prepupal and pupal stages. Attacins A and B are predicted to have different secondary structure in that attacin A has no tendency to form helices but attacin B contains a substantial number of helices. Attacin A is induced at a trace level in infected larvae, while attacin B is strongly induced against Gram-positive and negative bacteria, fungi, and viruses. The attacin B transcripts were detected in fat body, epidermis and hemocytes after injection with Escherichia coli, Citrobacter freundii, or Candida albicans, but not in the midgut and Malpighian tubule. Recombinant attacin A showed no antibacterial activity, while recombinant attacin B showed strong antibacterial activity in proportion to the amount of the protein injected.

Changing the protease specificity for activation of a flavivirus, tick-borne encephalitis virus

The infectivity of flavivirus particles depends on a maturation process that is triggered by the proteolytic cleavage of the precursor of the M protein (prM). This activation cleavage is naturally performed by ubiquitous cellular proteases of the furin family, which typically recognize the multibasic sequence motif R-X-R/K-R. Previously, we demonstrated that a tick-borne encephalitis virus (TBEV) mutant with an altered cleavage motif, R-X-R, produced immature, noninfectious particles that could be activated by exogenous trypsin, which cleaves after single basic residues. Here, we report the adaptation of this mutant to chymotrypsin, a protease specific for large, hydrophobic amino acid residues. Using selection pressure in cell culture, two different mutations conferring a chymotrypsin-dependent phenotype were identified. Surprisingly, one of these mutations (Ser85Phe) occurred three positions upstream of the natural cleavage site. The other mutation (Arg89His) arose at the natural cleavage position but involved a His residue, which is not a typical chymotrypsin cleavage site. Efficient cleavage of protein prM and activation by the heterologous protease were confirmed using various recombinant TBEV mutants. Mutants with only the originally selected mutations exhibited unimpaired export kinetics and were genotypically stable during at least six cell culture passages. However, in contrast to the wild-type virus or trypsin-dependent mutants, chymotrypsin-dependent mutants were not neurovirulent in suckling mice. Our results demonstrate that flaviviruses with altered protease specificities can be generated and suggest that this approach can be used for the construction of viral mutants or vectors that can be activated on demand and have restricted tissue tropism and virulence.

Source of high pathogenicity of an avian influenza virus H5N1: why H5 is better cleaved by furin

The origin of the high pathogenicity of an emerging avian influenza H5N1 due to the -RRRKK- insertion at the cleavage loop of the hemagglutinin H5, was studied using the molecular dynamics technique, in comparison with those of the noninserted H5 and H3 bound to the furin (FR) active site. The cleavage loop of the highly pathogenic H5 was found to bind strongly to the FR cavity, serving as a conformation suitable for the proteolytic reaction. With this configuration, the appropriate interatomic distances were found for all three reaction centers of the enzyme-substrate complex: the arrangement of the catalytic triad, attachment of the catalytic Ser(368) to the reactive S1-Arg, and formation of the oxyanion hole. Experimentally, the--RRRKK--insertion was also found to increase in cleavage of hemagglutinin by FR. The simulated data provide a clear answer to the question of why inserted H5 is better cleaved by FR than the other subtypes, explaining the high pathogenicity of avian influenza H5N1.

Phenylalanine-427 of anthrax protective antigen functions in both pore formation and protein translocation

The protective antigen (PA) moiety of anthrax toxin forms a heptameric pore in endosomal membranes of mammalian cells and translocates the enzymatic moieties of the toxin to the cytosol of these cells. Phenylalanine-427 (F427), a solvent-exposed residue in the lumen of the pore, was identified earlier as being crucial for the transport function of PA. The seven F427 residues were shown in electrophysiological studies to form a clamp that catalyzes protein translocation through the pore. Here, we demonstrate by a variety of tests that certain F427 mutations also profoundly inhibit the conformational transition of the heptameric PA prepore to the pore and thereby block pore formation in membranes. Lysine, arginine, aspartic acid, or glycine at position 427 strongly inhibited this acidic pH-induced conformational transition, whereas histidine, serine, and threonine had virtually no effect on this step, but inhibited translocation instead. Thus, it is possible to inhibit pore formation or translocation selectively, depending on the choice of the side chain at position 427; and the net inhibition of the PA transport function by any given F427 mutation is the product of its effects on both steps. Mutations inhibiting either or both steps elicited a strong dominant-negative phenotype. These findings demonstrate the dual functions of F427 and underline its central role in transporting the enzymatic moieties of anthrax toxin across membranes.

A novel cryptic peptide derived from the rat neuropeptide FF precursor reverses antinociception and conditioned place preference induced by morphine

Neuropeptide FF (NPFF) precursors from different species contain at least three known neuropeptides, i.e. FF (FLFQPQRF-NH(2)), AF (AGEGLSSPFWSLAAPQR-NH(2)) and SF (SLAAPQRF-NH(2)). We demonstrate that the rat NPFF precursor contains another bioactive sequence, NAWGPWSKEQLSPQA, spanning between positions 85 and 99. Synthetic NPFF precursor (85-99) (10 and 20 nmol, i.c.v.) blocked the expression of conditioned place preference induced by morphine (5 mg/kg, s.c.). This peptide alone (10 and 20 nmol, i.c.v.) had no influence on the baseline latency of a nociceptive reaction but reversed the antinociceptive activity of morphine (5 mg/kg, s.c.) in the tail-immersion test in rats. These data suggest the existence of a novel bioactive cryptic peptide within an already known NPFF precursor.

Functional analysis of four Gloverin-like genes in the silkworm, Bombyx mori

To identify genes involved in the innate immunity of the silkworm Bombyx mori, we constructed a cDNA library from the fat body of Escherichia coli-challenged B. mori larvae. Based on the expressed sequence tag (EST) data and whole genome shotgun sequence analysis, we found four Gloverin-like genes, BmGlov1-4, in the Bombyx genome. Northern blot and RT-PCR analysis showed that BmGlov1-4 were induced in the larval fat body after an immune challenge by the injection of E. coli; however, less induction was observed after the injection of a yeast Candida albicans. In silico sequence analysis revealed the presence of a motif homologous to NF-kappaB binding site in the upstream region of each BmGlov gene. Moreover, we expressed recombinant BmGlov1-4 proteins using the baculovirus expression system, and found that all the recombinant BmGlov1-4 significantly inhibited the growth of E. coli.

Mutagenic analysis of herpes simplex virus type 1 glycoprotein L reveals the importance of an arginine-rich region for function

Herpes simplex virus type 1 (HSV-1) glycoproteins H and L (gH and gL) are required for virus-induced membrane fusion. Expression of gH at the virion or infected cell surface is mediated by the chaperone-like activity of gL. We have previously shown that a region between amino acids 155 and 161 is critical for gL chaperone-like activity. Here, we conducted Ala substitution mutagenesis of residues in this region and found that substitution of Cys160, Arg156, Arg158, or Arg156/158/159 with Ala resulted in a gL mutant that bound gH but displayed a reduced ability in gH trafficking and membrane fusion. Substitution of Arg156 with another positively charged amino acid, Lys, restored function. Substitution of Arg158 with Lys restored function in gH trafficking and cell fusion but not virus entry. These results indicate that an arginine-rich region of gL is critical for function.

Inhibition of anthrax lethal toxin-induced cytolysis of RAW264.7 cells by celastrol

Background

Bacillus anthracis is the bacterium responsible for causing anthrax. The ability of B. anthracis to cause disease is dependent on a secreted virulence factor, lethal toxin, that promotes survival of the bacteria in the host by impairing the immune response. A well-studied effect of lethal toxin is the killing of macrophages, although the molecular mechanisms involved have not been fully characterized.

Methodology/Principal Findings

Here, we demonstrate that celastrol, a quinone methide triterpene derived from a plant extract used in herbal medicine, inhibits lethal toxin-induced death of RAW264.7 murine macrophages. Celastrol did not prevent cleavage of mitogen activated protein kinase kinase 1, a cytosolic target of the toxin, indicating that it did not inhibit the uptake or catalytic activity of lethal toxin. Surprisingly, celastrol conferred almost complete protection when it was added up to 1.5 h after intoxication, indicating that it could rescue cells in the late stages of intoxication. Since the activity of the proteasome has been implicated in intoxication using other pharmacological agents, we tested whether celastrol blocked proteasome activity. We found that celastrol inhibited the proteasome-dependent degradation of proteins in RAW264.7 cells, but only slightly inhibited proteasome-mediated cleavage of fluorogenic substrates in vitro. Furthermore, celastrol blocked stimulation of IL-18 processing, indicating that celastrol acted upstream of inflammasome activation.

Conclusions/Significance

This work identifies celastrol as an inhibitor of lethal toxin-mediated macrophage lysis and suggests an inhibitory mechanism involving inhibition of the proteasome pathway.

High-level expression of an antimicrobial peptide histonin as a natural form by multimerization and furin-mediated cleavage

Direct expression of an antimicrobial peptide (AMP) in Escherichia coli causes several problems such as the toxicity of AMP to the host cell, its susceptibility to proteolytic degradation, and decreased antimicrobial activity due to the additional residue(s) introduced after cleavage of AMPs from fusion partners. To overcome these problems and produce a large quantity of a potent AMP histonin (RAGLQFPVGKLLKKLLKRLKR) in E. coli, an efficient expression system was developed, in which the toxicity of histonin was neutralized by a fusion partner F4 (a truncated fragment of PurF protein) and the productivity was increased by a multimeric expression of a histonin gene. The expression level of the fusion proteins reached a maximum with a 12-mer of a histonin gene. In addition, because of the RLKR residues present at the C terminus of histonin, furin cleavage of the multimeric histonin expressed produces an intact, natural histonin. The AMP activity of the histonin produced in E. coli was identical to that of a synthetic histonin. With our expression system, 167 mg of histonin was obtained from 1 l of E. coli culture. These results may lead to a cost-effective solution for the mass production of AMPs that are toxic to a host.

Elevated furin levels in human cystic fibrosis cells result in hypersusceptibility to exotoxin A-induced cytotoxicity

Progressive pulmonary disease and infections with Pseudomonas aeruginosa remain an intractable problem in cystic fibrosis (CF). At the cellular level, CF is characterized by organellar hyperacidification, which results in altered protein and lipid glycosylation. Altered pH of the trans-Golgi network (TGN) may further disrupt the protein processing and packaging that occurs in this organelle. Here we measured activity of the major TGN endoprotease furin and demonstrated a marked upregulation in human CF cells. Increased furin activity was linked to elevated production in CF of the immunosuppressive and tissue remodeling cytokine TGF-beta and its downstream effects, including macrophage deactivation and augmented collagen secretion by epithelial cells. As furin is responsible for the proteolytic processing of a range of endogenous and exogenous substrates including growth factors and bacterial toxins, we determined that elevated furin-dependent activation of exotoxin A caused increased cell death in CF respiratory epithelial cells compared with genetically matched CF transmembrane conductance regulator-corrected cells. Thus elevated furin levels in CF respiratory epithelial cells contributes to bacterial toxin-induced cell death, fibrosis, and local immunosuppression. These data suggest that the use of furin inhibitors may represent a strategy for pharmacotherapy in CF.

EMILINs interact with anthrax protective antigen and inhibit toxin action in vitro

The informational spectrum method (ISM) is a virtual spectroscopy method for the fast analysis of potential protein-protein relationships. By applying the ISM approach to the GeneBank protein database the vascular proteins EMILIN1 (Elastin Microfibril Interface Located ProteIN), EMILIN2, MMN1, and MMN2 were identified as additional anthrax PA antigen interacting molecules. This virtual molecular interaction was formally proven by solid phase assays using recombinant proteins. The interaction is independent of the presence of divalent cations and does not involve PA aspartic residue at 683, a critical residue in receptor binding. In fact, the D683A point mutation fully prevented the cell intoxication ability of PA in the presence of Lethal Factor, but it was fully ineffective on the binding of mutated PA to EMILIN1 and EMILIN2. The ISM approach also led to the identification of the potential interaction sites between PA and EMILINs. A PA mutant with a deletion at residue D425 and solid phase protein-protein interaction studies as well as deletion mutant of EMILIN2 confirmed the hypothesized interaction site. Our findings imply that the PA-cell surface receptor interaction is not likely to provide the full explanation for the vascular lesions and prominent hemorrhages that follow Bacillus anthracis infection and spreading and call into play vascular associated proteins such as EMILINs as potential inhibitory proteins.

Inhibition of proprotein convertases: approaches to block squamous carcinoma development and progression

Most proprotein convertase (PC) inhibitors are compounds that act as competitive inhibitors. All of them contain the general cleavage motif RXK/RR that binds to the PC's active site impairing further interactions with their physiological substrates. The first inhibitors synthesized were the acyl-peptidyl-chloromethyl ketones that bind to the PC's active site through its peptidyl group and are able to transverse the plasma membrane due to the acyl moiety. For instance, one of the members of this family that exhibits reduced toxicity and has been widely used as an effective general PCs inhbitor is the derivative decanoyl-RVKR-chloromethylketone (CMK). Another approach to PC inhibition is based on proteins that contain either a natural or a bioengineered PC cleavage consensus site. In this context, the bioengineered serpin, alpha-1-antitrypsin Portland (alpha 1-PDX or PDX), proved to be a potent inhibitor of furin, the most studied of the cancer-related PCs. Both PDX and CMK were able to inhibit invasiveness of squamous cell carcinoma cell lines by blocking activation of cancer-associated PC substrates such as MT-MMPs, IGF-1R, and VEGF-C. A similar effect was produced by inhibiting PC-mediated processing using furin prosegment. PDX and CMK have also been assayed in vivo using skin carcinogenesis models. Newer promising small molecules and RNA interference approaches are also being developed to inhibit PCs.

Anthrax toxin: receptor binding, internalization, pore formation, and translocation

Anthrax toxin consists of three nontoxic proteins that self-assemble at the surface of receptor-bearing mammalian cells or in solution, yielding a series of toxic complexes. Two of the proteins, called Lethal Factor (LF) and Edema Factor (EF), are enzymes that act on cytosolic substrates. The third, termed Protective Antigen (PA), is a multifunctional protein that binds to receptors, orchestrates the assembly and internalization of the complexes, and delivers them to the endosome. There, the PA moiety forms a pore in the endosomal membrane and promotes translocation of LF and EF to the cytosol. Recent advances in understanding the entry process include insights into how PA recognizes its two known receptors and its ligands, LF and EF; how the PA:receptor interaction influences the pH-dependence of pore formation; and how the pore functions in promoting translocation of LF and EF across the endosomal membrane.

Anthrax protective antigen cleavage and clearance from the blood of mice and rats

Bacillus anthracis protective antigen (PA) is an 83-kDa (PA83) protein that is cleaved to the 63-kDa protein (PA63) as an essential step in binding and internalizing lethal factor (LF). To assess in vivo receptor saturating PA concentrations, we injected mice with PA variants and measured the PA remaining in the blood at various times using PA83- and PA63-specific enzyme-linked immunosorbent assays. We found that both wild-type PA (WT-PA) and a receptor-binding-defective mutant (Ub-PA) were cleaved to PA63 independent of their ability to bind cells. This suggested a PA-acting protease activity in the blood. The protease cleaved PA at the furin cleavage sequence because furin site-modified PA mutants were not cleaved. Cleavage measured in vitro was leupeptin sensitive and dependent on calcium. Cell surface cleavage was important for toxin clearance, however, as Ub-PA and uncleavable PA mutants were cleared at slower rates than WT-PA. The cell binding-independent cleavage of PA was also verified by using Ub-PA (which is still cleaved) to rescue mice from toxin challenge by competitively binding circulating LF. This mutant was able to rescue mice even when given 12 h before toxin challenge. Its therapeutic ability was comparable to that of dominant-negative PA, which binds cells but does not allow LF translocation, and to the protection afforded through receptor clearance by WT-PA and uncleavable receptor binding-competent mutants. The PA cleavage and clearance observed in mice did not appear to have a role in the differential mouse susceptibility as it occurred similarly in lethal toxin (LT)-resistant DBA/2J and LT-sensitive BALB/cJ mice. Interestingly, PA63 was not found in LT-resistant or -sensitive rats and PA83 clearance was slower in rats than in mice. Finally, to determine the minimum amount of PA required in circulation for LT toxicity in mice, we administered time-separated injections of PA and LF and showed that lethality of LF for mice after PA was no longer measurable in circulation, suggesting active PA sequestration at tissue surfaces.

Inactivation of small Rho GTPases by the multifunctional RTX toxin from Vibrio cholerae

Many bacterial toxins target small Rho GTPases in order to manipulate the actin cytoskeleton. The depolymerization of the actin cytoskeleton by the Vibrio cholerae RTX toxin was previously identified to be due to the unique mechanism of covalent actin cross-linking. However, identification and subsequent deletion of the actin cross-linking domain within the RTX toxin revealed that this toxin has an additional cell rounding activity. In this study, we identified that the multifunctional RTX toxin also disrupts the actin cytoskeleton by causing the inactivation of small Rho GTPases, Rho, Rac and Cdc42. Inactivation of Rho by RTX was reversible in the presence of cycloheximide and by treatment of cells with CNF1 to constitutively activate Rho. These data suggest that RTX targets Rho GTPase regulation rather than affecting Rho GTPase directly. A novel 548-amino-acid region of RTX was identified to be responsible for the toxin-induced inactivation of the Rho GTPases. This domain did not carry GAP or phosphatase activities. Overall, these data show that the RTX toxin reversibly inactivates Rho GTPases by a mechanism distinct from other Rho-modifying bacterial toxins.

Efficient Production of Bioactive Insulin from Human Epidermal Keratinocytes and Tissue-Engineered Skin Substitutes: Implications for Treatment of Diabetes

Despite many years of research, daily insulin injections remain the gold standard for diabetes treatment. Gene therapy may provide an alternative strategy by imparting the ability to secrete insulin from an ectopic site. The epidermis is a self-renewing tissue that is easily accessible and can provide large numbers of autologous cells to generate insulin-secreting skin substitutes. Here we used a recombinant retrovirus to modify human epidermal keratinocytes with a gene encoding for human proinsulin containing the furin recognition sequences at the A-C and B-C junctions. Keratinocytes were able to process proinsulin and secrete active insulin that promoted glucose uptake. Primary epidermal cells produced higher amounts of insulin than cell lines, suggesting that insulin secretion may depend on the physiological state of the producer cells. Modified cells maintained the ability to stratify into 3-dimensional skin equivalents that expressed insulin at the basal and suprabasal layers. Modifications at the furin recognition sites did not improve proinsulin processing, but a single amino acid substitution in the proinsulin B chain enhanced C-peptide secretion from cultured cells and bioengineered skin substitutes 10- and 28-fold, respectively. These results suggest that gene-modified bioengineered skin may provide an alternative means of insulin delivery for treatment of diabetes.

Anthrax toxin evades Toll-like receptor recognition, whereas its cell wall components trigger activation via TLR2/6 heterodimers

Bacillus anthracis is a Gram-positive bacillus that is the causative agent of anthrax. The virulence of the bacillus is partly due to the production of a tripartite virulence factor: protective antigen (PA), lethal factor (LF) and edema factor (EF). Recognition of the bacillus and its toxins by the innate immune system is likely to play a key role following infection. In this study we set out to investigate whether anthrax cell wall (ACW) components as well as the lethal toxin are sensed by Toll-like receptors (TLRs). Our data suggest that ACW components as well as PA are sensed by TLR2/6 heterodimers triggering an inflammatory response. This recognition takes place on the cell surface within specialized microdomains for ACW, whereas PA seems to trigger responses intracellularly. Interestingly, LF does not trigger a pro-inflammatory response, and when combined with PA, the complex is not sensed by the innate immune system. Overall our data suggest that TLR2/6 heterodimers are responsible for sensing the ACW and PA, whereas the formation of the subsequent toxin (LF + PA) seems to evade detection by the innate immune system contributing to the virulence of the toxin.

Cross-linked forms of the isolated N-terminal domain of the lethal factor are potent inhibitors of anthrax toxin

The proteins that comprise anthrax toxin self-assemble at the mammalian cell surface into a series of toxic complexes, each containing a heptameric form of protective antigen (PA) plus up to a total of three molecules of the enzymatic moieties of the toxin (lethal factor [LF] and edema factor [EF]). These complexes are trafficked to the endosome, where the PA heptamer forms a pore in the membrane under the influence of low pH, and bound LF and EF unfold and translocate through the pore to the cytosol. To explore the hypothesis that the PA pore can translocate multiple, cross-linked polypeptides simultaneously, we cross-linked LF(N), the N-terminal domain of LF, via an introduced cysteine at its N or C terminus and characterized the products. Both dimers and trimers of LF(N) retained the ability to bind to PA pores and block ion conductance, but they were unable to translocate across the membrane, even at high voltages or with a transmembrane pH gradient. The multimers were remarkably potent inhibitors of toxin action in mammalian cells (20- to 50-fold more potent than monomeric LF(N)) and in a zebrafish model system. These findings show that the PA pore cannot translocate multimeric, cross-linked polypeptides and demonstrate a new approach to generating potent inhibitors of anthrax toxin.

LRP5 and LRP6 are not required for protective antigen-mediated internalization or lethality of anthrax lethal toxin

Anthrax toxin (AnTx) plays a key role in the pathogenesis of anthrax. AnTx is composed of three proteins: protective antigen (PA), edema factor, and lethal factor (LF). PA is not toxic but serves to bind cells and translocate the toxic edema factor or LF moieties to the cytosol. Recently, the low-density lipoprotein receptor–related protein LRP6 has been reported to mediate internalization and lethality of AnTx. Based on its similarity to LRP6, we hypothesized that LRP5 may also play a role in cellular uptake of AnTx. We assayed PA-dependent uptake of anthrax LF or a cytotoxic LF fusion protein (FP59) in cells and mice harboring targeted deletions of Lrp5 or Lrp6. Unexpectedly, we observed that uptake was unaltered in the presence or absence of either Lrp5 or Lrp6 expression. Moreover, we observed efficient PA-mediated uptake into anthrax toxin receptor (ANTXR)–deficient Chinese hamster ovary cells (PR230) that had been stably engineered to express either human ANTXR1 or human ANTXR2 in the presence or absence of siRNA specific for LRP5 or LRP6. Our results demonstrate that neither LRP5 nor LRP6 is necessary for PA-mediated internalization or lethality of anthrax lethal toxin.

The effects of many pathogenic bacteria are caused by the toxins they release. The toxin released by bacteria that cause anthrax is particularly fascinating since it is made of three different proteins: edema factor, lethal factor, and protective antigen (PA). On their own, each of these proteins is harmless, but when combined, they are deadly. This is because edema factor and lethal factor can exert their poisonous effects only after they have been moved into cells by PA. Determining exactly how PA does this is seen as a critical step in developing medicines that will fight anthrax. That is why a recent report suggesting that LRP6, an outer cell protein, was needed for PA to move the other toxin proteins into cells, was greeted with such interest. However, we now show that mice or cells lacking LRP6, or a related protein called LRP5, are still susceptible to anthrax toxin. The discovery that PA can move lethal factor and edema factor into cells without the help of LRP6 presents a significant challenge to the previously published model. These findings will help focus the efforts of scientists working on new ways to treat anthrax.

Subtilisin-like proteases in nematodes

Cleavage by subtilisin-like proteases (subtilases) is an essential step in post-translational processing of proteins found in organisms ranging from yeast to mammals. Our knowledge of the diversity of this protease family in nematodes is aided by the rapid increase in sequence information, especially from the Brugia malayi genome project. Genetic studies of the subtilases in Caenorhabitis elegans give valuable insight into the biological function of these proteases in other nematode species. In this review, we focus on the subtilases in filarial nematodes as well as other parasitic and free-living nematodes in comparison to what is known in C. elegans. Topics to be addressed include expansion and diversity of the subtilase gene family during evolution, enhanced complexity created by alternative RNA splicing, molecular and biochemical characterization of the different subtilases and the challenges of designing subtilase-specific inhibitors for parasitic nematodes.

Differential P1 arginine and lysine recognition in the prototypical proprotein convertase Kex2

The high-resolution crystal structure of kexin (Kex2) in complex with a peptidyl-chloromethylketone inhibitor containing a noncognate lysine at the P(1) position provides the structural basis for the differential lysine/arginine selectivity that defines the prohormone (proprotein) convertase (PC) family. By comparison with the previous structures of Kex2 and furin, this structure of the acylated enzyme provides a basis for the observed decrease in the acylation rate with substrates containing a lysine at P(1) and the absence of an effect on the deacylation rate without involving mobility of the S(1) lid. The structure of the complex shows that a secondary subsite in the S(1) pocket is present, and that this site recognizes and binds the P(1) lysine in a more shallow fashion than arginine. This results in a displacement of the bound peptide away from the S385 nucleophile relative to substrates containing a P(1) arginine. It is concluded that this alternate binding site and resultant displacement of the scissile bond in the active site results in the observed decrease in the acylation rate. Studies of the inactivation kinetics of Kex2 by two peptidyl chloromethylketone inhibitors demonstrates that the selectivity between lysine and arginine at the P(1) position arises at the acylation step, consistent with what was observed with peptidyl substrates [Rockwell NC, Fuller RS (2001) J Biol Chem 276:38394-38399].

A high performance liquid chromatography assay for monitoring proprotein convertase activity

A rapid HPLC assay was developed for monitoring the activity of the two proprotein convertases, PACE-4 and furin. Six novel peptide substrates were synthesized containing the minimal PC recognition sequence (Arg-X-X-Arg), as well as tryptophan residue(s) for easy detection. Four of the peptides were cleaved by both PCs and their kinetic parameters determined. Two peptides were not cleaved but were shown to be good negative controls although not inhibitors of either PC. In addition, inhibition curves were plotted and IC(50) values calculated for PACE-4 and furin in the presence of two polyarginine peptides, hexa and deca-D-arginine.

Processing and trafficking of a prohormone convertase 2 active site mutant

Processing of most PC zymogens is required for successful folding and/or passage through the secretory pathway; active site mutants are retained in the ER and degraded. We here report that the active site serine mutant of PC2 (PC2-S383A) was efficiently secreted as the intact zymogen in CHO-K1 cells, suggesting that its propeptide can productively insert into the mutated binding pocket without causing misfolding. In AtT-20 cells, PC2-S383A was cleaved at the secondary cleavage site within the propeptide; this cleavage event was pH-dependent and was inhibited by a proprotein convertase inhibitor. In vitro digestion of PC2-S383A with various convertases indicates that this site is accessible to in trans cleavage. Abundant immunoreactive S383A PC2 was found in secretory granules, supporting the idea that this protein is efficiently trafficked through the secretory pathway.

Mechanistic differences between in vitro assays for hydrazone-based small molecule inhibitors of anthrax lethal factor

A systematically generated series of hydrazones were analyzed as potential inhibitors of anthrax lethal factor. The hydrazones were screened using one UV-based and two fluorescence-based in vitro assays. The study identified several inhibitors with IC50 values in the micromolar range, and importantly, significant differences in the types of inhibition were observed with the different assays.

Murine innate immune response to virulent toxigenic and nontoxigenic Bacillus anthracis strains

Effective treatment of anthrax is hampered by our limited understanding of the pathophysiology of Bacillus anthracis infection. We used a genetically complete (pXO1(+) pXO2(+)) virulent B. anthracis strain and four isogenic toxin-null mutants to determine the effects of the anthrax edema toxin (ET; edema factor [EF] plus protective antigen [PA]) and lethal toxin (LT; lethal factor [LF] plus PA) on the host innate response during systemic infection. Using the spleen as an indicator for host response, we found that intravenous inoculation of LT-deficient mutants into C57BL/6 mice significantly increased production of several cytokines over that observed after infection with the parent strain or an EF-deficient mutant. Bacteria producing one or both of the toxins were capable of inducing significant apoptosis of cells present in spleens, whereas apoptosis was greatly reduced in mice infected with nontoxigenic mutants. Mice infected with toxin-producing strains also showed increased splenic neutrophil recruitment compared to mice infected with nontoxigenic strains and neutrophil depletion prior to infection with toxin-producing strains, leading to decreased levels of apoptosis. Together, these studies indicate that anthrax LT suppresses cytokine secretion during infection, but both EF and LF play roles in inducing neutrophil recruitment and enhancing apoptosis. Interestingly, in the absence of LF the effect of EF-induced cell recruitment is further enhanced, perhaps because LF so effectively suppresses the secretion of chemokines.

The structure of the TGF-β latency associated peptide region determines the ability of the proprotein convertase furin to cleave TGF-βs

The TGF-beta family members are generated as latent pre-pro-polypeptides. The active mature peptides are cleaved from the latent forms by cellular proteases. TGF-beta 1, for instance, is predominantly processed by a substilisin-like proprotein convertase, furin. TGF-beta 2 has a consensus cleavage site for furin and therefore has been presumed to be cleaved by furin. However, TGF-beta 2 is often secreted as the latent form, which appears to be inconsistent with its postulated sensitivity to furin. We report here that both the regular (short) form of TGF-beta2 and its spliced variant with an additional exon (long form) are insensitive to furin. NIH 3T3 and CHO cells were transfected with expression vectors containing the short or long form of TGF-beta 2 or a chimeric TGF-beta consisting of the TGF-beta1 LAP region, the TGF-beta 2 cleavage site and the TGF-beta 2 mature peptide. The constructs included a c-myc epitope tag in the N-terminal region of the mature peptide. The TGF-betas produced by the transfected cells were analyzed with Western blots and immunocytochemistry. The intracellular proteins harvested from these cells were incubated with furin. Furin only inefficiently cleaved both the long and short forms of TGF-beta 2, but efficiently processed the chimeric TGF-beta. This indicates that the insensitivity of both forms of TGF-beta 2 to furin is a consequence of the tertiary structure of their LAP regions rather than their cleavage site. This differential processing of TGF-beta1 and -beta 2 may be part of the mechanism that generates isoform-specific functions of the TGF-betas.

Diseases involving the Golgi calcium pump

Secretory-pathway Ca2(+)-transport ATPases (SPCA) provide the Golgi apparatus with Ca2+ and Mn2+ needed for the normal functioning of this organelle. Loss of one functional copy of the human SPCA1 gene (ATP2C1) causes Hailey-Hailey disease, a rare skin disorder characterized by recurrent blisters and erosions in the flexural areas. Here, we will review the properties and functional role of the SPCAs. The relationship between Hailey-Hailey disease and its defective gene (ATP2C1) will be adressed as well.

Synthetic small molecule furin inhibitors derived from 2,5-dideoxystreptamine

Furin plays a crucial role in embryogenesis and homeostasis and in diseases such as Alzheimer's disease, cancer, and viral and bacterial infections. Thus, inhibition of furin may provide a feasible and promising approach for therapeutic intervention of furin-mediated disease mechanisms. Here, we report on a class of small molecule furin inhibitors based on 2,5-dideoxystreptamine. Derivatization of 2,5-dideoxystreptamine by the addition of guanidinylated aryl groups yielded a set of furin inhibitors with nanomolar range potency against furin when assayed in a biochemical cleavage assay. Moreover, a subset of these furin inhibitors protected RAW 264.7 macrophage cells from toxicity caused by furin-dependent processing of anthrax protective antigen. These inhibitors were found to behave as competitive inhibitors of furin and to be relatively specific for furin. Molecular modeling revealed that these inhibitors may target the active site of furin as they showed site occupancy similar to the alkylating inhibitor decanoyl-Arg-Val-Lys-Arg-CH(2)Cl. The compounds presented here are bona fide synthetic small molecule furin inhibitors that exhibit potency in the nanomolar range, suggesting that they may serve as valuable tools for studying furin action and potential therapeutics agents for furin-dependent diseases.

Insertion of anthrax protective antigen into liposomal membranes: effects of a receptor

Protective antigen (PA), the receptor-binding component of anthrax toxin, heptamerizes and inserts into the endosomal membrane at acidic pH, forming a pore that mediates translocation of the enzymic components of the toxin to the cytosol. When the heptameric pre-insertion form of PA (the prepore) is acidified in solution, it rapidly loses the ability to insert into membranes. To maximize insertion into model membranes, we examined two ways to bind the protein to large unilamellar vesicles (LUV). One involved attaching a His tag to the von Willebrand factor A domain of one of the PA receptors, ANTXR2, and using this protein as a bridge to bind PA to LUV containing a nickel-chelating lipid. The other involved using a His tag fused to the C terminus of PA to bind the protein directly to LUV containing the same lipid. Both ways enhanced pore formation at pH 5.0 strongly and about equally, as measured by the release of K+. Controls showed that pore formation in this system faithfully reproduced that in vivo. We also showed that binding unmodified ANTXR2 von Willebrand factor A to the prepore in solution enhanced its pore forming activity by slowing its inactivation at acidic pH. These findings indicate that an important role of PA receptors is to promote partitioning of PA into the bilayer by maintaining the prepore close to the target membrane and presumably in the optimal orientation as it undergoes the acidic pH-dependent conformational transition to the pore.

Regulation of Bone Morphogenetic Protein-4 Activity by Sequence Elements within the Prodomain

Bone morphogenetic protein-4 (BMP-4) is synthesized as a large precursor protein, which undergoes proprotein convertase-mediated proteolytic maturation along the secretory pathway to release the active ligand. Pro-BMP-4 is initially cleaved at a consensus furin motif adjacent to the mature ligand domain (the S1 site), and this allows for subsequent cleavage at an upstream motif (the S2 site). This sequential cleavage liberates a small, evolutionarily conserved, prodomain fragment (the linker peptide) of unknown fate and function. Here we show that the linker domain is essential for proper folding, exit from the endoplasmic reticulum, and thus cleavage of the BMP-4 precursor when overexpressed in Xenopus oocytes and embryos but not in cultured mammalian cells. Mature BMP-4 synthesized from a precursor in which the S1 site is non-cleavable, such that the linker domain remains covalently attached to the ligand, has little or no activity in vivo. Finally, analysis of folding, cleavage, and bioactivity of chimeric precursors containing the BMP-7 prodomain and BMP-4 mature domain, or vice versa, with or without the BMP-4 linker domain revealed that the linker domain is only functional in the context of the BMP-4 prodomain, and that differential cleavage around this domain can regulate the activity of a heterologous ligand.

Proteolytic processing of the Cryptosporidium glycoprotein gp40/15 by human furin and by a parasite-derived furin-like protease activity

The apicomplexan parasite Cryptosporidium causes diarrheal disease worldwide. Proteolytic processing of proteins plays a significant role in host cell invasion by apicomplexan parasites. In previous studies, we described gp40/15, a Cryptosporidium sp. glycoprotein that is proteolytically cleaved to yield two surface glycopeptides (gp40 and gp15), which are implicated in mediating infection of host cells. In the present study, we showed that biosynthetically labeled gp40/15 is processed in Cryptosporidium parvum-infected HCT-8 cells. We identified a putative furin cleavage site RSRR downward arrow in the deduced amino acid sequence of gp40/15 from C. parvum and from all Cryptosporidium hominis subtypes except subtype 1e. Both human furin and a protease activity present in a C. parvum lysate cleaved recombinant C. parvum gp40/15 protein into 2 peptides, identified as gp40 and gp15 by size and by immunoreactivity with specific antibodies. C. hominis gp40/15 subtype 1e, in which the RSRR sequence is replaced by ISKR, has an alternative furin cleavage site (KSISKR downward arrow) and was also cleaved by both furin and the C. parvum lysate. Site-directed mutagenesis of the C. parvum RSRR sequence to ASRR resulted in inhibition of cleavage by furin and the C. parvum lysate. Cleavage of recombinant gp40/15 and a synthetic furin substrate by the C. parvum lysate was inhibited by serine protease inhibitors, by the specific furin inhibitor decanoyl-Arg-Val-Lys-Arg-chloromethylketone (Dec-RVKR-cmk), and by calcium chelators, suggesting that the parasite expresses a Ca2+ dependent, furin-like protease activity. The furin inhibitor Dec-RVKR-cmk decreased C. parvum infection of HCT-8 cells, suggesting that a furin-like protease activity may be involved in mediating host-parasite interactions.

Characterization of human metapneumovirus F protein-promoted membrane fusion: critical roles for proteolytic processing and low pH

Human metapneumovirus (HMPV) is a recently described human pathogen of the pneumovirus subfamily within the paramyxovirus family. HMPV infection is prevalent worldwide and is associated with severe respiratory disease, particularly in infants. The HMPV fusion protein (F) amino acid sequence contains features characteristic of other paramyxovirus F proteins, including a putative cleavage site and potential N-linked glycosylation sites. Propagation of HMPV in cell culture requires exogenous trypsin, which cleaves the F protein, and HMPV, like several other pneumoviruses, is infectious in the absence of its attachment protein (G). However, little is known about HMPV F-promoted fusion, since the HMPV glycoproteins have yet to be analyzed separately from the virus. Using syncytium and luciferase reporter gene fusion assays, we determined the basic requirements for HMPV F protein-promoted fusion in transiently transfected cells. Our data indicate that proteolytic cleavage of the F protein is a stringent requirement for fusion and that the HMPV G protein does not significantly enhance fusion. Unexpectedly, we also found that fusion can be detected only when transfected cells are treated with trypsin and exposed to low pH, indicating that this viral fusion protein may function in a manner unique among the paramyxoviruses. We also analyzed the F protein cleavage site and three potential N-linked glycosylation sites by mutagenesis. Mutations in the cleavage site designed to facilitate endogenous cleavage did so with low efficiency, and our data suggest that all three N-glycosylation sites are utilized and that each affects cleavage and fusion to various degrees.

The Actin cross-linking domain of the Vibrio cholerae RTX toxin directly catalyzes the covalent cross-linking of actin

Vibrio cholerae is a Gram-negative bacterial pathogen that exports enterotoxins to alter host cells and to elicit diarrheal disease. Among the secreted toxins is the multifunctional RTX toxin, which causes cell rounding and actin depolymerization by covalently cross-linking actin monomers into dimers, trimers, and higher multimers. The region of the toxin responsible for cross-linking activity is the actin cross-linking domain (ACD). In this study, we further investigated the role of the ACD in the actin cross-linking reaction. We show that the RTX toxin cross-links actin independently of tissue transglutaminase, thus eliminating an indirect model of ACD activity. We demonstrate that a fusion protein of the ACD and the N-terminal portion of lethal factor from Bacillus anthracis (LF(N)ACD) has cross-linking activity in vivo and in crude cell extracts. Furthermore, we determined that LF(N)ACD directly catalyzes the formation of covalent linkages between actin molecules in vitro and that Mg(2+) and ATP are essential cofactors for the cross-linking reaction. In addition, G-actin is proposed as a cytoskeletal substrate of the RTX toxin in vivo. Future studies of the in vitro cross-linking reaction will facilitate characterization of the enzymatic properties of the ACD and contribute to our knowledge of the novel mechanism of covalent actin cross-linking.

Inhibition of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) infectivity by peptides analogous to the viral spike protein

Severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is the cause of an atypical pneumonia that affected Asia, North America and Europe in 2002-2003. The viral spike (S) glycoprotein is responsible for mediating receptor binding and membrane fusion. Recent studies have proposed that the carboxyl terminal portion (S2 subunit) of the S protein is a class I viral fusion protein. The Wimley and White interfacial hydrophobicity scale was used to identify regions within the CoV S2 subunit that may preferentially associate with lipid membranes with the premise that peptides analogous to these regions may function as inhibitors of viral infectivity. Five regions of high interfacial hydrophobicity spanning the length of the S2 subunit of SARS-CoV and murine hepatitis virus (MHV) were identified. Peptides analogous to regions of the N-terminus or the pre-transmembrane domain of the S2 subunit inhibited SARS-CoV plaque formation by 40-70% at concentrations of 15-30 microM. Interestingly, peptides analogous to the SARS-CoV or MHV loop region inhibited viral plaque formation by >80% at similar concentrations. The observed effects were dose-dependent (IC50 values of 2-4 microM) and not a result of peptide-mediated cell cytotoxicity. The antiviral activity of the CoV peptides tested provides an attractive basis for the development of new fusion peptide inhibitors corresponding to regions outside the fusion protein heptad repeat regions.

Correlation of serpin–protease expression by comparative analysis of real-time PCR profiling data

Imbalanced protease activity has long been recognized in the progression of disease states such as cancer and inflammation. Serpins, the largest family of endogenous protease inhibitors, target a wide variety of serine and cysteine proteases and play a role in a number of physiological and pathological states. The expression profiles of 20 serpins and 105 serine and cysteine proteases were determined across a panel of normal and diseased human tissues. In general, expression of serpins was highly restricted in both normal and diseased tissues, suggesting defined physiological roles for these protease inhibitors. A high correlation in expression for a particular serpin-protease pair in healthy tissues was often predictive of a biological interaction. The most striking finding was the dramatic change observed in the regulation of expression between proteases and their cognate inhibitors in diseased tissues. The loss of regulated serpin-protease matched expression may underlie the imbalanced protease activity observed in pathological states.

Recombinant expression, purification, and kinetic and inhibitor characterisation of human site-1-protease

Human site-1-protease (S1P, MEROPS S08.8063), also widely known as subtilisin/kexin isozyme 1 (SKI-1), is a membrane bound subtilisin-related serine protease, that belongs to a group of nine mammalian proprotein convertases. Among these proteases, S1P displays unique substrate specificity, by showing preferred cleavage after non-basic amino acids. S1P plays a key role in a proteolytic pathway that controls the cholesterol content of membranes, cells and blood. S1P also participates in the activation of viral coat glycoproteins of the lassa virus, the lympocytic choriomeningitis virus and the crimean congo hemorrhagic fever virus. We expressed recombinant human S1P using the baculovirus expression vector system and characterized the highly purified enzyme. Featuring a new chromogenic substrate (Acetyl-Arg-Arg-Leu-Leu-p-nitroanilide) we show that the enzymatic activity of S1P is not calcium dependent, but can be modulated by a variety of mono- and divalent cations. S1P displayed pronounced positive cooperativity with a substrate derived from the viral coat glycoprotein of the lassa virus. The screening of a limited number of protease inhibitors showed that S1P was not inhibited by specific inhibitors of other proprotein convertases or by Pefabloc SC (4-(2-aminoethyl) benzene sulphonyl fluoride, AEBSF). We found 3,4-dichloroisocoumarin (DCI) to be a potent slow binding inhibitor of human S1P, with a K(iapp) = 6.8 microM, thus representing a new small molecule inhibitor of S1P. These findings show that S1P differs significantly from other proprotein convertases with respect to kinetics, co-factor requirement and inhibition.

Selectively guanidinylated derivatives of neamine. Syntheses and inhibition of anthrax lethal factor protease

A series of mono-, di-, and tri-guanidinylated derivatives of neamine were prepared via selective guanidinylation of neamine. These molecules represent a novel scaffold as inhibitors of anthrax lethal factor zinc metalloprotease. Methods for the synthesis of these compounds are described, and structure-activity relationships among the series are analyzed. In addition, initial findings regarding the mechanism of LF inhibition for these molecules are presented.

Rabies virus glycoprotein as a carrier for anthrax protective antigen

Live viral vectors expressing foreign antigens have shown great promise as vaccines against viral diseases. However, safety concerns remain a major problem regarding the use of even highly attenuated viral vectors. Using the rabies virus (RV) envelope protein as a carrier molecule, we show here that inactivated RV particles can be utilized to present Bacillus anthracis protective antigen (PA) domain-4 in the viral membrane. In addition to the RV glycoprotein (G) transmembrane and cytoplasmic domains, a portion of the RV G ectodomain was required to express the chimeric RV G anthrax PA on the cell surface. The novel antigen was also efficiently incorporated into RV virions. Mice immunized with the inactivated recombinant RV virions exhibited seroconversion against both RV G and anthrax PA, and a second inoculation greatly increased these responses. These data demonstrate that a viral envelope protein can carry a bacterial protein and that a viral carrier can display whole polypeptides compared to the limited epitope presentation of previous viral systems.

A loop network within the anthrax toxin pore positions the phenylalanine clamp in an active conformation

Heptameric pores formed in the endosomal membrane by the protective antigen moiety of anthrax toxin serve as portals for entry of the enzymatic moieties of the toxin into the cytosol. In the aqueous lumen of each pore is a "Phe clamp," a heptad of narrowly apposed Phe residues (Phe-427), that catalyzes the unfolding and translocation of the enzymatic moieties across the membrane. Here, we provide evidence for a "loop swap" between neighboring protective antigen subunits, which is required for efficient translocation and is mediated by a salt bridge formed between the side chains of Lys-397 and Asp-426. We propose that the interaction between residues 397 and 426 creates a structural framework that positions Phe-427 within the pore lumen, forming a functional Phe clamp and, hence, a translocation-competent pore.

Cationic polyamines inhibit anthrax lethal factor protease

Background

Anthrax is a human disease that results from infection by the bacteria, Bacillus anthracis and has recently been used as a bioterrorist agent. Historically, this disease was associated with Bacillus spore exposure from wool or animal carcasses. While current vaccine approaches (targeted against the protective antigen) are effective for prophylaxis, multiple doses must be injected. Common antibiotics that block the germination process are effective but must be administered early in the infection cycle. In addition, new therapeutics are needed to specifically target the proteolytic activity of lethal factor (LF) associated with this bacterial infection.

Results

Using a fluorescence-based assay to identify and characterize inhibitors of anthrax lethal factor protease activity, we identified several chemically-distinct classes of inhibitory molecules including polyamines, aminoglycosides and cationic peptides. In these studies, spermine was demonstrated for the first time to inhibit anthrax LF with a K_i value of 0.9 ± 0.09 μM (mean ± SEM; n = 3). Additional linear polyamines were also active as LF inhibitors with lower potencies.

Conclusion

Based upon the studies reported herein, we chose linear polyamines related to spermine as potential lead optimization candidates and additional testing in cell-based models where cell penetration could be studied. During our screening process, we reproducibly demonstrated that the potencies of certain compounds, including neomycin but not neamine or spermine, were different depending upon the presence or absence of nucleic acids. Differential sensitivity to the presence/absence of nucleic acids may be an additional point to consider when comparing various classes of active compounds for lead optimization.

Self-association of the transmembrane domain of an anthrax toxin receptor

Protective antigen (PA), lethal factor (LF) and edema factor (EF) are secreted individually by Bacillus anthracis. These components of anthrax toxin must then assemble into complexes to intoxicate mammalian cells. Toxin assembly initiates when molecules of PA bind mammalian receptors ANTXR1/2 and are cleaved by surface proteases into 20 kDa and 63 kDa fragments. After PA20 dissociates, receptor-bound PA63 homo-oligomerizes into heptamers. Oligomeric PA63 binds EF and LF and these complexes are internalized into an acidic compartment where the two enzymatic components are translocated across the membrane by a channel formed by heptameric PA63. Since oligomerization of PA63 is required to bind and translocate the enzymatic components, we sought to determine whether interactions between toxin receptors could facilitate the assembly process. In the present work, we performed a co-immunoprecipitation experiment to demonstrate that ANTXR1 is oligomeric in mammalian cells. Computer modeling predicted the self-association of the ANTXR1 transmembrane domain and we detected oligomerization of ANTXR1 transmembrane domain peptides in the membrane-mimetic environment of SDS micelles using fluorescence resonance energy transfer. Furthermore, the ANTXR1 transmembrane domain mediated oligomerization of a reporter protein construct in a bacterial membrane. In both assays, mutations that disrupted the interaction were consistent with the interaction being mediated through an asymmetric binding interface. Mutations that impaired self-association of the transmembrane domain reduced the rate of PA63 heptamer formation on the mammalian cell surface. Our findings indicate that ANTXR1 transmembrane domains self-associate and that these interactions may stabilize intermediate oligomerization states of ANTXR1-PA63 complexes.

Heparin Activates β-Secretase (BACE1) of Alzheimer's Disease and Increases Autocatalysis of the Enzyme

BACE1 is an aspartic protease that generates the N-terminus of the beta-amyloid protein (Alphabeta) from the beta-amyloid precursor protein (APP). BACE1 is a key target for Alzheimer drug development. However, little is known about the physiological regulation of the enzyme. Heparin can promote beta-secretase cleavage of APP in neuroblastoma cells. However, heparin has also been reported to directly inhibit BACE1 activity in vitro. To clarify the role of heparin in regulating BACE1, we examined the effect of heparin on the activity of recombinant human BACE1 (rBACE1) in vitro. Low concentrations (1 microg/mL) of heparin were found to stimulate rBACE1, increasing enzyme V(max) and decreasing the K(M). In contrast, higher concentrations of heparin (10 or 100 microg/mL) were inhibitory. Heparin affinity chromatography demonstrated that heparin interacted strongly with the zymogen form of rBACE1 and bound to a peptide homologous to the N-terminal pro sequence of BACE1. Mature (pro sequence cleaved) enzyme lacked the capacity to be stimulated by heparin, indicating that the pro domain was necessary for the stimulation by heparin. Furthermore, in the presence of stimulatory concentrations of heparin, there was an increase in autocatalytic cleavage of the protease domain and a subsequent loss of enzyme activity in vitro. Our results strongly suggest that heparin stimulates the partially active BACE1 zymogen, and we propose that the activation is mediated by high-affinity binding of heparin to the pro domain. Our study provides evidence that heparan sulfate proteoglycans could regulate the rate of Alphabeta production in vivo.

Bridging neuropeptidomics and genomics with bioinformatics: Prediction of mammalian neuropeptide prohormone processing

Neuropeptides are an important class of cell to cell signaling molecules that are difficult to predict from genetic information because of their large number of post-translational modifications. The transition from prohormone genetic sequence information to the determination of the biologically active neuropeptides requires the identification of the cleaved basic sites, among the many possible cleavage sites, that exist in the prohormone. We report a binary logistic regression model trained on mammalian prohormones that is more sensitive than existing methods in predicting these processing sites, and demonstrate the application of this method to mammalian neuropeptidomic studies. By comparing the predictive abilities of a binary logistic model trained on molluscan prohormone cleavages with the reported model, we establish the need for phyla-specific models.

The roles of specific genes implicated as circulating factors involved in normal and disordered phosphate homeostasis: frizzled related protein-4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor 23

Normal serum phosphate (Pi) concentrations are relatively tightly controlled by endocrine mediators of Pi balance. Recent data involving several disorders of Pi homeostasis have shed new light on the regulation of serum Pi balance. It has been hypothesized that circulating phosphaturic factors, or phosphatonins, exist that, when present at high serum concentrations, directly act on the kidney to induce renal Pi wasting. This review will focus upon recently discovered factors that are overexpressed in tumors associated with tumor-induced osteomalacia and have reported activity consistent with effecting Pi balance in vivo. Currently, the best-characterized group of phosphatonin-like polypeptides includes secreted frizzled related protein-4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor-23. Our understanding of these factors will, in the short term, aid us in understanding normal Pi balance and, in the future, help to design novel therapeutic strategies for disorders of Pi handling.

Inhibition of mitogen-activated protein kinase signalling by Bacillus anthracis lethal toxin causes destabilization of interleukin-8 mRNA

Bacillus anthracis must overcome host innate immune defences to establish a systemic anthrax infection. This is facilitated in part by lethal toxin (LT), a secreted virulence factor that consists of a cell-binding moiety, protective antigen (PA), and an enzymatic moiety, lethal factor (LF). PA binds cells through protein receptors and mediates the delivery of LF to the cytosol. LF is a protease that cleaves amino-terminal fragments from mitogen-activated protein kinase kinases (MAPKKs), preventing phosphorylation of their downstream targets. Here we report that LT reduces the amount of interleukin (IL)-8 produced and secreted by human endothelial cells. The reduction of IL-8 levels by LT was not attributable to reduced expression from the IL-8 promoter, but resulted from destabilization of IL-8 mRNA. Destabilization by LT was mediated through the 3' untranslated region of the IL-8 transcript and could be mimicked by pharmacological inhibitors of MAPK pathways. LT diminished the induction of IL-8 mRNA and protein by lipopolysaccharide, indicating that the toxin can impair the ability of these cells to initiate an immune response. Destabilization of a cytokine transcript represents a new interference strategy used by either a bacterial or viral pathogen to reduce cytokine expression and may help B. anthracis to evade host immune defences.

Assembly and disassembly kinetics of anthrax toxin complexes

Proteolytic activation of the protective antigen (PA) component of anthrax toxin allows it to self-associate into a ring-shaped homoheptamer, [PA(63)](7), which can bind the enzymatic components lethal factor (LF) and edema factor (EF). [PA(63)](7) is a pore-precursor (prepore), and under the low-pH conditions of the endosome, it forms a transmembrane pore that allows LF and EF to enter the cytosol. PA was labeled with donor and acceptor fluorescent dyes, and Förster resonance energy transfer was used to measure the assembly and disassembly kinetics of the prepore complex in solution. The dissociation rate constant for [PA(63)](7) was 1 x 10(-)(6) s(-)(1) (t(1/2) approximately 7 days). In contrast, a ternary complex containing the PA-binding domain of LF (LF(N)) bound to a PA(63) dimer composed of two nonoligomerizing mutants dissociated rapidly (t(1/2) approximately 1 min). Thus, the substantial decrease in the rate of disassembly of [PA(63)](7) relative to the ternary complex is due to the cooperative interactions among neighboring subunits in the heptameric ring. Low concentrations of LF(N) promoted assembly of the prepore from proteolytically activated PA, whereas high concentrations inhibited assembly of both the prepore and the ternary complex. A self-assembly scheme of anthrax toxin complexes is proposed.