Plasmodium falciparum dipeptidyl aminopeptidase 3 activity is important for efficient erythrocyte invasion by the malaria parasite.

Parasite egress from infected erythrocytes and invasion of new red blood cells are essential processes for the exponential asexual replication of the malaria parasite. These two tightly coordinated events take place in less than a minute and are in part regulated and mediated by proteases. Dipeptidyl aminopeptidases (DPAPs) are papain-fold cysteine proteases that cleave dipeptides from the N-terminus of protein substrates. DPAP3 was previously suggested to play an essential role in parasite egress. However, little is known about its enzymatic activity, intracellular localization, or biological function. In this study, we recombinantly expressed DPAP3 and demonstrate that it has indeed dipeptidyl aminopeptidase activity, but contrary to previously studied DPAPs, removal of its internal prodomain is not required for activation. By combining super resolution microscopy, time-lapse fluorescence microscopy, and immunoelectron microscopy, we show that Plasmodium falciparum DPAP3 localizes to apical organelles that are closely associated with the neck of the rhoptries, and from which DPAP3 is secreted immediately before parasite egress. Using a conditional knockout approach coupled to complementation studies with wild type or mutant DPAP3, we show that DPAP3 activity is important for parasite proliferation and critical for efficient red blood cell invasion. We also demonstrate that DPAP3 does not play a role in parasite egress, and that the block in egress phenotype previously reported for DPAP3 inhibitors is due to off target or toxicity effects. Finally, using a flow cytometry assay to differentiate intracellular parasites from extracellular parasites attached to the erythrocyte surface, we show that DPAP3 is involved in the initial attachment of parasites to the red blood cell surface. Overall, this study establishes the presence of a DPAP3-dependent invasion pathway in malaria parasites.

Ferrous iron-dependent drug delivery enables controlled and selective release of therapeutic agents in vivo.

The precise targeting of cytotoxic agents to specific cell types or cellular compartments is of significant interest in medicine, with particular relevance for infectious diseases and cancer. Here, we describe a method to exploit aberrant levels of mobile ferrous iron (Fe(II)) for selective drug delivery in vivo. This approach makes use of a 1,2,4-trioxolane moiety, which serves as an Fe(II)-sensitive "trigger," making drug release contingent on Fe(II)-promoted trioxolane fragmentation. We demonstrate in vivo validation of this approach with the Plasmodium berghei model of murine malaria. Malaria parasites produce high concentrations of mobile ferrous iron as a consequence of their catabolism of host hemoglobin in the infected erythrocyte. Using activity-based probes, we successfully demonstrate the Fe(II)-dependent and parasite-selective delivery of a potent dipeptidyl aminopeptidase inhibitor. We find that delivery of the compound in its Fe(II)-targeted form leads to more sustained target inhibition with greatly reduced off-target inhibition of mammalian cathepsins. This selective drug delivery translates into improved efficacy and tolerability. These findings demonstrate the utility of a purely chemical means to achieve selective drug targeting in vivo. This approach may find useful application in parasitic infections and more broadly in any disease state characterized by aberrant production of reactive ferrous iron.

A coupled protein and probe engineering approach for selective inhibition and activity-based probe labeling of the caspases.

Caspases are cysteine proteases that play essential roles in apoptosis and inflammation. Unfortunately, their highly conserved active sites and overlapping substrate specificities make it difficult to use inhibitors or activity-based probes to study the function, activation, localization, and regulation of individual members of this family. Here we describe a strategy to engineer a caspase to contain a latent nucleophile that can be targeted by a probe containing a suitably placed electrophile, thereby allowing specific, irreversible inhibition and labeling of only the engineered protease. To accomplish this, we have identified a non-conserved residue on the small subunit of all caspases that is near the substrate-binding pocket and that can be mutated to a non-catalytic cysteine residue. We demonstrate that an active-site probe containing an irreversible binding acrylamide electrophile can specifically target this cysteine residue. Here we validate the approach using the apoptotic mediator, caspase-8, and the inflammasome effector, caspase-1. We show that the engineered enzymes are functionally identical to the wild-type enzymes and that the approach allows specific inhibition and direct imaging of the engineered targets in cells. Therefore, this method can be used to image localization and activation as well as the functional contributions of individual caspase proteases to the process of cell death or inflammation.

Coupling protein engineering with probe design to inhibit and image matrix metalloproteinases with controlled specificity.

Matrix metalloproteinases (MMPs) are zinc endopeptidases that play roles in numerous pathophysiological processes and therefore are promising drug targets. However, the large size of this family and a lack of highly selective compounds that can be used for imaging or inhibition of specific MMPs members has limited efforts to better define their biological function. Here we describe a protein engineering strategy coupled with small-molecule probe design to selectively target individual members of the MMP family. Specifically, we introduce a cysteine residue near the active-site of a selected protease that does not alter its overall activity or function but allows direct covalent modification by a small-molecule probe containing a reactive electrophile. This specific engineered interaction between the probe and the target protease provides a means to both image and inhibit the modified protease with absolute specificity. Here we demonstrate the feasibility of the approach for two distinct MMP proteases, MMP-12 and MT1-MMP (or MMP-14).

Plasmodium dipeptidyl aminopeptidases as malaria transmission-blocking drug targets.

The Plasmodium falciparum and P. berghei genomes each contain three dipeptidyl aminopeptidase (dpap) homologs. dpap1 and -3 are critical for asexual growth, but the role of dpap2, the gametocyte-specific homolog, has not been tested. If DPAPs are essential for transmission as well as asexual growth, then a DPAP inhibitor could be used for treatment and to block transmission. To directly analyze the role of DPAP2, a dpap2-minus P. berghei (Pbdpap2Δ) line was generated. The Pbdpap2Δ parasites grew normally, differentiated into gametocytes, and generated sporozoites that were infectious to mice when fed to a mosquito. However, Pbdpap1 transcription was >2-fold upregulated in the Pbdpap2Δ clonal lines, possibly compensating for the loss of Pbdpap2. The role of DPAP1 and -3 in the dpap2Δ parasites was then evaluated using a DPAP inhibitor, ML4118S. When ML4118S was added to the Pbdpap2Δ parasites just before a mosquito membrane feed, mosquito infectivity was not affected. To assess longer exposures to ML4118S and further evaluate the role of DPAPs during gametocyte development in a parasite that causes human malaria, the dpap2 deletion was repeated in P. falciparum. Viable P. falciparum dpap2 (Pfdpap2)-minus parasites were obtained that produced morphologically normal gametocytes. Both wild-type and Pfdpap2-negative parasites were sensitive to ML4118S, indicating that, unlike many antimalarials, ML4118S has activity against parasites at both the asexual and sexual stages and that DPAP1 and -3 may be targets for a dual-stage drug that can treat patients and block malaria transmission.

Activity-based protein profiling of human and plasmodium serine hydrolases and interrogation of potential antimalarial targets.

Malaria remains a global health issue requiring the identification of novel therapeutic targets to combat drug resistance. Metabolic serine hydrolases are druggable enzymes playing essential roles in lipid metabolism. However, very few have been investigated in malaria-causing parasites. Here, we used fluorophosphonate broad-spectrum activity-based probes and quantitative chemical proteomics to annotate and profile the activity of more than half of predicted serine hydrolases in P. falciparum across the erythrocytic cycle. Using conditional genetics, we demonstrate that the activities of four serine hydrolases, previously annotated as essential (or important) in genetic screens, are actually dispensable for parasite replication. Of importance, we also identified eight human serine hydrolases that are specifically activated at different developmental stages. Chemical inhibition of two of them blocks parasite replication. This strongly suggests that parasites co-opt the activity of host enzymes and that this opens a new drug development strategy against which the parasites are less likely to develop resistance.

Application of a Highly Selective Cathepsin S Two-step Activity-Based Probe in Multicolor Bio-Orthogonal Correlative Light-Electron Microscopy.

Cathepsin S is a lysosomal cysteine protease highly expressed in immune cells such as dendritic cells, B cells and macrophages. Its functions include extracellular matrix breakdown and cleavage of cell adhesion molecules to facilitate immune cell motility, as well as cleavage of the invariant chain during maturation of major histocompatibility complex II. The identification of these diverse specific functions has brought the challenge of delineating cathepsin S activity with great spatial precision, relative to related enzymes and substrates. Here, the development of a potent and highly selective two-step activity-based probe for cathepsin S and the application in multicolor bio-orthogonal correlative light-electron microscopy is presented. LHVS, which has been reported as a selective inhibitor of cathepsin S with nanomolar potency, formed the basis for our probe design. However, in competitive activity-based protein profiling experiments LHVS showed significant cross-reactivity toward Cat L. Introduction of an azide group in the P2 position expanded the selectivity window for cathepsin S, but rendered the probe undetectable, as demonstrated in bio-orthogonal competitive activity-based protein profiling. Incorporation of an additional azide handle for click chemistry on the solvent-exposed P1 position allowed for selective labeling of cathepsin S. This highlights the influence of click handle positioning on probe efficacy. This probe was utilized in multicolor bio-orthogonal confocal and correlative light-electron microscopy to investigate the localization of cathepsin S activity at an ultrastructural level in bone marrow-derived dendritic cells. The tools developed in this study will aid the characterization of the variety of functions of cathepsin S throughout biology.

Correction: Novel broad-spectrum activity-based probes to profile malarial cysteine proteases.

[This corrects the article DOI: 10.1371/journal.pone.0227341.].

Novel broad-spectrum activity-based probes to profile malarial cysteine proteases.

Clan CA cysteine proteases, also known as papain-like proteases, play important roles throughout the malaria parasite life cycle and are therefore potential drug targets to treat this disease and prevent its transmission. In order to study the biological function of these proteases and to chemically validate some of them as viable drug targets, highly specific inhibitors need to be developed. This is especially challenging given the large number of clan CA proteases present in Plasmodium species (ten in Plasmodium falciparum), and the difficulty of designing selective inhibitors that do not cross-react with other members of the same family. Additionally, any efforts to develop antimalarial drugs targeting these proteases will also have to take into account potential off-target effects against the 11 human cysteine cathepsins. Activity-based protein profiling has been a very useful tool to determine the specificity of inhibitors against all members of an enzyme family. However, current clan CA proteases broad-spectrum activity-based probes either target endopeptidases or dipeptidyl aminopeptidases, but not both subfamilies efficiently. In this study, we present a new series of dipeptydic vinyl sulfone probes containing a free N-terminal tryptophan and a fluorophore at the P1 position that are able to label both subfamilies efficiently, both in Plasmodium falciparum and in mammalian cells, thus making them better broad-spectrum activity-based probes. We also show that some of these probes are cell permeable and can therefore be used to determine the specificity of inhibitors in living cells. Interestingly, we show that the choice of fluorophore greatly influences the specificity of the probes as well as their cell permeability.

The partially folded homodimeric intermediate of Escherichia coli aspartate aminotransferase contains a "molten interface" structure.

The role of intersubunit side chain-side chain interactions in the stability of the Escherichia coli aspartate aminotransferase (eAATase) homodimer was investigated by directed mutagenesis at 10 different interface contacts. The urea-mediated unfolding pathway of this enzyme proceeds through the formation of a dimeric intermediate, D*, that retains only 40% of the native enzyme secondary structure as judged by circular dichroism. Disruption of any single intersubunit interaction results in a >2.6 kcal mol(-1) decrease in native state stability, independent of its location or nature. However, the stability of D* with respect to U, the unfolded monomer, is the same for all mutants. The stability of the eAATase interface cannot be ascribed to the contribution of a few hot spots, or to the accumulation of a large number of weak interactions, but only to the presence of multiple important and interconnected interactions. It is proposed that a "molten interface" structure, flexible enough to accommodate point mutations, accounts for the stability of D*. Nuclei of tertiary structure, which are not involved in native intersubunit contacts, likely provide a scaffold for the unstructured interface of D*. Such a scaffold would account for the cooperative unfolding of the intermediate.

Identification of Plasmodium dipeptidyl aminopeptidase allosteric inhibitors by high throughput screening.

Dipeptidyl aminopeptidases (DPAPs) are cysteine proteases that cleave dipeptides from the N-terminus of protein substrates and have been shown to play important roles in many pathologies including parasitic diseases such as malaria, toxoplasmosis and Chagas's disease. Inhibitors of the mammalian homologue cathepsin C have been used in clinical trials as potential drugs to treat chronic inflammatory disorders, thus proving that these enzymes are druggable. In Plasmodium species, DPAPs play important functions at different stages of parasite development, thus making them potential antimalarial targets. Most DPAP inhibitors developed to date are peptide-based or peptidomimetic competitive inhibitors. Here, we used a high throughput screening approach to identify novel inhibitor scaffolds that block the activity of Plasmodium falciparum DPAP1. Most of the hits identified in this screen also inhibit Plasmodium falciparum DPAP3, cathepsin C, and to a lesser extent other malarial clan CA proteases, indicating that these might be general DPAP inhibitors. Interestingly, our mechanism of inhibition studies indicate that most hits are allosteric inhibitors, which opens a completely new strategy to inhibit these enzymes, study their biological function, and potentially develop new inhibitors as starting points for drug development.

Characterization of P. falciparum dipeptidyl aminopeptidase 3 specificity identifies differences in amino acid preferences between peptide-based substrates and covalent inhibitors.

Malarial dipeptidyl aminopeptidases (DPAPs) are cysteine proteases important for parasite development thus making them attractive drug targets. In order to develop inhibitors specific to the parasite enzymes, it is necessary to map the determinants of substrate specificity of the parasite enzymes and its mammalian homologue cathepsin C (CatC). Here, we screened peptide-based libraries of substrates and covalent inhibitors to characterize the differences in specificity between parasite DPAPs and CatC, and used this information to develop highly selective DPAP1 and DPAP3 inhibitors. Interestingly, while the primary amino acid specificity of a protease is often used to develop potent inhibitors, we show that equally potent and highly specific inhibitors can be developed based on the sequences of nonoptimal peptide substrates. Finally, our homology modelling and docking studies provide potential structural explanations of the differences in specificity between DPAP1, DPAP3, and CatC, and between substrates and inhibitors in the case of DPAP3. Overall, this study illustrates that focusing the development of protease inhibitors solely on substrate specificity might overlook important structural features that can be exploited to develop highly potent and selective compounds.

Proteases as antimalarial targets: strategies for genetic, chemical, and therapeutic validation.

Malaria is a devastating parasitic disease affecting half of the world's population. The rapid emergence of resistance against new antimalarial drugs, including artemisinin-based therapies, has made the development of drugs with novel mechanisms of action extremely urgent. Proteases are enzymes proven to be well suited for target-based drug development due to our knowledge of their enzymatic mechanisms and active site structures. More importantly, Plasmodium proteases have been shown to be involved in a variety of pathways that are essential for parasite survival. However, pharmacological rather than target-based approaches have dominated the field of antimalarial drug development, in part due to the challenge of robustly validating Plasmodium targets at the genetic level. Fortunately, over the last few years there has been significant progress in the development of efficient genetic methods to modify the parasite, including several conditional approaches. This progress is finally allowing us not only to validate essential genes genetically, but also to study their molecular functions. In this review, I present our current understanding of the biological role proteases play in the malaria parasite life cycle. I also discuss how the recent advances in Plasmodium genetics, the improvement of protease-oriented chemical biology approaches, and the development of malaria-focused pharmacological assays, can be combined to achieve a robust biological, chemical and therapeutic validation of Plasmodium proteases as viable drug targets.

Development and Application of a Simple Plaque Assay for the Human Malaria Parasite Plasmodium falciparum.

Malaria is caused by an obligate intracellular protozoan parasite that replicates within and destroys erythrocytes. Asexual blood stages of the causative agent of the most virulent form of human malaria, Plasmodium falciparum, can be cultivated indefinitely in vitro in human erythrocytes, facilitating experimental analysis of parasite cell biology, biochemistry and genetics. However, efforts to improve understanding of the basic biology of this important pathogen and to develop urgently required new antimalarial drugs and vaccines, suffer from a paucity of basic research tools. This includes a simple means of quantifying the effects of drugs, antibodies and gene modifications on parasite fitness and replication rates. Here we describe the development and validation of an extremely simple, robust plaque assay that can be used to visualise parasite replication and resulting host erythrocyte destruction at the level of clonal parasite populations. We demonstrate applications of the plaque assay by using it for the phenotypic characterisation of two P. falciparum conditional mutants displaying reduced fitness in vitro.

The role of the conserved Lys68*:Glu265 intersubunit salt bridge in aspartate aminotransferase kinetics: multiple forced covariant amino acid substitutions in natural variants.

The role of the Lys68*:Glu265 intersubunit salt bridge that is conserved (Csb) in all known aspartate aminotransferases (AATases), except those of animal cytosolic, Ac (His68*:Glu265), and plant mitochondrial, Pm (Met68*:Gln265), origins, was evaluated in the Escherichia coli AATase. Two double-mutant cycles, to K68M/E265Q and the charge reversed K68E/E265K, were characterized with the context dependence (C) and impact (I) formalism, previously defined for functional chimeric analysis. Mutations of Lys68* with Glu265 fixed are generally more deleterious than the converse mutations of Glu265 with Lys68* fixed, showing that buried negative charges have greater effects than buried positive charges in this context. Replacement of the charged Lys68*:Glu265 with the K68M/E265Q neutral pair introduces relatively small effects on the kinetic parameters. The differential sensitivity of k(cat)/K(M, L-Asp) and k(cat)/K(M, alpha-KG) to salt bridge mutagenic replacements is shown by a linear-free energy relationship, in which the logarithms of the latter second order rate constants are generally decreased by a factor of two more than are those of the former. Thus, k(cat)/K(M, L-Asp) and k(cat)/K(M, alpha-KG) are 133 and 442 mM(-1)s(-1) for the wild-type (WT) enzyme, respectively, but their relative order is reversed in the more severely compromised mutants (14.8 and 5.3 mM(-1)s(-1) for K68E). A Venn diagram illustrates apparent forced covariances of groups of amino acids that accompany the naturally occurring salt bridge replacements in the Pm and Ac classes. The more deeply rooted tree indicates that the Csb variant was the ancestral specie.

New approaches for dissecting protease functions to improve probe development and drug discovery.

Proteases are well-established targets for pharmaceutical development because of their known enzymatic mechanism and their regulatory roles in many pathologies. However, many potent clinical lead compounds have been unsuccessful either because of a lack of specificity or because of our limited understanding of the biological roles of the targeted protease. In order to successfully develop protease inhibitors as drugs, it is necessary to understand protease functions and to expand the platform of inhibitor development beyond active site-directed design and in vitro optimization. Several newly developed technologies will enhance assessment of drug selectivity in living cells and animal models, allowing researchers to focus on compounds with high specificity and minimal side effects in vivo. In this review, we highlight advances in the development of chemical probes, proteomic methods and screening tools that we feel will help facilitate this paradigm shift in drug discovery.

Validation of the proteasome as a therapeutic target in Plasmodium using an epoxyketone inhibitor with parasite-specific toxicity.

The Plasmodium proteasome has been suggested to be a potential antimalarial drug target; however, toxicity of inhibitors has prevented validation of this enzyme in vivo. We report a screen of a library of 670 analogs of the recent US Food and Drug Administration-approved inhibitor, carfilzomib, to identify compounds that selectively kill parasites. We identified one compound, PR3, that has significant parasite killing activity in vitro but dramatically reduced toxicity in host cells. We found that this parasite-specific toxicity is not due to selective targeting of the Plasmodium proteasome over the host proteasome, but instead is due to a lack of activity against one of the human proteasome subunits. Subsequently, we used PR3 to significantly reduce parasite load in Plasmodium berghei infected mice without host toxicity, thus validating the proteasome as a viable antimalarial drug target.

The antimalarial natural product symplostatin 4 is a nanomolar inhibitor of the food vacuole falcipains.

The marine natural product symplostatin 4 (Sym4) has been recognized as a potent antimalarial agent. However, its mode of action and, in particular, direct targets have to date remained elusive. We report a chemical synthesis of Sym4 and show that Sym4-treatment of P. falciparum-infected red blood cells (RBCs) results in the generation of a swollen food vacuole phenotype and a reduction of parasitemia at nanomolar concentrations. We furthermore demonstrate that Sym4 is a nanomolar inhibitor of the P. falciparum falcipains in infected RBCs, suggesting inhibition of the hemoglobin degradation pathway as Sym4's mode of action. Finally, we reveal a critical influence of the unusual methyl-methoxypyrrolinone (mmp) group of Sym4 for potent inhibition, indicating that Sym4 derivatives with such a mmp moiety might represent viable lead structures for the development of antimalarial falcipain inhibitors.

Engineering homooligomeric proteins to detect weak intersite allosteric communication: aminotransferases, a case study.

The existence of low levels of intersubunit communication in homooligomeric enzymes is often difficult to discover, as the identical active sites cannot be probed individually to dissect their interdependent contributions. The homodimeric paralogs, E. coli aspartate- (AATase) and tyrosine aminotransferase (TATase), have not been demonstrated to show allostery. To address this question, we engineered a hybrid aminotransferase containing two distinct catalytic pockets: an AATase and a TATase site. The TATase/AATase hybrid was constructed by grafting an engineered TATase active site into one of the catalytic pockets of E. coli AATase. Each active site conserves its specific catalytic and inhibitor binding properties, and the hybrid catalyzes simultaneously each aminotransferase reaction at the respective site. Importantly, association of a selective inhibitor into one of the catalytic pockets decreases the activity of the second active site by up to 25%, thus proving unequivocally the existence of allosteric communication between active sites. The procedure may be applicable to other homologous sets of enzymes.

Biochemical characterization of Plasmodium falciparum dipeptidyl aminopeptidase 1.

Dipeptidyl aminopeptidase 1 (DPAP1) is an essential food vacuole enzyme with a putative role in hemoglobin catabolism by the erythrocytic malaria parasite. Here, the biochemical properties of DPAP1 have been investigated and compared to those of the human ortholog cathepsin C. To facilitate the characterization of DPAP1, we have developed a method for the production of purified recombinant DPAP1 with properties closely resembling those of the native enzyme. Like cathepsin C, DPAP1 is a chloride-activated enzyme that is most efficient in catalyzing amide bond hydrolysis at acidic pH values. The monomeric quaternary structure of DPAP1 differs from the homotetrameric structure of cathepsin C, which suggests that tetramerization is required for a cathepsin C-specific function. The S1 and S2 subsite preferences of DPAP1 and cathepsin C were profiled with a positional scanning synthetic combinatorial library. The S1 preferences bore close similarity to those of other C1-family cysteine peptidases. The S2 subsites of both DPAP1 and cathepsin C accepted aliphatic hydrophobic residues, proline, and some polar residues, yielding a distinct specificity profile. DPAP1 efficiently catalyzed the hydrolysis of several fluorogenic dipeptide substrates; surprisingly, however, a potential substrate with a P2-phenylalanine residue was instead a competitive inhibitor. Together, our biochemical data suggest that DPAP1 accelerates the production of amino acids from hemoglobin by bridging the gap between the endopeptidase and aminopeptidase activities of the food vacuole. Two reversible cathepsin C inhibitors potently inhibited both recombinant and native DPAP1, thereby validating the use of recombinant DPAP1 for future inhibitor discovery and characterization.