Development of protease inhibitors for protozoan infections

Antioxidant and anti-protease activities of diazepinomicin from the sponge-associated Micromonospora strain RV115

Diazepinomicin is a dibenzodiazepine alkaloid with an unusual structure among the known microbial metabolites discovered so far. Diazepinomicin was isolated from the marine sponge-associated strain Micromonospora sp. RV115 and was identified by spectroscopic analysis and by comparison to literature data. In addition to its interesting preclinical broad-spectrum antitumor potential, we report here new antioxidant and anti-protease activities for this compound. Using the ferric reducing antioxidant power (FRAP) assay, a strong antioxidant potential of diazepinomicin was demonstrated. Moreover, diazepinomicin showed a significant antioxidant and protective capacity from genomic damage induced by the reactive oxygen species hydrogen peroxide in human kidney (HK-2) and human promyelocytic (HL-60) cell lines. Additionally, diazepinomicin inhibited the proteases rhodesain and cathepsin L at an IC₅₀ of 70–90 µM. It also showed antiparasitic activity against trypomastigote forms of Trypanosoma brucei with an IC₅₀ of 13.5 µM. These results showed unprecedented antioxidant and anti-protease activities of diazepinomicin, thus further highlighting its potential as a future drug candidate.

Novel cruzain inhibitors for the treatment of Chagas' disease

The protozoan parasite Trypanosoma cruzi, the etiological agent of Chagas’ disease, affects millions of individuals and continues to be an important global health concern. The poor efficacy and unfavorable side effects of current treatments necessitate novel therapeutics. Cruzain, the major cysteine protease of T. cruzi, is one potential novel target. Recent advances in a class of vinyl sulfone inhibitors are encouraging; however, as most potential therapeutics fail in clinical trials and both disease progression and resistance call for combination therapy with several drugs, the identification of additional classes of inhibitory molecules is essential. Using an exhaustive virtual-screening and experimental validation approach, we identify several additional small-molecule cruzain inhibitors. Further optimization of these chemical scaffolds could lead to the development of novel drugs useful in the treatment of Chagas’ disease.

Host-parasite interaction: parasite-derived and -induced proteases that degrade human extracellular matrix

Parasitic protozoa are among the most important pathogens worldwide. Diseases such as malaria, leishmaniasis, amoebiasis, giardiasis, trichomoniasis, and trypanosomiasis affect millions of people. Humans are constantly threatened by infections caused by these pathogens. Parasites engage a plethora of surface and secreted molecules to attach to and enter mammalian cells. The secretion of lytic enzymes by parasites into host organs mediates critical interactions because of the invasion and destruction of interstitial tissues, enabling parasite migration to other sites within the hosts. Extracellular matrix is a complex, cross-linked structure that holds cells together in an organized assembly and that forms the basement membrane lining (basal lamina). The extracellular matrix represents a major barrier to parasites. Therefore, the evolution of mechanisms for connective-tissue degradation may be of great importance for parasite survival. Recent advances have been achieved in our understanding of the biochemistry and molecular biology of proteases from parasitic protozoa. The focus of this paper is to discuss the role of protozoan parasitic proteases in the degradation of host ECM proteins and the participation of these molecules as virulence factors. We divide the paper into two sections, extracellular and intracellular protozoa.

Structure- and ligand-based drug design approaches for neglected tropical diseases

Drug discovery has moved toward more rational strategies based on our increasing understanding of the fundamental principles of protein–ligand interactions. Structure- (SBDD) and ligand-based drug design (LBDD) approaches bring together the most powerful concepts in modern chemistry and biology, linking medicinal chemistry with structural biology. The definition and assessment of both chemical and biological space have revitalized the importance of exploring the intrinsic complementary nature of experimental and computational methods in drug design. Major challenges in this field include the identification of promising hits and the development of high-quality leads for further development into clinical candidates. It becomes particularly important in the case of neglected tropical diseases (NTDs) that affect disproportionately poor people living in rural and remote regions worldwide, and for which there is an insufficient number of new chemical entities being evaluated owing to the lack of innovation and R&D investment by the pharmaceutical industry. This perspective paper outlines the utility and applications of SBDD and LBDD approaches for the identification and design of new small-molecule agents for NTDs.

Microbial and fungal protease inhibitors--current and potential applications

Proteolytic enzymes play essential metabolic and regulatory functions in many biological processes and also offer a wide range of biotechnological applications. Because of their essential roles, their proteolytic activity needs to be tightly regulated. Therefore, small molecules and proteins that inhibit proteases can be versatile tools in the fields of medicine, agriculture and biotechnology. In medicine, protease inhibitors can be used as diagnostic or therapeutic agents for viral, bacterial, fungal and parasitic diseases as well as for treating cancer and immunological, neurodegenerative and cardiovascular diseases. They can be involved in crop protection against plant pathogens and herbivorous pests as well as against abiotic stress such as drought. Furthermore, protease inhibitors are indispensable in protein purification procedures to prevent undesired proteolysis during heterologous expression or protein extraction. They are also valuable tools for simple and effective purification of proteases, using affinity chromatography. Because there are such a large number and diversity of proteases in prokaryotes, yeasts, filamentous fungi and mushrooms, we can expect them to be a rich source of protease inhibitors as well.

Expression and characterization of the Babesia bigemina cysteine protease BbiCPL1

BbiCPL1 was the first papain-like cysteine protease from a piroplasm to be identified with proteolytic activity. Here we report the improved production of the active recombinant enzyme, and the biochemical characterization of this potential drug target. BbiCPL1 showed characteristic properties of its class, including hydrolysis of papain-family peptide substrates, an acidic pH optimum, requirement of a reducing environment for maximum activity, and inhibition by standard cysteine protease inhibitors such as E-64, leupeptin, ALLN and cystatin. The optimum pH for the protease activity against peptide substrates was 5.5, but enzymatic activity was observed between pH 4.0 and pH 9.0. At slightly basic pH 7.5, BbiCPL1 maintained 83% of maximum activity, suggesting a role in cytosol environment.

Current advances in detection and treatment of babesiosis

Babesiosis is a disease with a world-wide distribution affecting many species of mammals principally cattle and man. The major impact occurs in the cattle industry where bovine babesiosis has had a huge economic effect due to loss of meat and beef production of infected animals and death. Nowadays to those costs there must be added the high cost of tick control, disease detection, prevention and treatment. In almost a century and a quarter since the first report of the disease, the truth is: there is no a safe and efficient vaccine available, there are limited chemotherapeutic choices and few low-cost, reliable and fast detection methods. Detection and treatment of babesiosis are important tools to control babesiosis. Microscopy detection methods are still the cheapest and fastest methods used to identify Babesia parasites although their sensitivity and specificity are limited. Newer immunological methods are being developed and they offer faster, more sensitive and more specific options to conventional methods, although the direct immunological diagnoses of parasite antigens in host tissues are still missing. Detection methods based on nucleic acid identification and their amplification are the most sensitive and reliable techniques available today; importantly, most of those methodologies were developed before the genomics and bioinformatics era, which leaves ample room for optimization. For years, babesiosis treatment has been based on the use of very few drugs like imidocarb or diminazene aceturate. Recently, several pharmacological compounds were developed and evaluated, offering new options to control the disease. With the complete sequence of the Babesia bovis genome and the B. bigemina genome project in progress, the post-genomic era brings a new light on the development of diagnosis methods and new chemotherapy targets. In this review, we will present the current advances in detection and treatment of babesiosis in cattle and other animals, with additional reference to several apicomplexan parasites.

Structural basis for inhibition of cathepsin B drug target from the human blood fluke, Schistosoma mansoni

Schistosomiasis caused by a parasitic blood fluke of the genus Schistosoma afflicts over 200 million people worldwide. Schistosoma mansoni cathepsin B1 (SmCB1) is a gut-associated peptidase that digests host blood proteins as a source of nutrients. It is under investigation as a drug target. To further this goal, we report three crystal structures of SmCB1 complexed with peptidomimetic inhibitors as follows: the epoxide CA074 at 1.3 Å resolution and the vinyl sulfones K11017 and K11777 at 1.8 and 2.5 Å resolutions, respectively. Interactions of the inhibitors with the subsites of the active-site cleft were evaluated by quantum chemical calculations. These data and inhibition profiling with a panel of vinyl sulfone derivatives identify key binding interactions and provide insight into the specificity of SmCB1 inhibition. Furthermore, hydrolysis profiling of SmCB1 using synthetic peptides and the natural substrate hemoglobin revealed that carboxydipeptidase activity predominates over endopeptidolysis, thereby demonstrating the contribution of the occluding loop that restricts access to the active-site cleft. Critically, the severity of phenotypes induced in the parasite by vinyl sulfone inhibitors correlated with enzyme inhibition, providing support that SmCB1 is a valuable drug target. The present structure and inhibitor interaction data provide a footing for the rational design of anti-schistosomal inhibitors.

Cysteine peptidases of kinetoplastid parasites

We review Clan CA Family C1 peptidases of kinetoplastid parasites (Trypanosoma and Leishmania) with respect to biochemical and genetic diversity, genomic organization and stage-specificity and control of expression. We discuss their contributions to parasite metabolism, virulence and pathogenesis and modulation of the host's immune response. Their applications as vaccine candidates and diagnostic markers as well as their chemical and genetic validation as drug targets are also summarized.

Development of novel drugs for human African trypanosomiasis

Human African trypanosomiasis (HAT) or ‘sleeping sickness’ is a neglected tropical disease caused by the parasite Trypanosoma brucei. Novel models for funding pharmaceutical development against HAT are beginning to yield results. The Drugs for Neglected Diseases initiative (DNDi) rediscovered a nitroimidazole, fexinidazole, which is currently in Phase I clinical trials. Novel benzoxaboroles, discovered by Anacor, Scynexis and DNDi, have good pharmacokinetic properties in plasma and in the brain and are curative in a murine model of stage two HAT with brain infection. The Consortium for Parasitic Drug Development (CPDD) has identified a series of dicationic compounds that can cure a monkey model of stage two HAT. With other screening programs yielding hits, the pipeline for new HAT drugs might finally begin to fill.

The Plasmodium falciparum-infected red blood cell

Plasmodium falciparum, the most virulent of the human malaria parasites, causes up to one million deaths per year. The parasite spends part of its lifecycle inside the red blood cells (RBCs) of its host. As it grows it ingests the RBC cytoplasm, digesting it in an acidic vacuole. Free haem released during haemoglobin digestion is detoxified by conversion to inert crystals of haemozoin. Malaria pathology is evident during the blood stage of the infection and is exacerbated by adhesion of infected RBCs to blood vessel walls, which prevents splenic clearance of the infected cells. Cytoadherence is mediated by surface-exposed virulence proteins that bind to endothelial cell receptors. These 'adhesins' are exported to the RBC surface via an exomembrane system that is established outside the parasite in the host cell cytoplasm. Antimalarial drugs that interfere with haem detoxification, or target other parasite-specific processes, have been effective in the treatment of malaria, but their use has been dogged by the development of resistance. Similarly, efforts to develop an effective blood vaccine are hindered by the variability of surface-exposed antigens.

Protease expression by microorganisms and its relevance to crucial physiological/pathological events

The treatment of infections caused by fungi and trypanosomatids is difficult due to the eukaryotic nature of these microbial cells, which are similar in several biochemical and genetic aspects to host cells. Aggravating this scenario, very few antifungal and anti-trypanosomatidal agents are in clinical use and, therefore, therapy is limited by drug safety considerations and their narrow spectrum of activity, efficacy and resistance. The search for new bioactive agents against fungi and trypanosomatids has been expanded because progress in biochemistry and molecular biology has led to a better understanding of important and essential pathways in these microorganisms including nutrition, growth, proliferation, signaling, differentiation and death. In this context, proteolytic enzymes produced by these eukaryotic microorganisms are appointed and, in some cases, proven to be excellent targets for searching novel natural and/or synthetic pharmacological compounds, in order to cure or prevent invasive fungal/trypanosomatid diseases. With this task in mind, our research group and others have focused on aspartic-type proteases, since the activity of this class of hydrolytic enzymes is directly implicated in several facets of basic biological processes of both fungal and trypanosomatid cells as well as due to the participation in numerous events of interaction between these microorganisms and host structures. In the present paper, a concise revision of the beneficial effects of aspartic protease inhibitors, with emphasis on the aspartic protease inhibitors used in the anti-human immunodeficiency virus therapy, will be presented and discussed using our experience with the following microbial models: the yeast Candida albicans, the filamentous fungus Fonsecaea pedrosoi and the protozoan trypanosomatid Leishmania amazonensis.

Functional studies of Plasmodium falciparum dipeptidyl aminopeptidase I using small molecule inhibitors and active site probes

The widespread resistance of malaria parasites to all affordable drugs has made the identification of new targets urgent. Dipeptidyl aminopeptidases (DPAPs) represent potentially valuable new targets that are involved in hemoglobin degradation (DPAP1) and parasite egress (DPAP3). Here we use activity-based probes to demonstrate that specific inhibition of DPAP1 by a small molecule results in the formation of an immature trophozoite that leads to parasite death. Using computational methods, we designed stable, nonpeptidic covalent inhibitors that kill Plasmodium falciparum at low nanomolar concentrations. These compounds show signs of slowing parasite growth in a murine model of malaria, which suggests that DPAP1 might be a viable antimalarial target. Interestingly, we found that resynthesis and activation of DPAP1 after inhibition is rapid, suggesting that effective drugs would need to sustain DPAP1 inhibition for a period of 2-3 hr.

α-ketoheterocycles as inhibitors of Leishmania mexicana cysteine protease CPB

Cysteine proteases of the papain superfamily are present in nearly all eukaryotes and also play pivotal roles in the biology of parasites. Inhibition of cysteine proteases is emerging as an important strategy to combat parasitic diseases such as sleeping sickness, Chagas disease, and leishmaniasis. Inspired by the in vivo antiparasitic activity of the vinylsulfone-based cysteine protease inhibitors, a series of α-ketoheterocycles were developed as reversible inhibitors of a recombinant L. mexicana cysteine protease, CPB2.8. Three isoxazoles and especially one oxadiazole compound are potent reversible inhibitors of CPB2.8; however, in vitro whole-organism screening against a panel of protozoan parasites did not fully correlate with the observed inhibition of the cysteine protease.

State of the art in African trypanosome drug discovery

African sleeping sickness is endemic in sub-Saharan Africa where the WHO estimates that 60 million people are at risk for the disease. Human African trypanosomiasis (HAT) is 100% fatal if untreated and the current drug therapies have significant limitations due to toxicity and difficult treatment regimes. No new chemical agents have been approved since eflornithine in 1990. The pentamidine analog DB289, which was in late stage clinical trials for the treatment of early stage HAT recently failed due to toxicity issues. A new protocol for the treatment of late-stage T. brucei gambiense that uses combination nifurtomox/eflornithine (NECT) was recently shown to have better safety and efficacy than eflornithine alone, while being easier to administer. This breakthrough represents the only new therapy for HAT since the approval of eflornithine. A number of research programs are on going to exploit the unusual biochemical pathways in the parasite to identify new targets for target based drug discovery programs. HTS efforts are also underway to discover new chemical entities through whole organism screening approaches. A number of inhibitors with anti-trypanosomal activity have been identified by both approaches, but none of the programs are yet at the stage of identifying a preclinical candidate. This dire situation underscores the need for continued effort to identify new chemical agents for the treatment of HAT.

An atypical proprotein convertase in Giardia lamblia differentiation

Proteolytic activity is important in the lifecycles of parasites and their interactions with hosts. Cysteine proteases have been best studied in Giardia, but other protease classes have been implicated in growth and/or differentiation. In this study, we employed bioinformatics to reveal the complete set of putative proteases in the Giardia genome. We identified 73 peptidase homologs distributed over 5 catalytic classes in the genome. Serial analysis of gene expression of the G. lamblia lifecycle found thirteen protease genes with significant transcriptional variation over the lifecycle, with only one serine protease transcript upregulated late in encystation. The translated gene sequence of this encystation-specific transcript was most similar to eukaryotic subtilisin-like proprotein convertases (SPC), although the typical catalytic triad was not identified. Epitope-tagged gSPC protein expressed in Giardia under its own promoter was upregulated during encystation with highest expression in cysts and it localized to encystation-specific secretory vesicles (ESV). Total gSPC from encysting cells produced proteolysis in gelatin gels that co-migrated with the epitope-tagged protease in immunoblots. Immuno-purified gSPC also had gelatinase activity. To test whether endogenous gSPC activity is involved in differentiation, trophozoites and cysts were exposed to the specific serine proteinase inhibitor 4-(2-aminoethyl)-benzenesulfonyl fluoride hydrochloride (AEBSF). After 21 h encystation, a significant decrease in ESV was observed with 1mM AEBSF and by 42 h the number of cysts was significantly reduced, but trophozoite growth was not inhibited. Concurrently, levels of cyst wall proteins 1 and 2, and AU1-tagged gSPC protein itself were decreased. Excystation of G. muris cysts was also significantly reduced in the presence of AEBSF. These results support the idea that serine protease activity is essential for Giardia encystation and excystation.

The oligopeptidase B of Leishmania regulates parasite enolase and immune evasion

Proteases are a ubiquitous group of enzymes that play key roles in the life cycle of parasites, in the host-parasite relationship, and in the pathogenesis of parasitic diseases. Furthermore, proteases are targets for the development of new anti-parasitic therapy. Protozoan parasites like Leishmania predominantly express Clan CA cysteine proteases for key life cycle functions. It was therefore unexpected to find a high level of serine protease activity expressed by Leishmania donovani. Purification of this activity followed by mass spectrometry identified oligopeptidase B (OPB; Clan SC, family S9A) as the responsible enzyme. This was confirmed by gene knock-out of OPB, which resulted in the disappearance of the detected serine protease activity of Leishmania extracts. To delineate the specific role of OPB in parasite physiology, proteomic analysis was carried out on OPB(-/-) versus wild type parasites. Four protein species were significantly elevated in OPB(-/-) parasites, and all four were identified by mass spectrometry as enolase. This increased enolase was enzymatically inactive and associated with the parasite membrane. Aside from its classic role in carbohydrate metabolism, enolase was recently found to localize to membranes, where it binds host plasminogen and functions as a virulence factor for several pathogens. As expected, there was a striking alteration in macrophage responses to Leishmania when OPB was deleted. Whereas wild type parasites elicited little, if any, response from infected macrophages, OPB(-/-) parasites induced a massive up-regulation in gene transcription. Additionally, these OPB(-/-) parasites displayed decreased virulence in the murine footpad infection model.

Cathepsin B proteases of flukes: the key to facilitating parasite control?

Cysteine proteases are important virulence factors for parasites. This review will focus on the cathepsin B proteases of trematodes (also known as flukes) which are abundant in juvenile and immature flukes. Recent research, primarily in Fasciola, using inhibitors, RNA interference (RNAi) and vaccination studies indicates that cathepsin Bs play a key role in the biology of trematodes. As these proteases are largely expressed by infective parasite stages, their inactivation by chemotherapy or vaccination will greatly reduce the damage wrought by flukes as they invade host tissues. This validates cathepsin Bs as key strategic targets for fluke control.

Immunolocalization and characterization of two novel proteases in Leishmania donovani: putative roles in host invasion and parasite development

Two novel intracellular proteases having identical molecular mass (58 kDa) were purified from virulent Indian strain of Leishmania donovani by a combination of aprotinin-agarose affinity chromatography, ion exchange chromatography and finally continuous elution electrophoresis. Both of these proteases migrate in SDS-PAGE as a single homogeneous bands suggesting monomeric nature of these proteases. The enzyme activity of one of the proteases was inhibited by serine protease inhibitor aprotinin and another one was inhibited by metalloprotease inhibitor 1, 10 phenanthroline. The purified enzymes were thus of serine protease (SP-Ld) and metalloprotease (MP-Ld) type. The optimal pH for protease activity is 8.0 and 7.5 for SP-Ld and MP-Ld respectively. The temperature optimum for SP-Ld is 28 °C and for MP-Ld is 37 °C showing their thermostability upto 60 °C. Broad substrate (both natural and synthetic) specificity and the effect of Ca2+ upon these enzymes suggested novelty of these proteases. Kinetic data indicate that SP-Ld is of trypsin like as BAPNA appears to be the best substrate and MP-Ld seems to be collagenase type as it degrades azocoll with maximum efficiency. Both immunofluorescence and immune-gold electron microscopy studies revealed that the SP-Ld is localized in the flagellar pocket as well as at the surface of the parasite, whereas MP-Ld is located extensively near the flagellar pocket region. This work also suggests that the uses of anti SP-Ld and anti MP-Ld antibodies are quite significant in interfering with the process of parasite invasion and multiplication respectively. Thus the major role of SP-Ld could be predicted in invasion process as it down regulates the phagocytic activity of macrophages, and MP-Ld appears to play important roles in parasitic development.

A nonazole CYP51 inhibitor cures Chagas' disease in a mouse model of acute infection

Chagas' disease, the leading cause of heart failure in Latin America, is caused by the kinetoplastid protozoan Trypanosoma cruzi. The sterols of T. cruzi resemble those of fungi, both in composition and in biosynthesis. Azole inhibitors of sterol 14alpha-demethylase (CYP51) successfully treat fungal infections in humans, and efforts to adapt the success of antifungal azoles posaconazole and ravuconazole as second-use agents for Chagas' disease are under way. However, to address concerns about the use of azoles for Chagas' disease, including drug resistance and cost, the rational design of nonazole CYP51 inhibitors can provide promising alternative drug chemotypes. We report the curative effect of the nonazole CYP51 inhibitor LP10 in an acute mouse model of T. cruzi infection. Mice treated with an oral dose of 40 mg LP10/kg of body weight twice a day (BID) for 30 days, initiated 24 h postinfection, showed no signs of acute disease and had histologically normal tissues after 6 months. A very stringent test of cure showed that 4/5 mice had negative PCR results for T. cruzi, and parasites were amplified by hemoculture in only two treated mice. These results compare favorably with those reported for posaconazole. Electron microscopy and gas chromatography-mass spectrometry (GC-MS) analysis of sterol composition confirmed that treatment with LP10 blocked the 14alpha-demethylation step and induced breakdown of parasite cell membranes, culminating in severe ultrastructural and morphological alterations and death of the clinically relevant amastigote stage of the parasite.

Two approaches to discovering and developing new drugs for Chagas disease

This review will focus on two general approaches carried out at the Sandler Center, University of California, San Francisco, to address the challenge of developing new drugs for the treatment of Chagas disease. The first approach is target-based drug discovery, and two specific targets, cytochrome P450 CYP51 and cruzain (aka cruzipain), are discussed. A 'proof of concept' molecule, the vinyl sulfone inhibitor K777, is now a clinical candidate. The preclinical assessment compliance for filing as an Investigational New Drug with the United States Food and Drug Administration (FDA) is presented, and an outline of potential clinical trials is given. The second approach to identifying new drug leads is parasite phenotypic screens in culture. The development of an assay allowing high throughput screening of Trypanosoma cruzi amastigotes in skeletal muscle cells is presented. This screen has the advantage of not requiring specific strains of parasites, so it could be used with field isolates, drug resistant strains or laboratory strains. It is optimized for robotic liquid handling and has been validated through a screen of a library of FDA-approved drugs identifying 65 hits.

Diverse regulatory roles for lysosomal proteases in the immune response

The innate and adaptive immune system utilise endocytic protease activity to promote functional immune responses. Cysteine and aspartic proteases (cathepsins) constitute a subset of endocytic proteases, the immune function of which has been described extensively. Although historically these studies have focused on their role in processes such as antigen presentation and zymogen processing within the endocytic compartment, recent discoveries have demonstrated a critical role for these proteases in other intracellular compartments, and within the extracellular milieu. It has also become clear that their pattern of expression and substrate specificities are more diverse than was first envisaged. Here, we discuss recent advances addressing the role of lysosomal proteases in various aspects of the immune response. We pay attention to reports demonstrating cathepsin activity outside of its canonical endosome/lysosome microenvironment.

Design and evaluation of Trypanosoma brucei metacaspase inhibitors

Metacaspase (MCA) is an important enzyme in Trypanosoma brucei, absent from humans and differing significantly from the orthologous human caspases. Therefore MCA constitutes a new attractive drug target for antiparasitic chemotherapeutics, which needs further characterization to support the discovery of innovative drug candidates. A first series of inhibitors has been prepared on the basis of known substrate specificity and the predicted catalytic mechanism of the enzyme. In this Letter we present the first inhibitors of TbMCA2 with low micromolar enzymatic and antiparasitic activity in vitro combined with low cytotoxicity.

Development of NS3/4A protease-based reporter assay suitable for efficiently assessing hepatitis C virus infection

A cell culture system for the production of hepatitis C virus (HCV) whole virions has greatly accelerated studies of the virus life cycle and the discovery of anti-HCV agents. However, the quantification of the HCV titers in a whole-virus infection/replication system currently relies mostly on reverse transcription-PCR or immunofluorescence assay, which would be cumbersome for high-throughput drug screening. To overcome this problem, this study has generated a novel cell line, Huh7.5-EG(Delta4B5A)SEAP, that carries a dual reporter, EG(Delta4B5A)SEAP. The EG(Delta4B5A)SEAP reporter is a viral protease-cleavable fusion protein in which the enhanced green fluorescence protein is linked to secreted alkaline phosphatase (SEAP) in frame via Delta4B5A, a short peptide cleavage substrate for NS3/4A viral protease. This study demonstrates that virus replication/infection in the Huh7.5-EG(Delta4B5A)SEAP cells can be quantitatively indicated by measuring the SEAP activity in cell culture medium. The levels of SEAP released from HCV-infected Huh7.5-EG(Delta4B5A)SEAP cells correlated closely with the amounts of HCV in the inocula. The Huh7.5-EG(Delta4B5A)SEAP cells were also shown to be a suitable host for the discovery of anti-HCV inhibitors by using known compounds that target multiple stages of the HCV life cycle. The Z'-factor of this assay ranged from 0.64 to 0.74 in 96-well plates, indicating that this reporter system is suitable for high-throughput screening of prospective anti-HCV agents.

Post-translational modifications in Plasmodium: more than you think!

Recent evidences indicate that transcription in Plasmodium may be hard-wired and rigid, deviating from the classical model of transcriptional gene regulation. Thus, it is important that other regulatory pathways be investigated as a comprehensive effort to curb the deadly malarial parasite. Research in post-translational modifications in Plasmodium is an emerging field that may provide new venues for drug discovery and potential new insights into how parasitic protozoans regulate their life cycle. Here, we discuss the recent findings of post-translational modifications in Plasmodium.

Vinyl sulfones as antiparasitic agents and a structural basis for drug design

Cysteine proteases of the papain superfamily are implicated in a number of cellular processes and are important virulence factors in the pathogenesis of parasitic disease. These enzymes have therefore emerged as promising targets for antiparasitic drugs. We report the crystal structures of three major parasite cysteine proteases, cruzain, falcipain-3, and the first reported structure of rhodesain, in complex with a class of potent, small molecule, cysteine protease inhibitors, the vinyl sulfones. These data, in conjunction with comparative inhibition kinetics, provide insight into the molecular mechanisms that drive cysteine protease inhibition by vinyl sulfones, the binding specificity of these important proteases and the potential of vinyl sulfones as antiparasitic drugs.