Endotoxin-neutralizing effects of histidine-rich peptides

Low Prevalence of Plasmodium falciparum Histidine-Rich Protein 2 and 3 Gene Deletions—A Multiregional Study in Central and West Africa

Plasmodium falciparum parasites carrying deletions of histidine-rich protein 2 and 3 genes, pfhrp2 and pfhrp3, respectively, are likely to escape detection via HRP2-based rapid diagnostic tests (RDTs) and, consequently, treatment, posing a major risk to both the health of the infected individual and malaria control efforts. This study assessed the frequency of pfhrp2- and pfhrp3-deleted strains at four different study sites in Central Africa (number of samples analyzed: Gabon N = 534 and the Republic of Congo N = 917) and West Africa (number of samples analyzed: Nigeria N = 466 and Benin N = 120) using a highly sensitive multiplex qPCR. We found low prevalences for pfhrp2 (1%, 0%, 0.03% and 0) and pfhrp3 single deletions (0%, 0%, 0.03% and 0%) at all study sites (Gabon, the Republic of Congo, Nigeria and Benin, respectively). Double-deleted P. falciparum were only found in Nigeria in 1.6% of all internally controlled samples. The results of this pilot investigation do not point towards a high risk for false-negative RDT results due to pfhrp2/pfhrp3 deletions in Central and West African regions. However, as this scenario can change rapidly, continuous monitoring is essential to ensure that RDTs remain a suitable tool for the malaria diagnostic strategy.

Novel aspects of sepsis pathophysiology: NETs, plasma glycoproteins, endotheliopathy and COVID-19

In 2016, sepsis was newly defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Sepsis remains one of the crucial medical problems to be solved worldwide. Although the world health organization has made sepsis a global health priority, there remain no specific and effective therapy for sepsis so far. Indeed, over the previous decades almost all attempts to develop novel drugs have failed. This may be partly ascribable to the multifactorial complexity of the septic cascade and the resultant difficulties of identifying drug targets. In addition, there might still be missing links among dysregulated host responses in vital organs. In this review article, recent advances in understanding of the complex pathophysiology of sepsis are summarized, with a focus on neutrophil extracellular traps (NETs), the significant role of NETs in thrombosis/embolism, and the functional roles of plasma proteins, histidine-rich glycoprotein (HRG) and inter-alpha-inhibitor proteins (IAIPs). The specific plasma proteins that are markedly decreased in the acute phase of sepsis may play important roles in the regulation of blood cells, vascular endothelial cells and coagulation. The accumulating evidence may provide us with insights into a novel aspect of the pathophysiology of sepsis and septic ARDS, including that in COVID-19.

Histidine-Rich Glycoprotein Stimulates Human Neutrophil Phagocytosis and Prolongs Survival through CLEC1A

Histidine-rich glycoprotein (HRG) is a multifunctional plasma protein and maintains the homeostasis of blood cells and vascular endothelial cells. In the current study, we demonstrate that HRG and recombinant HRG concentration dependently induced the phagocytic activity of isolated human neutrophils against fluorescence-labeled Escherichia coli and Staphylococcus aureus through the stimulation of CLEC1A receptors, maintaining their spherical round shape. The phagocytosis-inducing effects of HRG were inhibited by a specific anti-HRG Ab and enhanced by opsonization of bacteria with diluted serum. HRG and C5a prolonged the survival time of isolated human neutrophils, in association with a reduction in the spontaneous production of extracellular ROS. In contrast, HRG maintained the responsiveness of neutrophils to TNF-α, zymosan, and E. coli with regard to reactive oxygen species production. The blocking Ab for CLEC1A and recombinant CLEC1A-Fc fusion protein significantly inhibited the HRG-induced neutrophil rounding, phagocytic activity, and prolongation of survival time, suggesting the involvement of the CLEC1A receptor in the action of HRG on human neutrophils. These results as a whole indicated that HRG facilitated the clearance of E. coli and S. aureus by maintaining the neutrophil morphology and phagocytosis, contributing to the antiseptic effects of HRG in vivo.

Histidine-rich glycoprotein possesses antioxidant activity through self-oxidation and inhibition of hydroxyl radical production via chelating divalent metal ions in Fenton's reaction

Sepsis is caused by infections associated with life-threatening multiple organ failure (MOF). Septic MOF appears to be closely related to circulatory failure due to immunothrombosis. This process involves the production of reactive oxygen spices (ROS) in inflammatory sites. Therefore, the detoxification of the systemic excess ROS is important for the improvement of the process in septic pathogenesis. Histidine-rich glycoprotein (HRG), a plasma glycoprotein, ameliorates a septic condition through the suppression of both excess ROS production from neutrophils and immunothrombosis. Hydroxyl radical is known as the most important species among ROS in pathogenesis; however, the direct influence of HRG on hydroxyl radical formation and ROS activity is poorly understood. In this study, we showed that HRG, in a concentration-dependent manner, efficiently inhibited the production of hydroxyl radical induced by the Fenton's reaction through chelation of the divalent iron. HRG also exhibited antioxidant activity against peroxyl radical by oxidation of HRG itself as a substrate; however, it did not show superoxide dismutase and catalase-like activities. Additionally, HRG enhanced glutathione peroxidase, a well-known antioxidant enzyme, activity. These results suggest that HRG may play a unique role in suppression of the production of hydroxyl radicals and subsequent tissue damage at inflammatory sites. Marked reduction in plasma HRG in sepsis might lose such an important protective mechanism. Thus, the present study provides evidence that inhibition of ROS and ROS-production systems by HRG may contribute to antiseptic effects in vivo and that HRG could be potential therapy for ROS-related diseases.

[Various functions of plasma histidine-rich glycoprotein and its clinical application as the biomarker and therapeutic drug for sepsis]

Histidine-rich glycoprotein (HRG) is a 75 kDa plasma glycoprotein synthesized in liver mainly, which exists at approximately 60-100 μg/ml in human plasma. HRG is known to bind to several ligands and cells, leading to exert coagulation, fibrinolysis, immune and inflammation regulatory activity in septic condition. Thus, decreased plasma HRG level induces the dysregulations of coagulation, fibrinolysis and immune system, resulting in disseminated intravascular coagulation and multiple organ failure. This article focuses on the physiological activity of HRG and the potential of HRG as the biomarker and therapeutic drug for sepsis.

Genetic polymorphism of histidine rich protein 2 in Plasmodium falciparum isolates from different infection sources in Yunnan Province, China

Background

Failed diagnoses of some falciparum malaria cases by RDTs are constantly reported in recent years. Plasmodium falciparum histidine-rich protein 2 (pfhpr2) gene deficiency has been found to be the major reason of RDTs failure in many countries. This article analysed the deletion of pfhpr2 gene of falciparum malaria cases isolated in Yunnan Province, China.

Methods

Blood samples from falciparum malaria cases diagnosed in Yunnan Province were collected. Plasmodium genomic DNA was extracted and the pfhrp2 gene exon2 region was amplified via nested PCR. The haplotype of the DNA sequence, the nucleic acid diversity index (PI) and expected heterozygosity (He) were analyzed. Count PfHRP2 amino acid peptide sequence repeat and its times, and predict the properties of PfHRP2 peptide chain reaction to RDTs testing.

Results

A total of 306 blood samples were collected, 84.9% (259/306) from which pfhrp2 PCR amplification products (gene exon2) were obtained, while the remaining 47 samples were false amplification. The length of the 250 DNA sequences ranged from 345 - 927 bp, with 151 haplotypes, with PI and He values of 0.169 and 0.983, respectively. The length of the PfHRP2 peptide chain translated from 250 DNA sequences ranged from 115 to 309 aa. All peptide chains had more than an amino acid codon deletion. All 250 PfHRP2 strands ended with a type 12 amino acid repeat, 98.0% (245/250) started with a type 1 repetition and 2.0% (5/250) with a type 2 repetition. The detection rate for type 2 duplicates was 100% (250/250). Prediction of RDT sensitivity of PfHRP2 peptide chains based on type 2 and type 7 repeats showed that 9.60% (24/250), 50.0% (125/250), 13.20% (33/250) and 27.20.5% (68/250) of the 250 peptide chains were very sensitive, sensitive, borderline and non-sensitive, respectively.

Conclusion

The diversified polymorphism of the pfhrp2 gene deletion from different infection sources in the Yunnan province are extremely complex. The cause of the failure of pfhrp2 exon2 amplification is still to be investigated. The results of this study appeal to Yunnan Province for a timely evaluation of the effectiveness and applicability of RDTs in the diagnosis of malaria.

High proportions of pfhrp2 gene deletion and performance of HRP2-based rapid diagnostic test in Plasmodium falciparum field isolates of Odisha

BACKGROUND

With the documentation of cases of falciparum malaria negative by rapid diagnostic tests (RDT), though at low frequency from natural isolates in a small pocket of Odisha, it became absolutely necessary to investigate the status of HRP-2 based RDT throughout the state and in different seasons of the year.

METHODS

Suspected individuals were screened for malaria infection by microscopy and RDT in 25/30 districts of Odisha, India. Discrepancies in results were confirmed by PCR. False negative RDT samples for Plasmodium falciparum mono-infection were evaluated for detection of HRP2 antigen in ELISA and genotyped for pfhrp2, pfhrp3 and their flanking genes. Multiplicity of infection was ascertained based on msp1 and msp2 genotyping and parasitaemia level was determined by microscopy.

RESULTS

Of the total 1058 patients suspected for malaria, 384 were microscopically confirmed for P. falciparum mono-infection and RDT failure was observed in 58 samples at varying proportion in different regions of the state. The failure in detection was due to undetectable level of HRP-2. Although most of these samples were screened during rainy season (45/345), significantly high proportion (9/17) of RDT negative samples were obtained during the summer compared to rainy season (P = 0.0002; OR = 7.5). PCR genotyping of pfhrp2 and pfhrp3 in RDT negative samples showed 38/58 (65.5) samples to be pfhrp2 negative and 24/58 (41.4) to be pfhrp3 negative including dual negative in 17/58 (29.3). Most of the RDT negative samples (39/58) were with single genotype infection and high proportions of pfhrp2 deletion (7/9) was observed in summer. No difference in parasitaemia level was observed between RDT positive and RDT negative patients.

CONCLUSION

High prevalence of parasites with pfhrp2 deletion including dual deletions (pfhrp2 and pfhrp3) is a serious cause of concern, as these patients could not be given a correct diagnosis and treatment. Therefore, HRP2-based RDT for diagnosing P. falciparum infection in Odisha is non-reliable and must be performed in addition to or replaced by other appropriate diagnostic tools for clinical management of the disease.

Histidine-Rich Glycoprotein Suppresses Hyperinflammatory Responses of Lung in a Severe Acute Pancreatitis Mouse Model

OBJECTIVES

Severe acute pancreatitis is a highly lethal disease caused by systemic inflammatory response syndrome, leading to multiple organ failure. We recently showed that histidine-rich glycoprotein (HRG) supplemental therapy ameliorated septic acute respiratory distress syndrome due to unnecessary neutrophil activation and immunothrombosis formation. Here, we evaluated the effect of HRG on lung inflammation followed by pancreatitis in a severe acute pancreatitis mouse model.

METHODS

Mice received intraperitoneal injections of cerulein 7 times (100 μg/kg each) at 1-hour intervals to induce acute pancreatitis. Immediately after the first cerulein injection, phosphate-buffered saline, human serum albumin (20 mg/kg), or HRG (20 mg/kg) was intravenously injected. One hour after the last cerulein injection, phosphate-buffered saline or lipopolysaccharide (5 mg/kg) was intravenously injected into the tail vein. We evaluated lung inflammatory level after pancreatitis.

RESULTS

We observed significantly decreased plasma HRG levels in an acute pancreatitis mouse model. Histidine-rich glycoprotein treatment inhibited lung edema and the accumulation of neutrophil in severe acute pancreatitis, but HRG did not directly affect pancreatitis. Moreover, HRG suppressed tumor necrosis factor α, inducible nitric oxide synthase, interleukin 6, and neutrophil elastase mRNA expression and myeloperoxidase activity in the lung.

CONCLUSIONS

These data suggested that HRG ameliorated lung inflammation secondary to pancreatitis.

Antimicrobial peptide–metal ion interactions – a potential way of activity enhancement

We discuss the potential correlation between the antimicrobial peptide–metal binding mode, structure, thermodynamics and mode of action.

Functional Regulation of the Plasma Protein Histidine-Rich Glycoprotein by Zn2+ in Settings of Tissue Injury

Divalent metal ions are essential nutrients for all living organisms and are commonly protein-bound where they perform important roles in protein structure and function. This regulatory control from metals is observed in the relatively abundant plasma protein histidine-rich glycoprotein (HRG), which displays preferential binding to the second most abundant transition element in human systems, Zinc (Zn²⁺). HRG has been proposed to interact with a large number of protein ligands and has been implicated in the regulation of various physiological and pathological processes including the formation of immune complexes, apoptotic/necrotic and pathogen clearance, cell adhesion, antimicrobial activity, angiogenesis, coagulation and fibrinolysis. Interestingly, these processes are often associated with sites of tissue injury or tumour growth, where the concentration and distribution of Zn²⁺ is known to vary. Changes in Zn²⁺ levels have been shown to modify HRG function by altering its affinity for certain ligands and/or providing protection against proteolytic disassembly by serine proteases. This review focuses on the molecular interplay between HRG and Zn²⁺, and how Zn²⁺ binding modifies HRG-ligand interactions to regulate function in different settings of tissue injury.

Divalent metal binding by histidine-rich glycoprotein differentially regulates higher order oligomerisation and proteolytic processing

The serum protein histidine-rich glycoprotein (HRG) has been implicated in tissue injury and tumour growth. Several HRG functions are regulated by the divalent metal Zn²⁺ , including ligand binding and proteolytic processing that releases active HRG fragments. Although HRG can bind divalent metals other than Zn²⁺ , the impact of these divalent metals on the biophysical properties of HRG remains poorly understood. We now show that HRG binds Zn²⁺ , Ni²⁺ , Cu²⁺ and Co²⁺ with micromolar affinities, but differing stoichiometries, and regulate the release of specific HRG fragments during proteolysis. Furthermore, HRG binding to Zn²⁺ promotes HRG dimer formation in a Zn²⁺ -concentration- and pH-dependent manner. Our data highlight the complex divalent metal-dependent regulatory mechanisms that govern HRG function.

Invited review: Mechanisms of endotoxin neutralization by synthetic cationic compounds

A basic challenge in the treatment of septic patients in critical care units is the release of bacterial pathogenicity factors such as lipopolysaccharide (LPS, endotoxin) from the cell envelope of Gram-negative bacteria due to killing by antibiotics. LPS aggregates may interact with serum and membrane proteins such as LBP (lipopolysaccharide-binding protein) and CD14 leading to the observed strong reaction of the immune system. Thus, an effective treatment of patients infected by Gram-negative bacteria must comprise beside bacterial killing the neutralization of endotoxins. Here, data are summarized for synthetic compounds indicating the stepwise development to very effective LPS-neutralizing agents. These data include synthetic peptides, based on the endotoxin-binding domains of natural binding proteins such as lactoferrin, Limulus anti-LPS factor, NK-lysin, and cathelicidins or based on LPS sequestering polyamines. Many of these compounds could be shown to act not only in vitro, but also in vivo (e.g . in animal models of sepsis), and might be useful in future clinical trials and in sepsis therapy.

Histidine-Rich Glycoprotein Prevents Septic Lethality through Regulation of Immunothrombosis and Inflammation

Sepsis is a major cause of death worldwide. We show that a plasma protein histidine-rich glycoprotein (HRG) was decreased significantly in septic mice with cecal ligation and puncture (CLP) and supplementary treatment of septic mice with exogenous HRG improved survival, with strong inhibition of tight attachment of neutrophils to pulmonary vasculatures, subsequent immunothrombosis, DIC state, lung inflammation, hypercytokinemia, and activation of vascular endothelial cells (VECs). In contrast, knockdown of HRG by siRNA exacerbated lethality. Purified human HRG reversibly induced morphological changes in human neutrophils in vitro; induction of spherical shape with reduced microvilli and adhesiveness to VECs. HRG maintained the passage of neutrophils through microcapillaries and abolished production of reactive oxygen species. These results suggested that the supplementary therapy with HRG may provide a novel strategy for the treatment of sepsis through suppression of excessive systemic inflammation and immunothrombosis by keeping circulating neutrophils quiescent and preventing uncontrolled activation of VECs.

•

A plasma protein HRG decreased markedly in septic mice with high lethality.

•

Supplementary treatment with HRG improved the survival of septic mice.

•

HRG treatment inhibited immunothrombosis, NETosis, and excessive inflammation.

•

HRG kept circulating neutrophils quiescent morphologically and functionally.

Although sepsis is a critical disease condition with high mortality and the main cause of death in intensive care units, there is no therapeutic drug for sepsis now. This research found that a plasma protein, histidine-rich glycoprotein (HRG), decreased dramatically in sepsis mouse model and that supplementary treatment of septic mice with purified human HRG remarkably improved the lethality of mice, associated with inhibition of tight attachment of neutrophils to pulmonary vasculatures, subsequent thrombosis, lung inflammation and activation of vascular endothelial cells. Thus, supplementary therapy with HRG may provide a novel strategy for the treatment of septic patients.

Structure-function relationships in mammalian histidine-proline-rich glycoprotein

Histidine-proline-rich glycoprotein (HPRG), or histidine-rich glycoprotein (HRG), is a serum protein that is synthesized in the liver and is actively internalised by different cells, including skeletal muscle. The multidomain arrangement of HPRG comprises two modules at the N-terminus that are homologous to cystatin but void of cysteine proteinase inhibitor function, and a second half consisting of a histidine-proline-rich region (HPRR) located between two proline-rich regions (PRR1 and PRR2), and a C-terminus domain. HPRG has been reported to bind various ligands and to modulate angiogenesis via the histidine residues of the HPRR. However, the secondary structure prediction of the HPRR reveals that more than 98% is disordered and the structural basis of the hypothesized functions remains unclear. Comparison of the PRR1 of several mammalian species indicates the presence of a conserved binding site that might coordinate the Zn(2+) ion with an amino acid arrangement compatible with the cysteine-containing site that has been identified experimentally for rabbit HPRG. This observation provides a structural basis to the function of HPRG as an intracellular zinc chaperone which has been suggested by the involvement of the protein in the maintenance of the quaternary structure of skeletal muscle AMP deaminase (AMPD). During Anthropoidea evolution, a change of the primary structure of the PRR1 Zn(2+) binding site took place, giving rise to the sequence M-S-C-S/L-S/R-C that resembles the MxCxxC motif characteristic of metal transporters and metallochaperones.

Potential biomarkers and their applications for rapid and reliable detection of malaria

Malaria has been responsible for the highest mortality in most malaria endemic countries. Even after decades of malaria control campaigns, it still persists as a disease of high mortality due to improper diagnosis and rapidly evolving drug resistant malarial parasites. For efficient and economical malaria management, WHO recommends that all malaria suspected patients should receive proper diagnosis before administering drugs. It is thus imperative to develop fast, economical, and accurate techniques for diagnosis of malaria. In this regard an in-depth knowledge on malaria biomarkers is important to identify an appropriate biorecognition element and utilize it prudently to develop a reliable detection technique for diagnosis of the disease. Among the various biomarkers, plasmodial lactate dehydrogenase and histidine-rich protein II (HRP II) have received increasing attention for developing rapid and reliable detection techniques for malaria. The widely used rapid detection tests (RDTs) for malaria succumb to many drawbacks which promotes exploration of more efficient economical detection techniques. This paper provides an overview on the current status of malaria biomarkers, along with their potential utilization for developing different malaria diagnostic techniques and advanced biosensors.

Crystal structure of histidine-rich glycoprotein N2 domain reveals redox activity at an interdomain disulfide bridge: implications for angiogenic regulation

Histidine-rich glycoprotein (HRG) is a plasma protein consisting of 6 distinct functional domains and is an important regulator of key cardiovascular processes, including angiogenesis and coagulation. The protein is composed of 2 N-terminal domains (N1 and N2), 2 proline-rich regions (PRR1 and PRR2) that flank a histidine-rich region (HRR), and a C-terminal domain. To date, structural information of HRG has largely come from sequence analysis and spectroscopic studies. It is thought that an HRG fragment containing the HRR, released via plasmin-mediated cleavage, acts as a negative regulator of angiogenesis in vivo. However, its release also requires cleavage of a disulphide bond suggesting that its activity is mediated by a redox process. Here, we present a 1.93 Å resolution crystal structure of the N2 domain of serum-purified rabbit HRG. The structure confirms that the N2 domain, which along with the N1 domain, forms an important molecular interaction site on HRG, possesses a cystatin-like fold composed of a 5-stranded antiparallel β-sheet wrapped around a 5-turn α-helix. A native N-linked glycosylation site was identified at Asn184. Moreover, the structure reveals the presence of an S-glutathionyl adduct at Cys185, which has implications for the redox-mediated release of the antiangiogenic cleavage product from HRG.

Histidine-rich glycoprotein binds fibrin(ogen) with high affinity and competes with thrombin for binding to the gamma'-chain

Histidine-rich glycoprotein (HRG) is an abundant protein that binds fibrinogen and other plasma proteins in a Zn(2+)-dependent fashion but whose function is unclear. HRG has antimicrobial activity, and its incorporation into fibrin clots facilitates bacterial entrapment and killing and promotes inflammation. Although these findings suggest that HRG contributes to innate immunity and inflammation, little is known about the HRG-fibrin(ogen) interaction. By immunoassay, HRG-fibrinogen complexes were detected in Zn(2+)-supplemented human plasma, a finding consistent with a high affinity interaction. Surface plasmon resonance determinations support this concept and show that in the presence of Zn(2+), HRG binds the predominant γ(A)/γ(A)-fibrinogen and the γ-chain elongated isoform, γ(A)/γ'-fibrinogen, with K(d) values of 9 nm. Likewise, (125)I-labeled HRG binds γ(A)/γ(A)- or γ(A)/γ'-fibrin clots with similar K(d) values when Zn(2+) is present. There are multiple HRG binding sites on fibrin(ogen) because HRG binds immobilized fibrinogen fragment D or E and γ'-peptide, an analog of the COOH terminus of the γ'-chain that mediates the high affinity interaction of thrombin with γ(A)/γ'-fibrin. Thrombin competes with HRG for γ'-peptide binding and displaces (125)I-HRG from γ(A)/γ'-fibrin clots and vice versa. Taken together, these data suggest that (a) HRG circulates in complex with fibrinogen and that the complex persists upon fibrin formation, and (b) by competing with thrombin for γ(A)/γ'-fibrin binding, HRG may modulate coagulation. Therefore, the HRG-fibrin interaction may provide a novel link between coagulation, innate immunity, and inflammation.

Histidine-rich glycoprotein: the Swiss Army knife of mammalian plasma

Histidine-rich glycoprotein (HRG), also known as histidine-proline-rich glyco-protein, is an abundant and well-characterized protein of vertebrate plasma. HRG has a multidomain structure that allows the molecule to interact with many ligands, including heparin, phospholipids, plasminogen, fibrinogen, immunoglobulin G, C1q, heme, and Zn2+. The ability of HRG to interact with various ligands simultaneously has suggested that HRG can function as an adaptor molecule and regulate numerous important biologic processes, such as immune complex/necrotic cell/pathogen clearance, cell adhesion, angiogenesis, coagulation, and fibrinolysis. The present review covers the proposed multifunctional roles of HRG with a focus on recent findings that have led to its emergence as a key regulator of immunity and vascular biology. Also included is a discussion of the striking functional similarities between HRG and other important multifunctional proteins found in plasma, such as C-reactive protein, C1q, β2 glycoprotein I, and thrombospondin-1.

Global sequence variation in the histidine-rich proteins 2 and 3 of Plasmodium falciparum: implications for the performance of malaria rapid diagnostic tests

Background

Accurate diagnosis is essential for prompt and appropriate treatment of malaria. While rapid diagnostic tests (RDTs) offer great potential to improve malaria diagnosis, the sensitivity of RDTs has been reported to be highly variable. One possible factor contributing to variable test performance is the diversity of parasite antigens. This is of particular concern for Plasmodium falciparum histidine-rich protein 2 (PfHRP2)-detecting RDTs since PfHRP2 has been reported to be highly variable in isolates of the Asia-Pacific region.

Methods

The pfhrp2 exon 2 fragment from 458 isolates of P. falciparum collected from 38 countries was amplified and sequenced. For a subset of 80 isolates, the exon 2 fragment of histidine-rich protein 3 (pfhrp3) was also amplified and sequenced. DNA sequence and statistical analysis of the variation observed in these genes was conducted. The potential impact of the pfhrp2 variation on RDT detection rates was examined by analysing the relationship between sequence characteristics of this gene and the results of the WHO product testing of malaria RDTs: Round 1 (2008), for 34 PfHRP2-detecting RDTs.

Results

Sequence analysis revealed extensive variations in the number and arrangement of various repeats encoded by the genes in parasite populations world-wide. However, no statistically robust correlation between gene structure and RDT detection rate for P. falciparum parasites at 200 parasites per microlitre was identified.

Conclusions

The results suggest that despite extreme sequence variation, diversity of PfHRP2 does not appear to be a major cause of RDT sensitivity variation.

Regulation of histidine-rich glycoprotein (HRG) function via plasmin-mediated proteolytic cleavage

The plasminogen/plasmin system is involved in a variety of normal physiological and pathological processes, including tissue remodelling, angiogenesis and tumour metastasis. Plasminogen activators and receptors for plasminogen/plasminogen activators are essential for the processing of plasminogen to form the active serine protease plasmin. Plasmin can in turn positively or negatively regulate further plasminogen activation via plasminmediated cleavage of receptors and activators. HRG (histidine-rich glycoprotein), a relatively abundant (approx. 100–150 μg/ml) plasma glycoprotein, has a multi-domain structure that can interact with many ligands, including Zn2+, heparin, HS (heparan sulfate) and plasminogen. HRG has been shown to function as an adaptor molecule to tether plasminogen to GAG (glycosaminoglycan)-bearing surfaces and to regulate plasminogen activation via various mechanisms. As HRG itself is sensitive to plasmin cleavage, the present study examines in detail the cleavage of human HRG by plasmin and the effect of this cleavage on various functions of HRG. HRG fragments, generated by plasmin cleavage, are held together by disulfide linkages and are not released from the molecule under non-reducing conditions. Plasmin-mediated cleavage partially inhibited HRG binding to cell surface HS, but enhanced HRG binding to necrotic cells and to plasminogen. However, both intact and plasmin-cleaved HRG enhanced the binding of plasminogen to heparin-coated surfaces to a similar extent. Furthermore, the presence of heparin, Zn2+ or acidic pH was found to protect HRG from plasmin cleavage. Thus proteolytic cleavage of HRG by plasmin may provide a feedback mechanism to regulate the effects of HRG on the plasminogen/plasmin system and other functions of HRG.

Arginine deficiency augments inflammatory mediator production by airway epithelial cells in vitro

Background

Previously we showed that reduced availability of the essential amino acid tryptophan per se attenuates post-transcriptional control of interleukin (IL)-6 and IL-8 leading to hyperresponsive production of these inflammatory mediators by airway epithelial cells. Availability of the non-essential amino acid arginine in the inflamed airway mucosa of patients with asthma is reduced markedly, but it is not known whether this can also lead to an exaggerated production of IL-6 and IL-8.

Methods

IL-6 and IL-8 were determined by ELISA in culture supernatants of NCI-H292 airway epithelial-like cells and normal bronchial epithelial (NHBE) cells that were exposed to TNF-α, LPS or no stimulus, in medium with or without arginine. Arginine deficiency may also result from exposure to poly-L-arginine or major basic protein (MBP), which can block arginine uptake. Epithelial cells were exposed to these polycationic proteins and L-¹⁴C-arginine uptake was assessed as well as IL-6 and IL-8 production. To determine the mode of action, IL-6 and IL-8 mRNA profiles over time were assessed as were gene transcription and post-transcriptional mRNA degradation.

Results

For both NCI-H292 and NHBE cells, low arginine concentrations enhanced basal epithelial IL-6 and IL-8 production and synergized with TNF-α-induced IL-6 and IL-8 production. Poly-L-arginine enhanced the stimulus-induced IL-6 and IL-8 production, however, blocking arginine uptake and the enhanced IL-6 and IL-8 production appeared unrelated. The exaggerated IL-6 and IL-8 production due to arginine deficiency and to poly-L-arginine depend on a post-transcriptional and a transcriptional process, respectively.

Conclusion

We conclude that both reduced arginine availability per se and the presence of polycationic proteins may promote airway inflammation by enhanced pro-inflammatory mediator production in airway epithelial cells, but due to distinct mechanisms.

Antimicrobial activity of histidine-rich peptides is dependent on acidic conditions

Synthetic peptides composed of multiples of the consensus heparin-binding Cardin and Weintraub sequences AKKARA and ARKKAAKA are antimicrobial. Replacement of lysine and arginine by histidine in these peptides completely abrogates their antimicrobial and heparin-binding activities at neutral pH. However, the antibacterial activity against Gram-negative (Escherichia coli, Pseudomonas aeruginosa) and Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) as well as the fungus Candida albicans, was restored at acidic conditions (pH 5.5). Fluorescence microscopy and FACS analysis showed that the binding of the histidine-rich peptides to E. coli and Candida was significantly enhanced at pH 5.5. Likewise, fluorescence studies for assessment of membrane permeation as well as electron microscopy analysis of peptide-treated bacteria, paired with studies of peptide effects on liposomes, demonstrated that the peptides induce membrane lysis only at acidic pH. No discernible hemolysis was noted for the histidine-rich peptides. Similar pH-dependent antimicrobial activities were demonstrated for peptides derived from histidine-rich and heparin-binding regions of human kininogen and histidine-rich glycoprotein. The results demonstrate that the presence of an acidic environment is an important regulator of the activity of histidine-rich antimicrobial peptides.

Targeting bacterial endotoxin: two sides of a coin

The term sepsis describes a potentially lethal clinical condition that develops as a result of a dysregulated host response to bacterial infection. The most common bacterial component implicated in initiating the septic syndrome is a cell wall molecule derived from Gram-negative bacteria, known as lipopolysaccharide (LPS) or endotoxin. Like all mammals, humans are equipped with an LPS-sensing machinery consisting, primarily, of LPS-binding protein (LBP), CD14, a glycosylphosphatidylinositol (GPI)-anchored monocyte differentiation antigen, and toll-like receptor 4 (TLR4), a signal-transducing integral membrane protein. Modest stimulation of TLR4 facilitates the elimination of invading microorganisms. Potent TLR4 stimulation, however, produces severe reactions in the host, often leading to multiple organ failure and death. The search for pharmaceuticals that reduce mortality in septic patients has been a painstaking process. Thus far, only a few compounds have been found to significantly reduce mortality rates. Perhaps one of the more promising therapeutic strategies currently pursued is based on the identification of synthetic or naturally occurring substances that neutralize LPS or inhibit LPS-mediated activation of host immune cells, such as monocytes and macrophages. Here, we describe a number of diverse molecular structures with a capacity to either enhance or blunt LPS-induced monocyte activation. The underlying molecular mechanisms are discussed.

Histidine-rich glycoprotein exerts antibacterial activity

Histidine-rich glycoprotein (HRGP), an abundant heparin-binding protein found in plasma and thrombocytes, exerts antibacterial effects against Gram-positive bacteria (Enterococcus faecalis and Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa). Fluorescence studies and electron microscopy to assess membrane permeation showed that HRGP induces lysis of E. faecalisbacteria in the presence of Zn2+ or at low pH. Heparin blocked binding of the protein to E. faecalis and abolished antibacterial activity. Furthermore, truncated HRGP, devoid of the heparin-binding and histidine-rich domain, was not antibacterial. It has previously been shown that peptides containing consensus heparin-binding sequences (Cardin and Weintraub motifs) are antibacterial. Thus, the peptide (GHHPH)4, derived from the histidine-rich region of HRGP and containing such a heparin-binding motif, was antibacterial for E. faecalis in the presence of Zn2+ or at low pH. The results show a previously undisclosed antibacterial activity of HRGP and suggest that the histidine-rich and heparin-binding domain of HRGP mediates the antibacterial activity of the protein.

Inactivation of Endotoxin by Human Plasma Gelsolin

Septic shock from bacterial endotoxin, triggered by the release of lipopolysaccharide (LPS) molecules from the outer wall of Gram-negative bacteria, is a major cause of human death for which there is no effective treatment once the complex inflammatory pathways stimulated by these small amphipathic molecules are activated. Here we report that plasma gelsolin, a highly conserved human protein, binds LPS from various bacteria with high affinity. Solid-phase binding assays, fluorescence measurements, and functional assays of actin depolymerizing effects show that gelsolin binds more tightly to LPS than it does to its other known lipid ligands, phosphatidylinositol 4,5-bisphosphate and lysophosphatidic acid. Gelsolin also competes with LPS-binding protein (LBP), a high-affinity carrier for LPS. One result of gelsolin-LPS binding is inhibition of the actin binding activity of gelsolin as well as the actin depolymerizing activity of blood serum. Simultaneously, effects of LPS on cellular functions, including cytoskeletal actin remodeling, and collagen-induced platelet activation by pathways independent of toll-like receptors (TLRs) are neutralized by gelsolin and by a peptide based on gelsolin residues 160-169 (GSN160-169) which comprise part of gelsolin's phosphoinositide binding site. Additionally, TLR-dependent NF-kappaB translocation in astrocytes appears to be blocked by gelsolin. These results show a strong effect of LPS on plasma gelsolin function and suggest that some effects of endotoxin in vivo may be mediated or inhibited by plasma gelsolin.

Peptide Motifs for Cell-Surface Intervention

The discovery of new antimicrobial and anticancer drugs, and overcoming the problem of resistance to current anti-infective and anticancer drug therapies require innovation in the pharmaceutical and scientific research community. A further challenge of drug design is to make the therapeutic agent specific, long lasting, of minimal toxicity, and affordable. Microbial and cancer cell surfaces present molecular features that can differentially prefocus drugs within the human host. This property can localize drugs near cell-surface targets, thereby reducing opportunities for adverse effects, or the emergence of drug resistance caused by intracellular drug and target modification and by the induction of drug efflux pumps. The solubility demands on cell-surface targeting drugs should also be less stringent than for those drugs requiring transmembrane transport or internalization in order to reach intracellular targets. Cationic peptides have provided an increasingly important research focus in this regard. Although the cationic antimicrobial peptides are distributed widely in nature and provide localized primary defenses against microbial attack, the susceptibility of L-peptides to proteolysis and the known properties of successful antimicrobials have led to a focus on circularized peptides, D,L-peptides, and peptides containing unusual amino acids. New on the scene as lead antifungal agents are D-octapeptides and their derivatives that were developed from a combinatorial library produced through solid-phase peptide synthesis protocols. These peptides contain an amidated C-terminal tri-arginine motif, which confers membrane impermeability and focuses the peptides near the fungal cell surface. To date, the octapeptides and their derivatives also require some aromaticity, preferably the indole ring of tryptophan. In some cases, a single 4-methoxy-2,3,6-trimethylbenzenesulfonyl moiety remaining on the peptide after incomplete cleavage of the peptide from the solid phase produces a peptide with activity, whereas the parent shows little or no activity in the screen. Recent research advances that support the polycationic cell surface approach include the RGD (Arg-Gly-Asp) tripeptide and its mimetics, as well as aminoglycoside arginine drugs (e.g. neomycin coupled to small arginine polymers) and prodrugs. In the case of polycationic peptides, D-peptides could be used for intravenous injection and direct-surface drug applications, but mimetics will probably be needed for oral delivery.