Identification of proteases that regulate erythrocyte rupture by the malaria parasite Plasmodium falciparum

The malarial serine protease SUB1 plays an essential role in parasite liver stage development

Transmission of the malaria parasite to its vertebrate host involves an obligatory exoerythrocytic stage in which extensive asexual replication of the parasite takes place in infected hepatocytes. The resulting liver schizont undergoes segmentation to produce thousands of daughter merozoites. These are released to initiate the blood stage life cycle, which causes all the pathology associated with the disease. Whilst elements of liver stage merozoite biology are similar to those in the much better-studied blood stage merozoites, little is known of the molecular players involved in liver stage merozoite production. To facilitate the study of liver stage biology we developed a strategy for the rapid production of complex conditional alleles by recombinase mediated engineering in Escherichia coli, which we used in combination with existing Plasmodium berghei deleter lines expressing Flp recombinase to study subtilisin-like protease 1 (SUB1), a conserved Plasmodium serine protease previously implicated in blood stage merozoite maturation and egress. We demonstrate that SUB1 is not required for the early stages of intrahepatic growth, but is essential for complete development of the liver stage schizont and for production of hepatic merozoites. Our results indicate that inhibitors of SUB1 could be used in prophylactic approaches to control or block the clinically silent pre-erythrocytic stage of the malaria parasite life cycle.

Malaria is caused by a single-celled parasite and is transmitted by the bite of an infected mosquito. The inoculated sporozoite forms of the parasite invade liver cells where they replicate, eventually releasing thousands of merozoites into the bloodstream to initiate the blood stage parasite life cycle which causes clinical malaria. The liver stage of the parasite life cycle is asymptomatic, so it is widely considered a potential target for prophylactic vaccine- or drug-based approaches designed to prevent infection. In this study, we use a robust, highly efficient gene engineering approach called recombineering, combined with a conditional gene deletion strategy to examine the function in liver stages of a parasite protease called SUB1, previously implicated in release of blood stage parasites. We show that SUB1 is expressed in the liver stage schizont and that the protease is essential for production of liver stage merozoites. Our results enhance our understanding of malarial liver stage biology, provide new tools for studying essential gene function in malaria, and suggest that inhibitors of SUB1 could be used as prophylactic drugs to prevent clinical malaria.

Bloodstream form pre-adaptation to the tsetse fly in Trypanosoma brucei

African trypanosomes are sustained in the bloodstream of their mammalian hosts by their extreme capacity for antigenic variation. However, for life cycle progression, trypanosomes also must generate transmission stages called stumpy forms that are pre-adapted to survive when taken up during the bloodmeal of the disease vector, tsetse flies. These stumpy forms are rather different to the proliferative slender forms that maintain the bloodstream parasitaemia. Firstly, they are non proliferative and morphologically distinct, secondly, they show particular sensitivity to environmental cues that signal entry to the tsetse fly and, thirdly, they are relatively robust such that they survive the changes in temperature, pH and proteolytic environment encountered within the tsetse midgut. These characteristics require regulated changes in gene expression to pre-adapt the parasite and the use of environmental sensing mechanisms, both of which allow the rapid initiation of differentiation to tsetse midgut procyclic forms upon transmission. Interestingly, the generation of stumpy forms is also regulated and periodic in the mammalian blood, this being governed by a density-sensing mechanism whereby a parasite-derived signal drives cell cycle arrest and cellular development both to optimize transmission and to prevent uncontrolled parasite multiplication overwhelming the host. In this review we detail recent developments in our understanding of the molecular mechanisms that underpin the production of stumpy forms in the mammalian bloodstream and their signal perception pathways both in the mammalian bloodstream and upon entry into the tsetse fly. These discoveries are discussed in the context of conserved eukaryotic signaling and differentiation mechanisms. Further, their potential to act as targets for therapeutic strategies that disrupt parasite development either in the mammalian bloodstream or upon their transmission to tsetse flies is also discussed.

Prison break: pathogens' strategies to egress from host cells

A wide spectrum of pathogenic bacteria and protozoa has adapted to an intracellular life-style, which presents several advantages, including accessibility to host cell metabolites and protection from the host immune system. Intracellular pathogens have developed strategies to enter and exit their host cells while optimizing survival and replication, progression through the life cycle, and transmission. Over the last decades, research has focused primarily on entry, while the exit process has suffered from neglect. However, pathogen exit is of fundamental importance because of its intimate association with dissemination, transmission, and inflammation. Hence, to fully understand virulence mechanisms of intracellular pathogens at cellular and systemic levels, it is essential to consider exit mechanisms to be a key step in infection. Exit from the host cell was initially viewed as a passive process, driven mainly by physical stress as a consequence of the explosive replication of the pathogen. It is now recognized as a complex, strategic process termed "egress," which is just as well orchestrated and temporally defined as entry into the host and relies on a dynamic interplay between host and pathogen factors. This review compares egress strategies of bacteria, pathogenic yeast, and kinetoplastid and apicomplexan parasites. Emphasis is given to recent advances in the biology of egress in mycobacteria and apicomplexans.

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 key role for Plasmodium subtilisin-like SUB1 protease in egress of malaria parasites from host hepatocytes

In their mammalian host, Plasmodium parasites have two obligatory intracellular development phases, first in hepatocytes and subsequently in erythrocytes. Both involve an orchestrated process of invasion into and egress from host cells. The Plasmodium SUB1 protease plays a dual role at the blood stage by enabling egress of the progeny merozoites from the infected erythrocyte and priming merozoites for subsequent erythrocyte invasion. Here, using conditional mutagenesis in P. berghei, we show that SUB1 plays an essential role at the hepatic stage. Stage-specific sub1 invalidation during prehepatocytic development showed that SUB1-deficient parasites failed to rupture the parasitophorous vacuole membrane and to egress from hepatocytes. Furthermore, mechanically released parasites were not adequately primed and failed to establish a blood stage infection in vivo. The critical involvement of SUB1 in both pre-erythrocytic and erythrocytic developmental phases qualifies SUB1 as an attractive multistage target for prophylactic and therapeutic anti-Plasmodium intervention strategies.

Proteolytic activity of Plasmodium falciparum subtilisin-like protease 3 on parasite profilin, a multifunctional protein

Subtilisin-like proteases of malaria parasite Plasmodium falciparum (PfSUB1, 2 and 3) are expressed at late asexual blood stages. PfSUB1 and 2 are considered important drug targets due to their essentiality for parasite blood stages and role in merozoite egress and invasion of erythrocytes. We have earlier shown the in vitro serine protease activity of PfSUB3 and its localization at asexual blood stages. In this study, we attempted to identify the biological substrate(s) of PfSUB3 and found parasite profilin (PfPRF) as a substrate of the protease. Eukaryotic profilins are multifunctional proteins with primary role in regulation of actin filament assembly. PfPRF possesses biochemical features of eukaryotic profilins and its rodent ortholog is essential in blood stages. Profilin from related apicomplexan parasite Toxoplasma gondii (TgPRF) is known to be involved in parasite motility, host cell invasion, active egress from host cell, immune evasion and virulence in mice. In this study, mature PfSUB3 proteolysed recombinant PfPRF in a dose-dependent manner in in vitro assays. Recombinant PfPRF was assessed for its proinflammatory activity and found to induce high level of TNF-α and low but significant level of IL-12 from mouse bone marrow-derived dendritic cells. Proteolysis of PfPRF by PfSUB3 is suggestive of the probable role of the protease in the processes of motility, virulence and immune evasion.

Recent insights into apicomplexan parasite egress provide new views to a kill

A hallmark of apicomplexan pathogens such as Plasmodium, Toxoplasma and Cryptosporidium is that they invade, replicate within, and then egress from their host cells. Egress usually results in lysis of the host cell, with deleterious consequences for the host. In the case of malaria, for example, much of the disease pathology is associated with cyclical waves of host erythrocyte destruction. This review highlights recent advances in mapping the signaling pathways that lead to egress and the parasite molecules involved in responding to and transmitting those signals. The review also discusses new findings for effector molecules that mediate disruption of the bounding membranes that enclose the intracellular parasite and the manner in which membrane rupture occurs to finally release invasive forms of the parasite.

Identification of a serine protease inhibitor which causes inclusion vacuole reduction and is lethal to Chlamydia trachomatis

The mechanistic details of the pathogenesis of Chlamydia, an obligate intracellular pathogen of global importance, have eluded scientists due to the scarcity of traditional molecular genetic tools to investigate this organism. Here we report a chemical biology strategy that has uncovered the first essential protease for this organism. Identification and application of a unique CtHtrA inhibitor (JO146) to cultures of Chlamydia resulted in a complete loss of viable elementary body formation. JO146 treatment during the replicative phase of development resulted in a loss of Chlamydia cell morphology, diminishing inclusion size, and ultimate loss of inclusions from the host cells. This completely prevented the formation of viable Chlamydia elementary bodies. In addition to its effect on the human Chlamydia trachomatis strain, JO146 inhibited the viability of the mouse strain, Chlamydia muridarum, both in vitro and in vivo. Thus, we report a chemical biology approach to establish an essential role for Chlamydia CtHtrA. The function of CtHtrA for Chlamydia appears to be essential for maintenance of cell morphology during replicative the phase and these findings provide proof of concept that proteases can be targeted for antimicrobial therapy for intracellular pathogens.

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.

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.

Malaria parasite cGMP-dependent protein kinase regulates blood stage merozoite secretory organelle discharge and egress

The malaria parasite replicates within an intraerythrocytic parasitophorous vacuole (PV). Eventually, in a tightly regulated process called egress, proteins of the PV and intracellular merozoite surface are modified by an essential parasite serine protease called PfSUB1, whilst the enclosing PV and erythrocyte membranes rupture, releasing merozoites to invade fresh erythrocytes. Inhibition of the Plasmodium falciparum cGMP-dependent protein kinase (PfPKG) prevents egress, but the underlying mechanism is unknown. Here we show that PfPKG activity is required for PfSUB1 discharge into the PV, as well as for release of distinct merozoite organelles called micronemes. Stimulation of PfPKG by inhibiting parasite phosphodiesterase activity induces premature PfSUB1 discharge and egress of developmentally immature, non-invasive parasites. Our findings identify the signalling pathway that regulates PfSUB1 function and egress, and raise the possibility of targeting PfPKG or parasite phosphodiesterases in therapeutic approaches to dysregulate critical protease-mediated steps in the parasite life cycle.

The role of serine-type serine repeat antigen in Plasmodium yoelii blood stage development

A key step for the survival of the malaria parasite is the release from and subsequent invasion of erythrocytes by the merozoite. Differences in the efficiency of these two linked processes have a direct impact on overall parasite burden in the host and thereby virulence. A number of parasite proteases have recently been shown to play important roles during both merozoite egress as well as merozoite invasion. The rodent malaria parasite Plasmodium yoelii has been extensively used to investigate the mechanisms of parasite virulence in vivo and a number of important proteins have been identified as being key contributors to pathology. Here we have utilized transcriptional comparisons to identify two protease-like SERAs as playing a potential role in virulence. We show that both SERAs are non-essential for blood stage development of the parasite though they provide a subtle but important growth advantage in vivo. In particular SERA2 appears to be an important factor in enabling the parasite to fully utilize the whole age repertoire of circulating erythrocytes. This work for the first time demonstrates the subtle contributions different protease-like SERAs make to provide the parasite with a maximal capacity to successfully maintain an infection in the host.

Identification of new peptide amides as selective cathepsin L inhibitors: the first step towards selective irreversible inhibitors?

A small library of peptide amides was designed to profile the cathepsin L active site. Within the cathepsin family of cysteine proteases, the first round of selection was on cathepsin L and cathepsin B, and then selected hits were further evaluated for binding to cathepsin K and cathepsin S. Five highly selective sequences with submicromolar affinities towards cathepsin L were identified. An acyloxymethyl ketone warhead was then attached to these sequences. Although these original irreversible inhibitors inactivate cathepsin L, it appears that the nature of the warhead drastically impact the selectivity profile of the resulting covalent inhibitors.

Molecular determinants of binding to the Plasmodium subtilisin-like protease 1

PfSUB1, a subtilisin-like protease of the human malaria parasite Plasmodium falciparum, is known to play important roles during the life cycle of the parasite and has emerged as a promising antimalarial drug target. In order to provide a detailed understanding of the origin of binding determinants of PfSUB1 substrates, we performed molecular dynamics simulations in combination with MM-GBSA free energy calculations using a homology model of PfSUB1 in complex with different substrate peptides. Key interactions, as well as residues that potentially make a major contribution to the binding free energy, are identified at the prime and nonprime side of the scissile bond and comprise peptide residues P4 to P2'. This finding stresses the requirement for peptide substrates to interact with both prime and nonprime side residues of the PfSUB1 binding site. Analyzing the energetic contributions of individual amino acids within the peptide-PfSUB1 complexes indicated that van der Waals interactions and the nonpolar part of solvation energy dictate the binding strength of the peptides and that the most favorable interactions are formed by peptide residues P4 and P1. Hot spot residues identified in PfSUB1 are dispersed over the entire binding site, but clustered areas of hot spots also exist and suggest that either the S4-S2 or the S1-S2' binding site should be exploited in efforts to design small molecule inhibitors. The results are discussed with respect to which binding determinants are specific to PfSUB1 and, therefore, might allow binding selectivity to be obtained.

Malaria parasites tolerate a broad range of ionic environments and do not require host cation remodelling

Malaria parasites grow within erythrocytes, but are also free in host plasma between cycles of asexual replication. As a result, the parasite is exposed to fluctuating levels of Na(+) and K(+) , ions assumed to serve important roles for the human pathogen, Plasmodium falciparum. We examined these assumptions and the parasite's ionic requirements by establishing continuous culture in novel sucrose-based media. With sucrose as the primary osmoticant and K(+) and Cl(-) as the main extracellular ions, we obtained parasite growth and propagation at rates indistinguishable from those in physiological media. These conditions abolish long-known increases in intracellular Na(+) via parasite-induced channels, excluding a requirement for erythrocyte cation remodelling. We also dissected Na(+) , K(+) and Cl(-) requirements and found that unexpectedly low concentrations of each ion meet the parasite's demands. Surprisingly, growth was not adversely affected by up to 148 mM K(+) , suggesting that low extracellular K(+) is not an essential trigger for erythrocyte invasion. At the same time, merozoite egress and invasion required a threshold ionic strength, suggesting critical electrostatic interactions between macromolecules at these stages. These findings provide insights into transmembrane signalling in malaria and reveal fundamental differences between host and parasite ionic requirements.

The cellular and molecular basis for malaria parasite invasion of the human red blood cell

Malaria is a major disease of humans caused by protozoan parasites from the genus Plasmodium. It has a complex life cycle; however, asexual parasite infection within the blood stream is responsible for all disease pathology. This stage is initiated when merozoites, the free invasive blood-stage form, invade circulating erythrocytes. Although invasion is rapid, it is the only time of the life cycle when the parasite is directly exposed to the host immune system. Significant effort has, therefore, focused on identifying the proteins involved and understanding the underlying mechanisms behind merozoite invasion into the protected niche inside the human erythrocyte.

Cytoplasmic free Ca2+ is essential for multiple steps in malaria parasite egress from infected erythrocytes

Background

Egress of Plasmodium falciparum, from erythrocytes at the end of its asexual cycle and subsequent parasite invasion into new host cells, is responsible for parasite dissemination in the human body. The egress pathway is emerging as a coordinated multistep programme that extends in time for tens of minutes, ending with rapid parasite extrusion from erythrocytes. While the Ca²⁺ regulation of the invasion of P. falciparum in erythrocytes is well established, the role of Ca²⁺ in parasite egress is poorly understood. This study analysed the involvement of cytoplasmic free Ca²⁺ in infected erythrocytes during the multistep egress programme of malaria parasites.

Methods

Live-cell fluorescence microscopy was used to image parasite egress from infected erythrocytes, assessing the effect of drugs modulating Ca²⁺ homeostasis on the egress programme.

Results

A steady increase in cytoplasmic free Ca²⁺ is found to precede parasite egress. This increase is independent of extracellular Ca²⁺ for at least the last two hours of the cycle, but is dependent upon Ca²⁺ release from internal stores. Intracellular BAPTA chelation of Ca²⁺ within the last 45 minutes of the cycle inhibits egress prior to parasitophorous vacuole swelling and erythrocyte membrane poration, two characteristic morphological transformations preceding parasite egress. Inhibitors of the parasite endoplasmic reticulum (ER) Ca²⁺-ATPase accelerate parasite egress, indicating that Ca²⁺ stores within the ER are sufficient in supporting egress. Markedly accelerated egress of apparently viable parasites was achieved in mature schizonts using Ca²⁺ ionophore A23187. Ionophore treatment overcomes the BAPTA-induced block of parasite egress, confirming that free Ca²⁺ is essential in egress initiation. Ionophore treatment of immature schizonts had an adverse effect inducing parasitophorous vacuole swelling and killing the parasites within the host cell.

Conclusions

The parasite egress programme requires intracellular free Ca²⁺ for egress initiation, vacuole swelling, and host cell cytoskeleton digestion. The evidence that parasitophorous vacuole swelling, a stage of unaffected egress, is dependent upon a rise in intracellular Ca²⁺ suggests a mechanism for ionophore-inducible egress and a new target for Ca²⁺ in the programme liberating parasites from the host cell. A regulatory pathway for egress that depends upon increases in intracellular free Ca²⁺ is proposed.

Target deconvolution techniques in modern phenotypic profiling

The past decade has seen rapid growth in the use of diverse compound libraries in classical phenotypic screens to identify modulators of a given process. The subsequent process of identifying the molecular targets of active hits, also called 'target deconvolution', is an essential step for understanding compound mechanism of action and for using the identified hits as tools for further dissection of a given biological process. Recent advances in 'omics' technologies, coupled with in silico approaches and the reduced cost of whole genome sequencing, have greatly improved the workflow of target deconvolution and have contributed to a renaissance of 'modern' phenotypic profiling. In this review, we will outline how both new and old techniques are being used in the difficult process of target identification and validation as well as discuss some of the ongoing challenges remaining for phenotypic screening.

A host GPCR signaling network required for the cytolysis of infected cells facilitates release of apicomplexan parasites

Following intracellular replication, the apicomplexan parasites Plasmodium falciparum and Toxoplasma gondii cause host cell cytolysis to facilitate parasite release and disease progression. Parasite exit from infected cells requires the interplay of parasite-derived proteins and host actin cytoskeletal changes; however, the host proteins underlying these changes remain obscure. We report the identification of a Gα(q)-coupled host-signaling cascade required for the egress of both P. falciparum and T. gondii. Gα(q)-coupled signaling results in protein kinase C (PKC)-mediated loss of the host cytoskeletal protein adducin and weakening of the cellular cytoskeleton. This cytoskeletal compromise induces catastrophic Ca(2+) influx mediated by the mechanosensitive cation channel TRPC6, which activates host calpain that proteolyzes the host cytoskeleton allowing parasite release. Reinforcing the feasibility of targeting host proteins as an antiparasitic strategy, mammalian PKC inhibitors demonstrated activity in murine models of malaria and toxoplasmosis. Importantly, an orally bioavailable PKC inhibitor prolonged survival in an experimental cerebral malaria model.

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.

Proteolytic activation of the essential parasitophorous vacuole cysteine protease SERA6 accompanies malaria parasite egress from its host erythrocyte

The malaria parasite replicates within an intraerythrocytic parasitophorous vacuole (PV). The PV and host cell membranes eventually rupture, releasing merozoites in a process called egress. Certain inhibitors of serine and cysteine proteases block egress, indicating a crucial role for proteases. The Plasmodium falciparum genome encodes nine serine-repeat antigens (SERAs), each of which contains a central domain homologous to the papain-like (clan CA, family C1) protease family. SERA5 and SERA6 are indispensable in blood-stage parasites, but the function of neither is known. Here we show that SERA6 localizes to the PV where it is precisely cleaved just prior to egress by an essential serine protease called PfSUB1. Mutations that replace the predicted catalytic Cys of SERA6, or that block SERA6 processing by PfSUB1, could not be stably introduced into the parasite genomic sera6 locus, indicating that SERA6 is an essential enzyme and that processing is important for its function. We demonstrate that cleavage of SERA6 by PfSUB1 converts it to an active cysteine protease. Our observations reveal a proteolytic activation step in the malarial PV that may be required for release of the parasite from its host erythrocyte.

Novel antimalarial drug targets: hope for new antimalarial drugs

Malaria is a major global threat, that results in more than 2 million deaths each year. The treatment of malaria is becoming extremely difficult due to the emergence of drug-resistant parasites, the absence of an effective vaccine, and the spread of insecticide-resistant vectors. Thus, malarial therapy needs new chemotherapeutic approaches leading to the search for new drug targets. Here, we discuss different approaches to identifying novel antimalarial drug targets. We have also given due attention to the existing validated targets with a view to develop novel, rationally designed lead molecules. Some of the important parasite proteins are claimed to be the targets; however, further in vitro or in vivo structure-function studies of such proteins are crucial to validate these proteins as suitable targets. The interactome analysis among apicoplast, mitochondrion and genomic DNA will also be useful in identifying vital pathways or proteins regulating critical pathways for parasite growth and survival, and could be attractive targets. Molecules responsible for parasite invasion to host erythrocytes and ion channels of infected erythrocytes, essential for intra-erythrocyte survival and stage progression of parasites are also becoming attractive targets. This review will discuss and highlight the current understanding regarding the potential antimalarial drug targets, which could be utilized to develop novel antimalarials.

Screening for small molecule inhibitors of Toxoplasma gondii

INTRODUCTION

Toxoplasma gondii, the agent that causes toxoplasmosis, is an opportunistic parasite that infects many mammalian species. It is an obligate intracellular parasite that causes severe congenital neurological and ocular disease mostly in immunocompromised humans. The current regimen of therapy includes only a few medications that often lead to hypersensitivity and toxicity. In addition, there are no vaccines available to prevent the transmission of this agent. Therefore, safer and more effective medicines to treat toxoplasmosis are urgently needed.

AREAS COVERED

The author presents in silico and in vitro strategies that are currently used to screen for novel targets and unique chemotypes against T. gondii. Furthermore, this review highlights the screening technologies and characterization of some novel targets and new chemical entities that could be developed into highly efficacious treatments for toxoplasmosis.

EXPERT OPINION

A number of diverse methods are being used to design inhibitors against T. gondii. These include ligand-based methods, in which drugs that have been shown to be efficacious against other Apicomplexa parasites can be repurposed to identify lead molecules against T. gondii. In addition, structure-based methods use currently available repertoire of structural information in various databases to rationally design small-molecule inhibitors of T. gondii. Whereas the screening methods have their advantages and limitations, a combination of methods is ideally suited to design small-molecule inhibitors of complex parasites such as T. gondii.

Quinolylhydrazones as novel inhibitors of Plasmodium falciparum serine protease PfSUB1

Plasmodium falciparum subtilisin-like protease 1 (PfSUB1) is a serine protease that plays key roles in the egress of the parasite from red blood cells and in preparing the released merozoites for the subsequent invasion of new erythrocytes. The development of potent and selective PfSUB1 inhibitors could pave the way to the discovery of potential antimalarial drugs endowed with an innovative mode of action and consequently able to overcome the current problems of resistance to established chemotherapies. Through the screening of a proprietary library of compounds against PfSUB1, we identified hydrazone 2 as a hit compound. Here we report a preliminary investigation of the structure-activity relationships for a class of PfSUB1 inhibitors related to our identified hit.

Signal transduction in Plasmodium-Red Blood Cells interactions and in cytoadherence

Malaria is responsible for more than 1.5 million deaths each year, especially among children (Snow et al. 2005). Despite of the severity of malaria situation and great effort to the development of new drug targets (Yuan et al. 2011) there is still a relative low investment toward antimalarial drugs. Briefly there are targets classes of antimalarial drugs currently being tested including: kinases, proteases, ion channel of GPCR, nuclear receptor, among others (Gamo et al. 2010). Here we review malaria signal transduction pathways in Red Blood Cells (RBC) as well as infected RBCs and endothelial cells interactions, namely cytoadherence. The last process is thought to play an important role in the pathogenesis of severe malaria. The molecules displayed on the surface of both infected erythrocytes (IE) and vascular endothelial cells (EC) exert themselves as important mediators in cytoadherence, in that they not only induce structural and metabolic changes on both sides, but also trigger multiple signal transduction processes, leading to alteration of gene expression, with the balance between positive and negative regulation determining endothelial pathology during a malaria infection.

How chemoproteomics can enable drug discovery and development

Creating first-in-class medications to treat human disease is an extremely challenging endeavor. While genome sequencing and genetics are making direct connections between mutations and human disorders at an unprecedented rate, matching molecular targets with a suitable therapeutic indication must ultimately be achieved by pharmacology. Here, we discuss how the integration of chemical proteomic platforms (such as activity-based protein profiling) into the earliest stages of the drug discovery process has the potential to greatly expand the scope of proteins that can be pharmacologically evaluated in living systems, and, through doing so, promote the identification and prioritization of new therapeutic targets.

Moving "Forward" in Plasmodium Genetics through a Transposon-Based Approach

The genome sequence of the human malaria parasite, Plasmodium falciparum, was released almost a decade ago. A majority of the Plasmodium genome, however, remains annotated to code for hypothetical proteins with unknown functions. The introduction of forward genetics has provided novel means to gain a better understanding of gene functions and their associated phenotypes in Plasmodium. Even with certain limitations, the technique has already shown significant promise to increase our understanding of parasite biology needed for rationalized drug and vaccine design. Further improvements to the mutagenesis technique and the design of novel genetic screens should lead us to some exciting discoveries about the critical weaknesses of Plasmodium, and greatly aid in the development of new disease intervention strategies.

The role of cGMP signalling in regulating life cycle progression of Plasmodium

The 3′-5′-cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG) is the main mediator of cGMP signalling in the malaria parasite. This article reviews the role of PKG in Plasmodium falciparum during gametogenesis and blood stage schizont rupture, as well as the role of the Plasmodium berghei orthologue in ookinete differentiation and motility, and liver stage schizont development. The current views on potential effector proteins downstream of PKG and the mechanisms that may regulate cyclic nucleotide levels are presented.

Plasmodium subtilisin-like protease 1 (SUB1): insights into the active-site structure, specificity and function of a pan-malaria drug target

Release of the malaria merozoite from its host erythrocyte (egress) and invasion of a fresh cell are crucial steps in the life cycle of the malaria pathogen. Subtilisin-like protease 1 (SUB1) is a parasite serine protease implicated in both processes. In the most dangerous human malarial species, Plasmodium falciparum, SUB1 has previously been shown to have several parasite-derived substrates, proteolytic cleavage of which is important both for egress and maturation of the merozoite surface to enable invasion. Here we have used molecular modelling, existing knowledge of SUB1 substrates, and recombinant expression and characterisation of additional Plasmodium SUB1 orthologues, to examine the active site architecture and substrate specificity of P. falciparum SUB1 and its orthologues from the two other major human malaria pathogens Plasmodium vivax and Plasmodium knowlesi, as well as from the rodent malaria species, Plasmodium berghei. Our results reveal a number of unusual features of the SUB1 substrate binding cleft, including a requirement to interact with both prime and non-prime side residues of the substrate recognition motif. Cleavage of conserved parasite substrates is mediated by SUB1 in all parasite species examined, and the importance of this is supported by evidence for species-specific co-evolution of protease and substrates. Two peptidyl alpha-ketoamides based on an authentic PfSUB1 substrate inhibit all SUB1 orthologues examined, with inhibitory potency enhanced by the presence of a carboxyl moiety designed to introduce prime side interactions with the protease. Our findings demonstrate that it should be possible to develop ‘pan-reactive’ drug-like compounds that inhibit SUB1 in all three major human malaria pathogens, enabling production of broad-spectrum antimalarial drugs targeting SUB1.

Eimeripain, a cathepsin B-like cysteine protease, expressed throughout sporulation of the apicomplexan parasite Eimeria tenella

The invasion and replication of Eimeria tenella in the chicken intestine is responsible for avian coccidiosis, a disease that has major economic impacts on poultry industries worldwide. E. tenella is transmitted to naïve animals via shed unsporulated oocysts that need contact with air and humidity to form the infectious sporulated oocysts, which contain the first invasive form of the parasite, the sporozoite. Cysteine proteases (CPs) are major virulence factors expressed by protozoa. In this study, we show that E. tenella expresses five transcriptionally regulated genes encoding one cathepsin L, one cathepsin B and three cathepsin Cs. Biot-LC-LVG-CHN₂, a cystatin derived probe, tagged eight polypeptides in unsporulated oocysts but only one in sporulated oocysts. CP-dependant activities were found against the fluorescent substrates, Z-FR-AMC and Z-LR-AMC, throughout the sporulation process. These activities corresponded to a cathepsin B-like enzyme since they were inhibited by CA-074, a specific cathepsin B inhibitor. A 3D model of the catalytic domain of the cathepsin B-like protease, based on its sequence homology with human cathepsin B, further confirmed its classification as a papain-like protease with similar characteristics to toxopain-1 from the related apicomplexan parasite, Toxoplasma gondii; we have, therefore, named the E. tenella cathepsin B, eimeripain. Following stable transfection of E. tenella sporozoites with a plasmid allowing the expression of eimeripain fused to the fluorescent protein mCherry, we demonstrated that eimeripain is detected throughout sporulation and has a punctate distribution in the bodies of extra- and intracellular parasites. Furthermore, CA-074 Me, the membrane-permeable derivative of CA-074, impairs invasion of epithelial MDBK cells by E. tenella sporozoites. This study represents the first characterization of CPs expressed by a parasite from the Eimeria genus. Moreover, it emphasizes the role of CPs in transmission and dissemination of exogenous stages of apicomplexan parasites.

Activity-based probes for the study of proteases: recent advances and developments

Proteases are important targets for the treatment of human disease. Several protease inhibitors have failed in clinical trials due to a lack of in vivo specificity, indicating the need for studies of protease function and inhibition in complex, disease-related models. The tight post-translational regulation of protease activity complicates protease analysis by traditional proteomics methods. Activity-based protein profiling is a powerful technique that can resolve this issue. It uses small-molecule tools-activity-based probes-to label and analyze active enzymes in lysates, cells, and whole animals. Over the last twelve years, a wide variety of protease activity-based probes have been developed. These synthetic efforts have enabled techniques ranging from real-time in vivo imaging of protease activity to high-throughput screening of uncharacterized proteases. This Review introduces the general principles of activity-based protein profiling and describes the recent advancements in probe design and analysis techniques, which have increased the knowledge of protease biology and will aid future protease drug discovery.

Inhibitory potential of prodomain of Plasmodium falciparum protease serine repeat antigen 5 for asexual blood stages of parasite

Plasmodium falciparum serine repeat antigen 5 (SERA5) is a target for both drug and vaccine intervention against malaria. SERA5 is secreted in the parasitophorous vacuole where it is proteolytically processed before schizont rupture. Among the processed products is a 50.8-kDa central domain of the protease, which possesses chymotrypsin-like activity and consists of a 28.9-kDa catalytic domain with a 21.9-kDa N-terminal prodomain, which remain attached together. Because SERA5 has been implicated in merozoite egress from host erythrocytes, the effect of the prodomain and a heptapeptide derived from its C-terminus spanning from D(560) to F(566) (DNSDNMF) on parasite growth was studied. When E. coli-expressed prodomain was incubated with parasite culture, a significant delay in transition from schizont to ring stages was observed up to nanomolar concentrations. The peptide, DNSDNMF also showed similar effects but at nearly 1000-fold higher concentrations. The peptide was also found to interact with the catalytic domain. These data demonstrate the crucial role of SERA5 prodomain for the egress process. Given the inhibitory potential of the prodomain for the parasite, we suggest that peptidomimetic inhibitors based on SERA5 prodomain sequences can be developed as future therapeutics against malaria.

Expression and characterization of catalytic domain of Plasmodium falciparum subtilisin-like protease 3

PfSUB3 is the third subtilisin-like protease annotated in Plasmodium genome database "PlasmoDB". The other two members, PfSUB1 and PfSUB2 have been implicated in merozoite egress and invasion in asexual blood stages. In this study, we recombinantly expressed a region of PfSUB3 spanning from Asn(334) to Glu(769) (PfSUB3c) which encompassed the predicted catalytic domain with all the active site residues and predicted mature region spanning from Thr(516) to Glu(769) (PfSUB3m) in E. coli. PfSUB3m showed PMSF-sensitive proteolytic activity in in vitro assays. Replacement of active site serine with alanine in PfSUB3m resulted in inactive protein. We found that PfSUB3c and PfSUB3m undergo truncation to produce a 25-kDa species which was sufficient for proteolytic activity. Quantitative real-time PCR, immnufluorescence assay and Western blot analyses revealed that PfSUB3 is expressed at late asexual blood stages. Serine protease activity of PfSUB3 and its expression in the late stages of erythrocytic schizogony are indicative of some possible role of the protease in merozoite egress and/or invasion processes.

Geographic differentiation of polymorphism in the Plasmodium falciparum malaria vaccine candidate gene SERA5

SERA5 is regarded as a promising malaria vaccine candidate of the most virulent human malaria parasite Plasmodium falciparum. SERA5 is a 120 kDa abundantly expressed blood-stage protein containing a papain-like protease. Since substantial polymorphism in blood-stage vaccine candidates may potentially limit their efficacy, it is imperative to fully investigate polymorphism of the SERA5 gene (sera5). In this study, we performed evolutionary and population genetic analysis of sera5. The level of inter-species divergence (kS=0.076) between P. falciparum and Plasmodium reichenowi, a closely related chimpanzee malaria parasite is comparable to that of housekeeping protein genes. A signature of purifying selection was detected in the proenzyme and enzyme domains. Analysis of 445 near full-length P. falciparum sera5 sequences from nine countries in Africa, Southeast Asia, Oceania and South America revealed extensive variations in the number of octamer repeat (OR) and serine repeat (SR) regions as well as substantial level of single nucleotide polymorphism (SNP) in non-repeat regions (2562 bp). Remarkably, a 14 amino acid sequence of SERA5 (amino acids 59-72) that is known to be the in vitro target of parasite growth inhibitory antibodies was found to be perfectly conserved in all 445 worldwide isolates of P. falciparum evaluated. Unlike other major vaccine target antigen genes such as merozoite surface protein-1, apical membrane antigen-1 or circumsporozoite protein, no strong evidence for positive selection was detected for SNPs in the non-repeat regions of sera5. A biased geographical distribution was observed in SNPs as well as in the haplotypes of the sera5 OR and SR regions. In Africa, OR- and SR-haplotypes with low frequency (<5%) and SNPs with minor allele frequency (<5%) were abundant and were mostly continent-specific. Consistently, significant genetic differentiation, assessed by the Wright's fixation index (Fst) of inter-population variance in allele frequencies, was detected for SNPs and both OR- and SR-haplotypes among almost all parasite populations. The exception was parasite populations between Tanzania and Ghana, suggesting frequent gene flow in Africa. The present study points to the importance of investigating whether biased geographical distribution for SNPs and repeat variants in the OR and SR regions affect the reactivity of human serum antibodies to variants.

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.

Proteases as regulators of pathogenesis: examples from the Apicomplexa

The diverse functional roles that proteases play in basic biological processes make them essential for virtually all organisms. Not surprisingly, proteolysis is also a critical process required for many aspects of pathogenesis. In particular, obligate intracellular parasites must precisely coordinate proteolytic events during their highly regulated life cycle inside multiple host cell environments. Advances in chemical, proteomic and genetic tools that can be applied to parasite biology have led to an increased understanding of the complex events centrally regulated by proteases. In this review, we outline recent advances in our knowledge of specific proteolytic enzymes in two medically relevant apicomplexan parasites: Plasmodium falciparum and Toxoplasma gondii. Efforts over the last decade have begun to provide a map of key proteotolyic events that are essential for both parasite survival and propagation inside host cells. These advances in our molecular understanding of proteolytic events involved in parasite pathogenesis provide a foundation for the validation of new networks and enzyme targets that could be exploited for therapeutic purposes. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Screening inhibitors of P. berghei blood stages using bioluminescent reporter parasites

We describe two improved assays for in vitro and in vivo screening of inhibitors and chemicals for antimalarial activity against blood stages of the rodent malaria parasite, Plasmodium berghei. These assays are based on the determination of bioluminescence in small blood samples that is produced by reporter parasites expressing luciferase. Luciferase production increases as the parasite develops in a red blood cell and as the numbers of parasites increase during an infection. In the first assay, in vitro drug luminescence (ITDL) assay, the in vitro development of ring-stage parasites into mature schizonts in the presence and absence of candidate inhibitor(s) is quantified by measuring luciferase activity after the parasites have been allowed to mature into schizonts in culture. In the second assay, the in vivo drug luminescence (IVDL) assay, in vivo parasite growth (using a standard 4-day suppressive drug test) is quantified by measuring the luciferase activity of circulating parasites in samples of tail blood of drug-treated mice.

Protease-associated cellular networks in malaria parasite Plasmodium falciparum

Background

Malaria continues to be one of the most severe global infectious diseases, responsible for 1-2 million deaths yearly. The rapid evolution and spread of drug resistance in parasites has led to an urgent need for the development of novel antimalarial targets. Proteases are a group of enzymes that play essential roles in parasite growth and invasion. The possibility of designing specific inhibitors for proteases makes them promising drug targets. Previously, combining a comparative genomics approach and a machine learning approach, we identified the complement of proteases (degradome) in the malaria parasite Plasmodium falciparum and its sibling species [1-3], providing a catalog of targets for functional characterization and rational inhibitor design. Network analysis represents another route to revealing the role of proteins in the biology of parasites and we use this approach here to expand our understanding of the systems involving the proteases of P. falciparum.

Results

We investigated the roles of proteases in the parasite life cycle by constructing a network using protein-protein association data from the STRING database [4], and analyzing these data, in conjunction with the data from protein-protein interaction assays using the yeast 2-hybrid (Y2H) system [5], blood stage microarray experiments [6-8], proteomics [9-12], literature text mining, and sequence homology analysis. Seventy-seven (77) out of 124 predicted proteases were associated with at least one other protein, constituting 2,431 protein-protein interactions (PPIs). These proteases appear to play diverse roles in metabolism, cell cycle regulation, invasion and infection. Their degrees of connectivity (i.e., connections to other proteins), range from one to 143. The largest protease-associated sub-network is the ubiquitin-proteasome system which is crucial for protein recycling and stress response. Proteases are also implicated in heat shock response, signal peptide processing, cell cycle progression, transcriptional regulation, and signal transduction networks.

Conclusions

Our network analysis of proteases from P. falciparum uses a so-called guilt-by-association approach to extract sets of proteins from the proteome that are candidates for further study. Novel protease targets and previously unrecognized members of the protease-associated sub-systems provide new insights into the mechanisms underlying parasitism, pathogenesis and virulence.

Centenary celebrations article: Cysteine proteases of human malaria parasites

There is an urgent need for new drugs against malaria, which takes millions of lives annually. Cysteine proteases are potential new drug targets, especially when current drugs are showing resistance. Falcipains and vivapains are well characterized cysteine proteases of P. falciparum and P. vivax, respectively. Studies with cysteine protease inhibitors and manipulating cysteine proteases specific genes have suggested their roles in hemoglobin hydrolysis. In P. falciparum, falcipain-2 and falcipain-3 are major hemoglobinases that hydrolyze host erythrocyte hemoglobin in the parasite food vacuole. It is confirmed that disruption of the falcipain-2 gene led to a transient block in hemoglobin hydrolysis, and disruption of falcipain-3 gene was not possible, suggesting that protease is essential for erythrocytic parasites. On the other hand, vivapain-2, vivapain-3 and vivapain-4 are important cysteine proteases of P. vivax, which shared a number of features with falcipain-2 and falcipain-3. A recent study indicates that vivapains and aspartic protease of P. vivax works collaboratively to enhance the parasites' ability to hydrolyze host erythrocyte hemoglobin. Studies also indicate that falcipains and vivapains also hydrolyse the erythrocyte cytoskeleton proteins and involved in rupture of red blood cell. Structural and biochemical analysis of falcipains and vivapains showed that they have unique domains for specific functions. Overall, the complexes of cysteine proteases with small and macromolecular inhibitors provide structural insight to facilitate the drug design. Therefore, giving due importance to the cysteine proteases, this review will briefly focus the recent advancement in the field of cysteine proteases of human malaria parasites.

Phosphatidylinositol 3,4,5-trisphosphate activity probes for the labeling and proteomic characterization of protein binding partners

Phosphatidylinositol polyphosphate lipids, such as phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P₃], regulate critical biological processes, many of which are aberrant in disease. These lipids often act as site-specific ligands in interactions that enforce membrane association of protein binding partners. Herein, we describe the development of bifunctional activity probes corresponding to the headgroup of PI(3,4,5)P₃ that are effective for identifying and characterizing protein binding partners from complex samples, namely cancer cell extracts. These probes contain both a photoaffinity tag for covalent labeling of target proteins and a secondary handle for subsequent detection or manipulation of labeled proteins. Probes bearing different secondary tags were exploited, either by direct attachment of a fluorescent dye for optical detection or by using an alkyne that can be derivatized after protein labeling via click chemistry. First, we describe the design and modular synthetic strategy used to generate multiple probes with different reporter tags of use for characterizing probe-labeled proteins. Next, we report initial labeling studies using purified protein, the PH domain of Akt, in which probes were found to label this target, as judged by in-gel detection. Furthermore, protein labeling was abrogated by controls including competition with an unlabeled PI(3,4,5)P₃ headgroup analogue as well as through protein denaturation, indicating specific labeling. In addition, probes featuring linkers of different lengths between the PI(3,4,5)P₃ headgroup and photoaffinity tag led to variations in protein labeling, indicating that a shorter linker was more effective in this case. Finally, proteomic labeling studies were performed using cell extracts; labeled proteins were observed by in-gel detection and characterized using postlabeling with biotin, affinity chromatography, and identification via tandem mass spectrometry. These studies yielded a total of 265 proteins, including both known and novel candidate PI(3,4,5)P₃-binding proteins.

The malaria parasite progressively dismantles the host erythrocyte cytoskeleton for efficient egress

Plasmodium falciparum is an obligate intracellular pathogen responsible for worldwide morbidity and mortality. This parasite establishes a parasitophorous vacuole within infected red blood cells wherein it differentiates into multiple daughter cells that must rupture their host cells to continue another infectious cycle. Using atomic force microscopy, we establish that progressive macrostructural changes occur to the host cell cytoskeleton during the last 15 h of the erythrocytic life cycle. We used a comparative proteomics approach to determine changes in the membrane proteome of infected red blood cells during the final steps of parasite development that lead to egress. Mass spectrometry-based analysis comparing the red blood cell membrane proteome in uninfected red blood cells to that of infected red blood cells and postrupture vesicles highlighted two temporally distinct events; (Hay, S. I., et al. (2009). A world malaria map: Plasmodium falciparum endemicity in 2007. PLoS Med. 6, e1000048) the striking loss of cytoskeletal adaptor proteins that are part of the junctional complex, including α/β-adducin and tropomyosin, correlating temporally with the emergence of large holes in the cytoskeleton seen by AFM as early ~35 h postinvasion, and (Maier, A. G., et al. (2008) Exported proteins required for virulence and rigidity of Plasmodium falciparum-infected human erythrocytes. Cell 134, 48-61) large-scale proteolysis of the cytoskeleton during rupture ~48 h postinvasion, mediated by host calpain-1. We thus propose a sequential mechanism whereby parasites first remove a selected set of cytoskeletal adaptor proteins to weaken the host membrane and then use host calpain-1 to dismantle the remaining cytoskeleton, leading to red blood cell membrane collapse and parasite release.

Bestatin-based chemical biology strategy reveals distinct roles for malaria M1- and M17-family aminopeptidases

Malaria causes worldwide morbidity and mortality, and while chemotherapy remains an excellent means of malaria control, drug-resistant parasites necessitate the discovery of new antimalarials. Peptidases are a promising class of drug targets and perform several important roles during the Plasmodium falciparum erythrocytic life cycle. Herein, we report a multidisciplinary effort combining activity-based protein profiling, biochemical, and peptidomic approaches to functionally analyze two genetically essential P. falciparum metallo-aminopeptidases (MAPs), PfA-M1 and Pf-LAP. Through the synthesis of a suite of activity-based probes (ABPs) based on the general MAP inhibitor scaffold, bestatin, we generated specific ABPs for these two enzymes. Specific inhibition of PfA-M1 caused swelling of the parasite digestive vacuole and prevented proteolysis of hemoglobin (Hb)-derived oligopeptides, likely starving the parasite resulting in death. In contrast, inhibition of Pf-LAP was lethal to parasites early in the life cycle, prior to the onset of Hb degradation suggesting that Pf-LAP has an essential role outside of Hb digestion.