Two forms of factor C from the amoebocytes of Carcinoscorpius rotundicauda: purification and characterisation

Host Defense Proteins and Peptides with Lipopolysaccharide-Binding Activity from Marine Invertebrates and Their Therapeutic Potential in Gram-Negative Sepsis

Sepsis is a life-threatening complication of an infectious process that results from the excessive and uncontrolled activation of the host's pro-inflammatory immune response to a pathogen. Lipopolysaccharide (LPS), also known as endotoxin, which is a major component of Gram-negative bacteria's outer membrane, plays a key role in the development of Gram-negative sepsis and septic shock in humans. To date, no specific and effective drug against sepsis has been developed. This review summarizes data on LPS-binding proteins from marine invertebrates (ILBPs) that inhibit LPS toxic effects and are of interest as potential drugs for sepsis treatment. The structure, physicochemical properties, antimicrobial, and LPS-binding/neutralizing activity of these proteins and their synthetic analogs are considered in detail. Problems that arise during clinical trials of potential anti-endotoxic drugs are discussed.

Production of codon-optimized Factor C fragment from Tachypleus gigas in the Pichia pastoris GS115 expression system for endotoxin detection

BACKGROUND

Factor C (FC) is widely used as a standard material for endotoxin testing. It functions as a zymogenic serine protease and serve as a biosensor that detects lipopolysaccharides. Prior investigations involving molecular docking and molecular dynamics simulations of FC demonstrated an interaction between the C-type lectin domain (CLECT) and the ligand lipopolysaccharide (lipid A). In this study, our aim was to assess the stability of the interaction between fragment FC and the lipid A ligand using protein modeling approaches, molecular docking, molecular dynamics simulation, and gene construction into the pPIC9K expression vector.

METHODS AND RESULTS

The FC structure was modelled by online tools. In this case, both molecular docking and MD simulations were applied to identify the interaction between protein and ligand (lipid A) including its complex stability. The FC structure model using three modeling websites has varied values, according to a Ramachandran plot study. When compared to other models, AlphaFold server modeling produced the best Ramachandran findings, with residues in the most advantageous area at 88.3%, followed by ERRAT values at 89.83% and 3D Verify at 71.93%. From the docking simulation of FC fragments with three ligands including diphosphoryl lipid A, FC-Core lipid A, and Kdo2 lipid A can be an activator of FC protein by binding to receptor regions to form ligand-receptor complexes. MD simulations were performed on all three complexes to assess their stability in water solvents showing that all complexes were stable during the simulation. The optimization of recombinant protein expression in Pichia pastoris was conducted by assessing the OD value and protease activity. Induction was carried out using 1% (v/v) methanol in BMMY media at 30°C for 72 h.

CONCLUSIONS

Protein fragments of Factor C has been proven to detect endotoxins and serve as a potential biomarker. Molecular docking simulation and MD simulation were employed to study the complex formation of protein fragments FC with ligands. The expression of FC fragments was successfully achieved through heterologous expression. We propose optimizing the expression of FC fragments by inducing them with 1% methanol at 30°C and incubating them for 72 h. These optimized conditions are well-suited for upscaling the production of recombinant FC fragments using a bioreactor.

Characterization, protein modeling, and molecular docking of factor C from Indonesian horseshoe crab (Tachypleus gigas)

BACKGROUND

Horseshoe crab (Tachypleus gigas) amebocytes are useful biomedical components for endotoxin detection, and their growing needs for biomedical purposes cause the horseshoe crab population to decline. Factor C synthesis via genetic engineering offers a solution to replace natural horseshoe crab's factor C and prevent its excessive harvest from nature. In response to these concerns, this study aimed to characterize the amebocyte lysates and factor C protein modeling of T. gigas originated from Banyuasin South Sumatra Estuary.

METHODS AND RESULTS

Sampling of T. gigas was carried out in Banyuasin South Sumatra Estuary, Indonesia. The endotoxin test or TAL (Tachypleus amebocyte lysates) assay was performed using gel coagulation method. Protein characterization of protease enzyme was conducted by protease activity, SDS-PAGE, and zymogram analysis. The cDNA of mitochondrial COI gene was amplified for molecular identification followed by cDNA cloning of factor C. Protein modeling was investigated by molecular docking and molecular dynamic (MD) simulation. Endotoxin test results showed that TAL-35 had endotoxin sensitivity in a range of 0.0156-1 EU/ml, while TAL 36 had a sensitivity between 00,625 and 1 EU/ml. T. gigas amebocytes have protease activity in molecular mass sizes less than 60 kDa, with 367 U/ml for TAL 35 and 430 U/ml for TAL 36. The molecular identification revealed 98.68% identity similarity to T. gigas. The docking results suggested three ligands; i.e., diphosphoryl lipid A, core lipid A, and Kdo2 lipid A can be activators of the factor C protein by binding to the region of the receptor to form a ligand-receptor complex.

CONCLUSIONS

Endotoxins can be detected using horseshoe crab amebocytes. The presence of proteases is considered responsible for this ability, as evidenced by casein zymogram results. According to docking and MD analysis, we found that lipopolysaccharides (LPS) participate to the binding site of factor C.

Citizen science frontiers horseshoe crab population regain at their spawning beach in East Peninsular Malaysia

Carcinoscorpius rotundicauda and Tachypleus gigas may co-exist and share common spawning grounds elsewhere but at Balok (East Coast of Peninsular Malaysia), C. rotundicauda is an understudied species. Neglected as research candidate because of inaccessible spawning grounds, smaller size and less commercial value than T. gigas and also, difficulty to attain from the wild has made C. rotundicauda population status remaining unidentified at Balok. This standpoint drove the present attempt because anthropic activities like structure placement and mining are point-source for runoffs that load sediments into Balok River. While erosion-accretion events have altered Balok River width, the shore sediments in Balok Beach were transitioned between medium-fine and fine sand between years 2012 and 2016. Eventually by year 2016, the C. rotundicauda were depositing 5117 eggs in 91 nests from 200 to 1000 m range along this corridor facing South China Sea. From this yield, C. rotundicauda released 2880 eggs in 56 nests during the Southwest monsoon, 1254 eggs in 19 nests during the Northeast monsoon and 983 eggs in 16 nests during the Inter-monsoon seasons. Though female C. rotundicauda opted to lay their eggs in shallow burrows at lower shorelines, the absence of erosion and substantial silt and clay (>20%) deposition facilitates C. rotundicauda embryogenesis with brief periods of temperature and salinity shocks during day-time falling tides. This encourages C. rotundicauda to emerge with increasing abundance and carry out bi-monthly spawning at Balok Beach. In short, shore restoration initiatives like systematic boat docking, proper disposal of nets and waste and, periodic fish-catching operations were effectively led by the Balok fisher citizen scientist. This successful community joint-cooperation proves that citizen-led caretaking of degraded beaches offers marine life protection and are practical for coastal area management especially at areas where other oviparous animals such as turtles and crocodiles are harboured.

Alpha-2-macroglobulin is a modulator of prophenoloxidase system in pacific white shrimp Litopenaeus vannamai

The shrimp multifunctional protein alpha-2-macroglobulin (A2M) is abundantly expressed in plasma, highly up-regulated upon microbial infection and involved in several immune pathways such as blood clotting system, phagocytosis and melanization. Herein, the function of LvA2M from Litopenaeus vannamei on the prophenoloxidase (proPO) system is reported. The recombinant (r)LvA2M produced strongly and specifically inhibited trypsin and the PO activity in shrimp plasma in a dose-dependent manner. Silencing of LvA2M led to an increase in the PO activity in shrimp plasma although the expression of proPO-associated genes, proPO-activating enzyme (PPAE) and prophenoloxidase (proPO) but not the proPO-activating factor (PPAF) was down-regulated. In Vibrio parahaemolyticus AHPND-infected shrimp, the LvA2M activity was suppressed in an early phase of infection while the PO activity was increased. Thus, the proPO-activating system was regulated by the LvA2M.

Genetic engineering approach to develop next-generation reagents for endotoxin quantification

The bacterial endotoxin test, which uses amebocyte lysate reagents of horseshoe crab origin, is a sensitive, reproducible and simple assay to measure endotoxin concentration. To develop sustainable raw materials for lysate reagents that do not require horseshoe crabs, three recombinant protease zymogens (factor C, derived from mammalian cells; factor B; and the proclotting enzyme derived from insect cells) were prepared using a genetic engineering technique. Recombinant cascade reagents (RCRs) were then prepared to reconstruct the reaction cascade in the amebocyte lysate reagent. The protease activity of the RCR containing recombinant factor C was much greater than that of recombinant factor C alone, indicating the efficiency of signal amplification in the cascade. Compared with the RCR containing the insect cell-derived factor C, those containing mammalian cell-derived factor C, which features different glycosylation patterns, were less susceptible to interference by the injectable drug components. The standard curve of the RCR containing mammalian cell-derived recombinant factor C had a steeper slope than the curves for those containing natural lysate reagents, suggesting a greater sensitivity to endotoxin. The present study supports the future production of recombinant reagents that do not require the use of natural resources.

Both systemic and local lipopolysaccharide (LPS) burden are associated with knee OA severity and inflammation

OBJECTIVE

The microbiome is recognized as a new frontier in medicine with connections to a variety of diseases. We aimed to evaluate the association of lipopolysaccharide (LPS), a key pro-inflammatory product of the microbiome, with severity of inflammation, symptoms and radiographic abnormalities of knee osteoarthritis (OA).

DESIGN

LPS was measured using a recombinant Factor C (rFC) assay, carefully optimized for systemic and synovial fluid (SF) analyses. LPS binding protein (LBP) was tested in both serum and SF of 25 patients (31 knees) from the Etarfolatide cohort for association with OA phenotypic outcomes. Models were adjusted for age, gender and body mass index.

RESULTS

Based on LPS spike-and-recovery, both serum and SF dilutions of 0.1% were required to achieve recovery rates of at least 75% in all test specimens. Low coefficients of variation (CVs) (<10%) were achieved with both serum and SF dilutions <0.2%. Serum LPS and LBP were associated with the abundance of activated macrophages in the knee joint capsule and synovium. SF LPS and LBP were associated with the abundance of activated macrophages in the synovium. Serum LPS, LBP and SF LPS were associated with knee osteophyte severity. SF LPS was positively associated with knee joint space narrowing (JSN) severity and total WOMAC score. SF LBP was positively associated with self-reported knee pain score.

CONCLUSION

These data strongly support a role for LPS in the pathogenesis and severity of structural abnormalities and symptoms of knee OA.

Expression of full length and deletion homologues of Carcinoscorpius rotundicauda Factor C in Saccharomyces cerevisiae: immunoreactivity and endotoxin binding

Deletion homologues of the cloned Factor C cDNAs from the horseshoe crab Carcinoscorpius rotundicauda were engineered to express in Saccharomyces cerevisiae under the regulation of a galactose-inducible promoter. Expression cassettes were constructed in the vectors: pEMBLyex4 and YEpsec1 to direct, respectively, the intracellular expression, and the secretion of the protein into the culture medium using a heterologous signal sequence. The effect of insert size on the efficiency of expression and the functionality of the resulting recombinant Factor C (rFC) were studied by creating expression constructs bearing various deletion and/or hybrid fragments of Factor C. Removal of the long 5' UTR from the Factor C cDNA improved expression of the rFC. 3' Deletions of up to 84%, or internal deletions of 65% of the Factor C cDNA resulted in either the lack of detectable amounts of Factor C or loss of immunoreactivity. Depending on the construct, full length or partial rFC-related proteins were correspondingly expressed intracellularly, regardless of the vector. The rFC partitioned with the insoluble cell fraction, was solubilised with either SDS or Triton X-100, and found to be immunoreactive. The rFCs were functionally active, being able to bind Gram-negative bacterial endotoxin, provided critical regions of the endotoxin-binding domain were preserved.

The isolation and characterization of lipopolysaccharides from Microcystis aeruginosa, a prominent toxic water bloom forming cyanobacteria

Massive toxic blooms of cyanobacteria represent a major threat to water supplies worldwide, yet serious gaps exist in understanding their complex toxic effects, including the role of lipopolysaccharides (LPS). The present comparative study focused on the levels and biological activities of LPS isolated from Microcystis aeruginosa, which is one of the most globally distributed toxic species. Using hot phenol extraction, LPS was isolated from 3 laboratory cultures and 11 natural water blooms. It formed 0.2-0.7% of the original dry biomass of the cyanobacteria, based on gravimetry. Additional analyses by commercial anti-LPS ELISA were correlated with gravimetry but showed concentrations that were about 7-times lower, which indicated either impurities in isolated LPS or the poor cross-reactivity of the antibodies used. LPS isolates from M. aeruginosa were potent pyrogens in the traditional Limulus amebocyte lysate (LAL)-test, but comparison with the PyroGene test demonstrated the limited selectivity of LAL with several interferences. The determined pyrogenicity (endotoxin units, EU) ranged from very low values in laboratory cultures (less than 0.003 up to 0.008-EU per 100 pg LPS) to higher values in complex bloom samples (0.01-0.078 EU per 100 pg of LPS), which suggested the role of bloom-associated bacteria in the overall effects. Potent pro-inflammatory effects of the studied LPS from both cultures and bloom samples were observed in a highly-relevant ex vivo human blood model by studying reactive oxygen species production in phagocytes as well as increased productions of interleukin 8, IL-8, and tumor necrosis factor α, TNF-α. LPS from M. aeruginosa seem to modulate several pathways involved in the regulation of both innate immunity and specific responses. In comparison to the standard pathogenic bacterial LPS (World Health Organization Escherichia coli O113:10 endotoxin; activity 1 EU per 100 pg), the studied cyanobacterial samples had pyrogenicity potencies that were at least 12-times lower. However, the health risks associated with LPS from M. aeruginosa should not be underestimated, especially with respect to diverse biological effects observed ex vivo and in the case of massive blooms in drinking water reservoirs, where the estimated pyrogenicity can reach up to 46,000 EU per mL of water.

Evaluation of lot-to-lot repeatability and effect of assay media choice in the recombinant Factor C assay

Measurement of environmental endotoxin exposures is complicated by variability encountered using current biological assay methods arising in part from lot-to-lot variability of the Limulus-amebocyte lysate (LAL) reagents. Therefore, we investigated the lot-to-lot repeatability of commercially available recombinant Factor C (rFC) kits as an alternative to LAL. Specifically, we compared endotoxin estimates obtained from rFC assay of twenty indoor dust samples, using four different extraction and assay media, to endotoxin estimates previously obtained by Limulus amebocyte lysate (LAL) assay and amounts of 3-hydroxy fatty acids (3-OHFA) in lipopolysaccharide (LPS) using gas-chromatography mass spectroscopy (GC-MS). We found that lot-to-lot variability of the rFC assay kits does not significantly alter endotoxin estimates in house dust samples when performed using three of the four assay media tested and that choice of assay media significantly altered endotoxin estimates obtained by rFC assay of house dust samples. Our findings demonstrate lot-to-lot reproducibility of rFC assay of environmental samples and suggest that use of rFC assay performed with Tris buffer or water as the extraction and assay medium for measurement of endotoxin in dust samples may be a suitable choice for developing a standardized methodology.

Structural basis for dual-inhibition mechanism of a non-classical Kazal-type serine protease inhibitor from horseshoe crab in complex with subtilisin

Serine proteases play a crucial role in host-pathogen interactions. In the innate immune system of invertebrates, multi-domain protease inhibitors are important for the regulation of host-pathogen interactions and antimicrobial activities. Serine protease inhibitors, 9.3-kDa CrSPI isoforms 1 and 2, have been identified from the hepatopancreas of the horseshoe crab, Carcinoscorpius rotundicauda. The CrSPIs were biochemically active, especially CrSPI-1, which potently inhibited subtilisin (Ki = 1.43 nM). CrSPI has been grouped with the non-classical Kazal-type inhibitors due to its unusual cysteine distribution. Here we report the crystal structure of CrSPI-1 in complex with subtilisin at 2.6 Å resolution and the results of biophysical interaction studies. The CrSPI-1 molecule has two domains arranged in an extended conformation. These two domains act as heads that independently interact with two separate subtilisin molecules, resulting in the inhibition of subtilisin activity at a ratio of 1:2 (inhibitor to protease). Each subtilisin molecule interacts with the reactive site loop from each domain of CrSPI-1 through a standard canonical binding mode and forms a single ternary complex. In addition, we propose the substrate preferences of each domain of CrSPI-1. Domain 2 is specific towards the bacterial protease subtilisin, while domain 1 is likely to interact with the host protease, Furin. Elucidation of the structure of the CrSPI-1: subtilisin (1∶2) ternary complex increases our understanding of host-pathogen interactions in the innate immune system at the molecular level and provides new strategies for immunomodulation.

Modifying the substrate specificity of Carcinoscorpius rotundicauda serine protease inhibitor domain 1 to target thrombin

Protease inhibitors play a decisive role in maintaining homeostasis and eliciting antimicrobial activities. Invertebrates like the horseshoe crab have developed unique modalities with serine protease inhibitors to detect and respond to microbial and host proteases. Two isoforms of an immunomodulatory two-domain Kazal-like serine protease inhibitor, CrSPI-1 and CrSPI-2, have been recently identified in the hepatopancreas of the horseshoe crab, Carcinoscorpius rotundicauda. Full length and domain 2 of CrSPI-1 display powerful inhibitory activities against subtilisin. However, the structure and function of CrSPI-1 domain-1 (D1) remain unknown. Here, we report the crystal structure of CrSPI-1-D1 refined up to 2.0 Å resolution. Despite the close structural homology of CrSPI-1-D1 to rhodniin-D1 (a known thrombin inhibitor), the CrSPI-1-D1 does not inhibit thrombin. This prompted us to modify the selectivity of CrSPI-1-D1 specifically towards thrombin. We illustrate the use of structural information of CrSPI-1-D1 to modify this domain into a potent thrombin inhibitor with IC₅₀ of 26.3 nM. In addition, these studies demonstrate that, besides the rigid conformation of the reactive site loop of the inhibitor, the sequence is the most important determinant of the specificity of the inhibitor. This study will lead to the significant application to modify a multi-domain inhibitor protein to target several proteases.

Endotoxin detection--from limulus amebocyte lysate to recombinant factor C

Gram negative bacterial endotoxin is a biological pyrogen that causes fever when introduced intravenously. The endotoxin, also known as lipopolysaccharide (LPS), is found in the outer membrane of Gram-negative bacteria. During Gram-negative sepsis, endotoxin stimulates host macrophages to release inflammatory cytokines. However, excessive inflammation causes multiple organ failure and death. Endotoxins, which are ubiquitous pathogenic molecules, are a bane to the pharmaceutical industry and healthcare community. Thus early and sensitive detection of endotoxin is crucial to prevent endotoxaemia. The limulus amebocyte lysate (LAL) has been widely used for ~30 years for the detection of endotoxin in the quality assurance of injectable drugs and medical devices. The LAL constitutes a cascade of serine proteases which are triggered by trace levels of endotoxin, culminating in a gel clot at the end of the reaction. The Factor C, which normally exists as a zymogen, is the primer of this coagulation cascade. In vivo, Factor C is the perfect biosensor, which alerts the horseshoe crab of the presence of a Gram-negative invader. The hemostatic end-point entraps the invader, killing it and limiting further infection. However, as an in vitro endotoxin detection tool, variations in the sensitivity and specificity of LAL to endotoxin, and the dwindling supply of horseshoe crabs are posing increasing challenges to the biotechnology industry. This has necessitated the innovation of an alternative test for endotoxin. Thus, Factor C became the obvious, albeit tricky target for the recombinant technology effort. This chapter documents the backwater of mining the natural blood lysate of the endangered species to the monumental effort of genetic engineering, to produce recombinant Factor C (rFC). The rFC is a 132 kDa molecule, which was produced as a proenzyme inducible by the presence of trace levels of endotoxin. The rFC forms the basis of the "PyroGene" kit, which is a novel micro-enzymatic endotoxin diagnostic assay for high-throughput screens of endotoxin. Using the rFC, Lonza Inc. has spawned the "PyroSense" which serves as checkpoints of the biotechnology production line. Thus, from cloning to commercial applications, the rFC has initiated a new era in endotoxin-testing for the quality assurance of biomedical products and for the healthcare industry, whilst sparing the endangered horseshoe crabs.

Rapid reprogramming of haemoglobin structure-function exposes multiple dual-antimicrobial potencies

The intrinsic cytotoxicity of cell-free haemoglobin (Hb) has hampered the development of reliable Hb-based blood substitutes for over seven decades. Notably, recent evidence shows that the Hb deploys this cytotoxic attack against invading microbes, albeit, through an unknown mechanism. Here, we unraveled a rapid molecular reprogramming of the Hb structure-function triggered by virulent haemolytic pathogens that feed on the haem-iron. On direct contact with the microbe, the Hb unveils its latent antimicrobial potency, where multiple antimicrobial fragments are released, each harbouring coordinated 'dual-action centres': microbe binding and pseudoperoxidase (POX) cycle activity. The activated Hb fragments anchor onto the microbe while the juxtaposed POX instantly unleashes a localized oxidative shock, killing the pathogen-in-proximity. This concurrent action conceivably restricts the diffusion of free radicals. Furthermore, the host astutely protects itself from self-cytotoxicity by simultaneously releasing endogenous antioxidants. We found that this decryption mechanism of antimicrobial potency is conserved in the ancient invertebrate respiratory protein, indicating its fundamental significance. Our definition of dual-antimicrobial centres in the Hb provides vital clues for designing a safer Hb-based oxygen carrier blood substitute.

The antioxidants dimethylsulfoxide and dimethylthiourea affect the immediate adhesion responses of larval haemocytes from 3 lepidopteran insect species

Antioxidants, dimethylsulfoxide (DMSO) and dimethylthiourea (DMTU), at concentrations not affecting the viability of blood cells (haemocytes) from the larval stage of 3 lepidopteran insects - Galleria mellonella, Lymantria dispar, and Malacosoma disstria - differed in their influence on the innate binding of haemocytes to glass, bacteria to haemocytes, and on humoral responses to alien materials. In vitro DMSO had little effect, whereas DMTU substantially impaired the adhesion of the haemocyte types, the plasmatocytes and granular cells, to slides as well as the attachment of Bacillus subtilis to these haemocytes. Although both antioxidants increased lysozyme and phenoloxidase activities, there was no correlation of enzyme activity and haemocyte adhesion responses, possibly reflecting sequestered radicals. Nitric oxide and hydroxyl radicals offset the DMTU effect. In the absence of antioxidants, inactivate protein kinases A (PKA) and C (PKC) enhanced haemocyte aggregation. In general, DMSO, as opposed to DMTU, did not alter the effects of PKA and PKC activators and inhibitors on haemocyte aggregation or of PKC and PKA activities. High concentrations of DMSO and all levels of DMTU, although inhibiting PKA and PKC, inhibited haemocyte adhesion to slides. Comparable results occurred for DMTU-treated haemocytes incubated with B. subtilis. In vivo DMSO, unlike DMTU, did not impair plasmatocyte or granular cell responses to foreign materials, including bacterial removal from the haemolymph and nodulation.

Recombinant factor C competes against LBP to bind lipopolysaccharide and neutralizes the endotoxicity

Endotoxin-mediated inflammation and septic shock remains a grave challenge to human healthcare management. It is, therefore, a worthwhile effort to develop anti-lipopolysaccharide (LPS) strategies to prevent downstream effects. Here, we demonstrate that purified recombinant Factor C (rFC) cloned from the horseshoe crab, binds LPS with high affinity, preventing it from binding a peptide derived from the human LPS-binding protein (LBP). Factor C is an innate immune defense protein present in the horseshoe crab hemocytes. The full-length rFC was found to be more efficacious in blocking LBP-mediated downstream effects than either of the individual LPS-binding peptides (Sushi 1 and Sushi 3) derived from rFC. When added to human macrophage culture, rFC blocks the LPS-induced phosphorylation of p38, which, in turn, inhibits the consequential overexpression of TNF-alpha and IL-8. The tandem arrangement of the LPS-binding Sushi domains in the Factor C molecule appears to be required for the synergy and amplification of LPS-binding in vivo to achieve such high affinity for LPS. Thus, rFC binds and neutralizes LPS to arrest signal transduction via the p38 pathway. The rFC does not show acute cytotoxicity and could be a potential lead for further development into an endotoxin-antagonist to inhibit inflammation and septic shock.

Respiratory protein–generated reactive oxygen species as an antimicrobial strategy

The evolution of the host-pathogen relationship comprises a series of invasive-defensive tactics elicited by both participants. The stereotype is that the antimicrobial immune response requires multistep processes. Little is known about the primordial immunosurveillance system, which probably has components that directly link sensors and effectors. Here we found that the respiratory proteins of both the horseshoe crab and human were directly activated by microbial proteases and were enhanced by pathogen-associated molecular patterns, resulting in the production of more reactive oxygen species. Hemolytic virulent pathogens, which produce proteases as invasive factors, are more susceptible to this killing mechanism. This 'shortcut' antimicrobial strategy represents a fundamental and universal mode of immunosurveillance, which has been in existence since before the split of protostomes and deuterostomes and still persists today.

Recombinant factor C assay for measuring endotoxin in house dust: comparison with LAL, and (1 --> 3)-beta-D-glucans

BACKGROUND

Measurement of exposure to environmental endotoxin is frequently performed using a Limulus amebocyte lysate (LAL) based assay. Recently, a new method has become available with similar sensitivity and potentially greater specificity using recombinant Factor C (rFC) from the horseshoe crab Carcinoscorpius rotundicauda. A preliminary study was carried out to determine the comparability of LAL and rFC in measuring endotoxins in house dust for large scale epidemiologic studies.

METHODS

House dust samples were collected from family rooms by vacuuming 1 m2 of the center of the room. Sixty sieved house dust samples were assayed for endotoxin by LAL (Cambrex, KQCL lysate) and rFC (Pyrogene, Cambrex) and for (1 --> 3)-beta-D-glucans by ELISA. The resistant parallel line estimation was used for data analysis of LAL and rFC. A four-parameter logistic fit with inverse prediction was used to calculate (1 --> 3)-beta-D-glucan levels of the samples.

RESULTS

The spike recovery was 113.63% (95% CI = 101.69, 125.57%) for LAL and 99.69% (95% CI = 90.14, 109.24%) for rFC assays. The LAL assay gave higher endotoxin estimates compared with rFC. The LAL and rFC estimates were highly correlated (r = 0.86, P < 0.0001). The difference between LAL and rFC endotoxin estimates correlated with the LAL estimates (r = 0.51, P < 0.0001). However, the difference was not correlated with (1 --> 3)-beta-D-glucans.

CONCLUSION

LAL and rFC gave comparable results, hence either assay can be used for studies of endotoxin exposure. The current study shows that (1 --> 3)-beta-D-glucan is not a major factor interfering with endotoxin measurements in house dust using a Cambrex KQCL LAL preparation.

Evidence for the ancient origin of the NF-kappaB/IkappaB cascade: its archaic role in pathogen infection and immunity

The evolutionary conservation of the NF-kappaB transcription factors, from Drosophila to humans, underscores its pivotal role in immune response. Unexpectedly, the canonical NF-kappaB signaling pathway is not functional in the immune system of Caenorhabditis elegans. Therefore, the ancient origin of the NF-kappaB signaling pathway is still unknown. Here, we report the discovery and characterization of a primitive and functional NF-kappaB/IkappaB pathway in the immune defense of a "living fossil," the horseshoe crab, Carcinoscorpius rotundicauda. The ancient NF-kappaB/IkappaB homologues, CrNFkappaB, CrRelish, and CrIkappaB, share numerous signature motifs with their vertebrate orthologues. CrNFkappaB recognizes both horseshoe crab and mammalian kappaB response elements. CrIkappaB interacts with CrNFkappaB and inhibits its nuclear translocation and DNA-binding activity. The activation of the CrNFkappaB is autoregulated by a feedback mechanism mediated by CrIkappaB, the natural inhibitor of CrNFkappaB. We further show that Gram-negative bacteria infection causes rapid degradation of CrIkappaB and nuclear translocation of CrNFkappaB. Infection also leads to an increase in the kappaB-binding activity and up-regulation of immune-related gene expression, like inducible nitric oxide synthase and Factor C, an LPS-activated serine protease. Altogether, our study shows that, although absent in C. elegans, the NF-kappaB/IkappaB signaling cascade remains well conserved from horseshoe crab to humans, playing an archaic but fundamental role in regulating the expression of critical immune defense molecules.

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

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

The ancient origin of the complement system

The complement system has been thought to originate exclusively in the deuterostomes. Here, we show that the central complement components already existed in the primitive protostome lineage. A functional homolog of vertebrate complement 3, CrC3, has been isolated from a 'living fossil', the horseshoe crab (Carcinoscorpius rotundicauda). CrC3 resembles human C3 and shows closest homology to C3 sequences of lower deuterostomes. CrC3 and plasma lectins bind a wide range of microbes, forming the frontline innate immune defense system. Additionally, we identified CrC2/Bf, a homolog of vertebrate C2 and Bf that participates in C3 activation, and a C3 receptor-like sequence. Furthermore, complement-mediated phagocytosis of bacteria by the hemocytes of horseshoe crab was also observed. Thus, a primitive yet complex opsonic complement defense system is revealed in the horseshoe crab, a protostome species. Our findings demonstrate an ancient origin of the critical complement components and the opsonic defense mechanism in the Precambrian ancestor of bilateral animals.

Perturbation of Lipopolysaccharide (LPS) Micelles by Sushi 3 (S3) Antimicrobial Peptide

S3 peptide, derived from the Sushi 3 domain of Factor C, which is the lipopolysaccharide (LPS)-sensitive serine protease of the horseshoe crab coagulation cascade, was shown previously to harbor antimicrobial activity against Gram-negative bacteria. However, the mechanism of action remains poorly understood at the molecular level. Here we demonstrate that the intermolecular disulfide bonding of S3 resulting in S3 dimers is indispensable for its interaction with LPS. The binding properties of the S3 monomer and dimer to LPS were analyzed by several approaches including enzyme-linked immunosorbent assay (ELISA)-based assay, surface plasmon resonance, and fluorescence correlation spectroscopy (FCS). It is evident that the S3 dimer exhibits stronger binding to LPS, demonstrating 50% LPS-neutralizing capability at a concentration of 1 mum. Circular dichroism spectrometry revealed that the S3 peptide undergoes conformational change in the presence of a disulfide bridge, transitioning from a random coil to beta-sheet structure. Using a fluorescence correlation spectroscopy monitoring system, we describe a novel approach for examining the mechanism of peptide interaction with LPS in the native environment. The strategy shows that intermolecular disulfide bonding of S3 into dimers plays a critical role in its propensity to disrupt LPS micelles and consequently neutralize LPS activity. S3 dimers display detergent-like properties in disrupting LPS micelles. Considering intermolecular disulfide bonds as an important parameter in the structure-activity relationship, this insight provides clues for the future design of improved LPS-binding and -neutralizing peptides.

Transcriptional regulation of limulus factor C: repression of an NFkappaB motif modulates its responsiveness to bacterial lipopolysaccharide

Serine proteases play fundamental roles in invertebrate development, hemostasis, and innate immunity. This is exemplified by the limulus Factor C, which is a serine protease that binds a pathogen-associated molecule, lipopolysaccharide (LPS) to trigger a blood coagulation cascade. As a central molecule in the limulus innate immunity and hemostasis, Factor C gene expression has been detected in two major immune defense tissues, the amebocytes and hepatopancreas. Infection of the limulus with live Gram-negative bacteria induces a 2-3-fold increase in mRNA transcripts in both tissues. However, in vitro studies in Drosophila cell lines using Factor C promoter-reporter chimera DNA constructs, and site-directed mutagenesis of the promoter demonstrated that a proximal kappaB binding site, aided by an adjacent dorsal-like binding motif responds dramatically to LPS and dorsal transcription factor overexpression. Electrophoretic mobility shift assay further confirmed a strong interaction of the limulus kappaB motif with Rel proteins. However, deletion constructs of the Factor C promoter harboring different numbers of dorsal-like binding sites upstream of the kappaB motif as well as the electrophoretic mobility shift assay of these motifs with Rel proteins strongly suggest that the up-regulation of Factor C gene expression is attenuated during microbial challenge. The repression of the dramatic activation of this pathogen-responsive gene by LPS is probably effected via competition between the dorsal-like motifs over the proximal LPS-responsive kappaB unit, or through inhibition from the upstream repressive element(s), which accounts for the gene expression pattern observed in vivo. Our findings demonstrate that blood coagulation and innate immune response are integrated at the transcriptional level in this ancient organism, and that this LPS-responsive serine protease is controlled by an evolutionarily conserved NFkappaB pathway.

High therapeutic index of factor C Sushi peptides: potent antimicrobials against Pseudomonas aeruginosa

Factor C protein isolated from the horseshoe crab, Carcinoscorpius rotundicauda, has endotoxin binding capability. Synthetic peptides of 34 amino acids based on the sequence of two regions of factor C (Sushi 1 and Sushi 3) as well as their corresponding mutants exhibited activities against 30 clinical isolates of Pseudomonas aeruginosa. Collectively, all four peptides demonstrated exceptionally effective bactericidal activity against P. aeruginosa with 90% minimal bactericidal concentrations (MBC(90)s) in the range of 0.06 to 0.25 microg/ml (16 to 63 nM). Viable bacteria were reduced by 90% after 7 min and were totally eradicated within 40 to 50 min. These peptides are minimally hemolytic against both rabbit and human erythrocytes even at concentrations up to 1,600-fold their MBC(90)s. Both in vitro and in vivo studies indicate that cytotoxic effects are small even at 1,000-fold their MBC(90)s. Furthermore, the Sushi peptides are tolerant of high-salt and adverse pH conditions. These findings demonstrate the promising therapeutic potential of the Sushi peptides.

High-performance affinity capture-removal of bacterial pyrogen from solutions

Synthetic peptide S3delta has high affinity for bacterial endotoxin or lipopolysaccharide (LPS). Under tested conditions of pH 5-9 and 0-0.4 M NaCl, the affinity constant, KD ranged from 2.10(-6) to 2 x 10(-9) M(-1). A novel affinity matrix based on peptide S3delta was developed for removal of LPS from solutions such as: water; buffers with a wide range of ionic strength and pH; medium for cell culture; and protein solutions under optimized conditions. At a starting LPS of approximately 100 EU/ml, a post-purification level below 0.005 EU/ml was achieved.

Interpretation of biological activity data of bacterial endotoxins by simple molecular models of mechanism of action

Lipid A moiety has been identified as the bioactive component of bacterial endotoxins (lipopolysaccharides). However, the molecular mechanism of biological activity of lipid A is still not fully understood. This paper contributes to understanding of the molecular mechanism of action of bacterial endotoxins by comparing molecular modelling results for two possible mechanisms with the underlying experimental data. Mechanisms of action involving specific binding of lipid A to a protein receptor as well as nonspecific intercalation into phospholipid membrane of a host cell were modelled and analysed. As the cellular receptor for endotoxin has not been identified, a model of a peptidic pseudoreceptor was proposed, based on molecular structure, symmetry of the lipid A moiety and the observed character of endotoxin-binding sites in proteins. We have studied the monomeric form of lipid A from Escherichia coli and its seven synthetic analogues with varying numbers of phosphate groups and correlated them with known biological activities determined by the Limulus assay. Gibbs free energies associated with the interaction of lipid A with the pseudoreceptor model and intercalation into phospholipid membrane calculated by molecular mechanics and molecular dynamics methods were used to compare the two possible mechanisms of action. The results suggest that specific binding of lipid A analogues to the peptidic pseudoreceptor carrying an amphipathic cationic binding pattern BHPHB (B, basic; H, hydrophobic; P, polar residue, respectively) is energetically more favourable than intercalation into the phospholipid membrane. In addition, binding affinities of lipid A analogues to the best minimum binding sequence KFSFK of the pseudoreceptor correlated with the experimental Limulus activity parameter. This correlation enabled us to rationalize the observed relationship between the number and position of the phosphate groups in the lipid A moiety and its biological activity in terms of specific ligand-receptor interactions. If lipid A-receptor interaction involves formation of phosphate-ammonium ion-pair(s) with cationic amino-acid residues, the specific mechanism of action was fully consistent with the underlying experimental data. As a consequence, recognition of lipid A variants by an amphipathic binding sequence BHPHB of a host-cell protein receptor might represent the initial and/or rate-determining molecular event of the mechanism of action of lipid A (or endotoxin). The insight into the molecular mechanism of action and the structure of the lipid A-binding pattern have potential implications for rational drug design strategies of endotoxin-neutralizing agents or binding factors.