Identification and Validation of Small-Gatekeeper Kinases as Drug Targets in Giardia lamblia

Ultrastructural Alterations of the Human Pathogen Giardia intestinalis after Drug Treatment

This review presents the main cell characteristics altered after in vitro incubation of the parasite with commercial drugs used to treat the disease caused by Giardia intestinalis. This important intestinal parasite primarily causes diarrhea in children. Metronidazole and albendazole are the primary compounds used in therapy against Giardia intestinalis. However, they provoke significant side effects, and some strains have developed resistance to metronidazole. Benzimidazole carbamates, such as albendazole and mebendazole, have shown the best activity against Giardia. Despite their in vitro efficacy, clinical treatment with benzimidazoles has yielded conflicting results, demonstrating lower cure rates. Recently, nitazoxanide has been suggested as an alternative to these drugs. Therefore, to enhance the quality of chemotherapy against this parasite, it is important to invest in developing other compounds that can interfere with key steps of metabolic pathways or cell structures and organelles. For example, Giardia exhibits a unique cell structure called the ventral disc, which is crucial for host adhesion and pathogenicity. Thus, drugs that can disrupt the adhesion process hold promise for future therapy against Giardia. Additionally, this review discusses new drugs and strategies that can be employed, as well as suggestions for developing novel drugs to control the infection caused by this parasite.

Antigiardial Activity of Novel Guanidine Compounds

From four focused compound libraries based on the known anticoccidial agent robenidine, 44 compounds total were synthesised and screened for antigiardial activity. All active compounds were counter-screened for antibiotic and cytotoxic action. Of the analogues examined, 21 displayed IC₅₀ <5 μM, seven with IC₅₀ <1.0 μM. Most active were 2,2'-bis{[4-(trifluoromethoxy)phenyl]methylene}carbonimidic dihydrazide hydrochloride (30), 2,2'-bis{[4-(trifluoromethylsulfanyl)phenyl]methylene}carbonimidic dihydrazide hydrochloride (32), and 2,2'-bis[(2-bromo-4,5-dimethoxyphenyl)methylene]carbonimidic dihydrazide hydrochloride (41) with IC₅₀ =0.2 μM. The maximal observed activity was a 5 h IC₅₀ value of 0.2 μM for 41. The clinically used metronidazole was inactive at this timepoint at a concentration of 25 μM. Robenidine off-target effects at bacteria and cell line toxicity were removed. Analogue 41 was well tolerated in mice treated orally (100 mg/kg). Following 5 h treatment with 41, no Giardia regrowth was noted after 48 h.

Unusual Cell Structures and Organelles in Giardia intestinalis and Trichomonas vaginalis Are Potential Drug Targets

This review presents the main cell organelles and structures of two important protist parasites, Giardia intestinalis, and Trichomonas vaginalis; many are unusual and are not found in other eukaryotic cells, thus could be good candidates for new drug targets aimed at improvement of the chemotherapy of diseases caused by these eukaryotic protists. For example, in Giardia, the ventral disc is a specific structure to this parasite and is fundamental for the adhesion and pathogenicity to the host. In Trichomonas, the hydrogenosome, a double membrane-bounded organelle that produces ATP, also can be a good target. Other structures include mitosomes, ribosomes, and proteasomes. Metronidazole is the most frequent compound used to kill many anaerobic organisms, including Giardia and Trichomonas. It enters the cell by passive diffusion and needs to find a highly reductive environment to be reduced to the nitro radicals to be active. However, it provokes several side effects, and some strains present metronidazole resistance. Therefore, to improve the quality of the chemotherapy against parasitic protozoa is important to invest in the development of highly specific compounds that interfere with key steps of essential metabolic pathways or in the functional macromolecular complexes which are most often associated with cell structures and organelles.

Repurposing the Kinase Inhibitor Mavelertinib for Giardiasis Therapy

A phenotypic screen of the ReFRAME compound library was performed to identify cell-active inhibitors that could be developed as therapeutics for giardiasis. A primary screen against Giardia lamblia GS clone H7 identified 85 cell-active compounds at a hit rate of 0.72%. A cytotoxicity counterscreen against HEK293T cells was carried out to assess hit compound selectivity for further prioritization. Mavelertinib (PF-06747775), a third-generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI), was identified as a potential new therapeutic based on indication, activity, and availability after reconfirmation. Mavelertinib has in vitro efficacy against metronidazole-resistant 713-M3 strains. Other EGFR-TKIs screened in follow-up assays exhibited insignificant inhibition of G. lamblia at 5 μM, suggesting that the primary molecular target of mavelertinib may have a different mechanistic binding mode from human EGFR-tyrosine kinase. Mavelertinib, dosed as low as 5 mg/kg of body weight or as high as 50 mg/kg, was efficacious in the acute murine Giardia infection model. These results suggest that mavelertinib merits consideration for repurposing and advancement to giardiasis clinical trials while its analogues are further developed.

Oxygen levels are key to understanding "Anaerobic" protozoan pathogens with micro-aerophilic lifestyles

Publications abound on the physiology, biochemistry and molecular biology of "anaerobic" protozoal parasites as usually grown under "anaerobic" culture conditions. The media routinely used are poised at low redox potentials using techniques that remove O₂ to "undetectable" levels in sealed containers. However there is growing understanding that these culture conditions do not faithfully resemble the O₂ environments these organisms inhabit. Here we review for protists lacking oxidative energy metabolism, the oxygen cascade from atmospheric to intracellular concentrations and relevant methods of measurements of O₂, some well-studied parasitic or symbiotic protozoan lifestyles, their homeodynamic metabolic and redox balances, organism-drug-oxygen interactions, and the present and future prospects for improved drugs and treatment regimes.

Antimicrobial resistance of the enteric protozoon Giardia duodenalis - A narrative review

Introduction

As therapy-refractory giardiasis is an emerging health issue, this review aimed at summarizing mechanisms of reduced antimicrobial susceptibility in Giardia duodenalis and strategies to overcome this problem.

Methods

A narrative review on antimicrobial resistance in G. duodenalis was based upon a selective literature research.

Results

Failed therapeutic success has been observed for all standard therapies of giardiasis comprising nitroimidazoles like metronidazole or tinidazole as first line substances but also benznidazoles like albendazole and mebendazole, the nitrofuran furazolidone, the thiazolide nitazoxanide, and the aminoglycoside paromomycin. Multicausality of the resistance phenotypes has been described, with differentiated gene expression due to epigenetic and post-translational modifications playing a considerable bigger role than mutational base exchanges in the parasite DNA. Standardized resistance testing algorithms are not available and clinical evidence for salvage therapies is scarce in spite of research efforts targeting new giardicidal drugs.

Conclusion

In case of therapeutic failure of first line nitroimidazoles, salvage strategies including various options for combination therapy exist in spite of limited evidence and lacking routine diagnostic-compatible assays for antimicrobial susceptibility testing in G. duodenalis. Sufficiently powered clinical and diagnostic studies are needed to overcome both the lacking evidence regarding salvage therapy and the diagnostic neglect of antimicrobial resistance.

Methionyl-tRNA synthetase inhibitor has potent in vivo activity in a novel Giardia lamblia luciferase murine infection model

BACKGROUND

Methionyl-tRNA synthetase (MetRS) inhibitors are under investigation for the treatment of intestinal infections caused by Giardia lamblia.

OBJECTIVES

To properly analyse the therapeutic potential of the MetRS inhibitor 1717, experimental tools including a robust cell-based assay and a murine model of infection were developed based on novel strains of G. lamblia that employ luciferase reporter systems to quantify viable parasites.

METHODS

Systematic screening of Giardia-specific promoters and luciferase variants led to the development of a strain expressing the click beetle green luciferase. Further modifying this strain to express NanoLuc created a dual reporter strain capable of quantifying parasites in both the trophozoite and cyst stages. These strains were used to develop a high-throughput cell assay and a mouse infection model. A library of MetRS inhibitors was screened in the cell assay and Compound-1717 was tested for efficacy in the mouse infection model.

RESULTS

Cell viability in in vitro compound screens was quantified via bioluminescence readouts while infection loads in mice were monitored with non-invasive whole-animal imaging and faecal analysis. Compound-1717 was effective in clearing mice of Giardia infection in 3 days at varying doses, which was supported by data from enzymatic and phenotypic cell assays.

CONCLUSIONS

The new in vitro and in vivo assays based on luciferase expression by engineered G. lamblia strains are useful for the discovery and development of new therapeutics for giardiasis. MetRS inhibitors, as validated by Compound-1717, have promising anti-giardiasis properties that merit further study as alternative therapeutics.

A Curious Case for Development of Kinase Inhibitors as Antigiardiasis Treatments Using Advanced Drug Techniques

Giardiasis is a neglected parasitic diarrheal disease that is particularly associated with poverty. Current treatment options are limited in the face of growing resistance, but the reduced kinome of Giardia lamblia increases the likelihood of identifying nonredundant essential kinases as potential drug targets. Repurposing known and newly identified kinase inhibitors in drug development programs for novel giardiasis therapeutics could therefore be a cost-effective and time saving approach. Innovative improvements to physiologically-based pharmacokinetic modeling coupled with emerging imaging technologies and a CRISPR-interference method could accelerate progress toward the goal of more effective giardiasis therapeutics based on kinase inhibition.

Calculating Transfer Entropy from Variance-Covariance Matrices Provides Insight into Allosteric Communication in ERK2

We develop an approach by which reliable estimates of the transfer entropy can be obtained from the variance-covariance matrix of atomic fluctuations, which converges quickly and retains sensitivity to the full chemical profile of the biomolecular system. We validate our method on ERK2, a well-studied kinase involved in the MAPK signaling cascade for which considerable computational, experimental, and mutation data are available. We present the results of transfer entropy analysis on data obtained from molecular dynamics simulations of wild-type active and inactive ERK2, along with mutants Q103A, I84A, L73P, and G83A. We show that our method is systematically consistent within the context of other approaches for calculating transfer entropy, and we provide a method for interpreting networks of interconnected residues in the protein from a perspective of allosteric coupling. We introduce new insights about possible allosteric activity of the extreme N-terminal region of the kinase, and we describe evidence that suggests that activation may occur by different paths or routes in different mutants. Our results highlight systematic advantages and disadvantages of each method for calculating transfer entropy and show the important role of transfer entropy analysis for understanding allosteric behavior in biomolecular systems.

Anti-Giardia Drug Discovery: Current Status and Gut Feelings

Giardia parasites are ubiquitous protozoans of global importance that impact a wide range of animals including humans. They are the most common enteric pathogen of cats and dogs in developed countries and infect ∼1 billion people worldwide. While Giardia infections can be asymptomatic, they often result in severe and chronic diseases. There is also mounting evidence that they are linked to postinfection disorders. Despite growing evidence of the widespread morbidity associated with Giardia infections, current treatment options are limited to compound classes with broad antimicrobial activity. Frontline anti-Giardia drugs are also associated with increasing drug resistance and treatment failures. To improve the health and well-being of millions, new selective anti-Giardia drugs are needed alongside improved health education initiatives. Here we discuss current treatment options together with recent advances and gaps in drug discovery. We also propose criteria to guide the discovery of new anti-Giardia compounds.

Nek8445, a protein kinase required for microtubule regulation and cytokinesis in Giardia lamblia

Giardia has 198 Nek kinases whereas humans have only 11. Giardia has a complex microtubule cytoskeleton that includes eight flagella and several unique microtubule arrays that are utilized for parasite attachment and facilitation of rapid mitosis and cytokinesis. The need to regulate these structures may explain the parallel expansion of the number of Nek family kinases. Here we use live and fixed cell imaging to uncover the role of Nek8445 in regulating Giardia cell division. We demonstrate that Nek8445 localization is cell cycle regulated and this kinase has a role in regulating overall microtubule organization. Nek8445 depletion results in short flagella, aberrant ventral disk organization, loss of the funis, defective axoneme exit, and altered cell shape. The axoneme exit defect is specific to the caudal axonemes, which exit from the posterior of the cell, and this defect correlates with rounding of the cell posterior and loss of the funis. Our findings implicate a role for the funis in establishing Giardia’s cell shape and guiding axoneme docking. On a broader scale our results support the emerging view that Nek family kinases have a general role in regulating microtubule organization.

Drug resistance in Giardia: Mechanisms and alternative treatments for Giardiasis

The use of chemotherapeutic drugs is the main resource against clinical giardiasis due to the lack of approved vaccines. Resistance of G. duodenalis to the most used drugs to treat giardiasis, metronidazole and albendazole, is a clinical issue of growing concern and yet unknown impact, respectively. In the search of new drugs, the completion of the Giardia genome project and the use of biochemical, molecular and bioinformatics tools allowed the identification of ligands/inhibitors for about one tenth of ≈150 potential drug targets in this parasite. Further, the synthesis of second generation nitroimidazoles and benzimidazoles along with high-throughput technologies have allowed not only to define overall mechanisms of resistance to metronidazole but to screen libraries of repurposed drugs and new pharmacophores, thereby increasing the known arsenal of anti-giardial compounds to some hundreds, with most demonstrating activity against metronidazole or albendazole-resistant Giardia. In particular, cysteine-modifying agents which include omeprazole, disulfiram, allicin and auranofin outstand due to their pleiotropic activity based on the extensive repertoire of thiol-containing proteins and the microaerophilic metabolism of this parasite. Other promising agents derived from higher organisms including phytochemicals, lactoferrin and propolis as well as probiotic bacteria/fungi have also demonstrated significant potential for therapeutic and prophylactic purposes in giardiasis. In this context the present chapter offers a comprehensive review of the current knowledge, including commonly prescribed drugs, causes of therapeutic failures, drug resistance mechanisms, strategies for the discovery of new agents and alternative drug therapies.

Screening of the Pathogen Box for inhibitors with dual efficacy against Giardia lamblia and Cryptosporidium parvum

There is need for a more efficient cell-based assay amenable to high-throughput drug screening against Giardia lamblia. Here, we report the development of a screening method utilizing G. lamblia engineered to express red-shifted firefly luciferase. Parasite growth and replication were quantified using D-luciferin as a substrate in a bioluminescent read-out plateform. This assay was validated for reproducibility and reliability against the Medicines for Malaria Venture (MMV) Pathogen Box compounds. For G. lamblia, forty-three compounds showed ≥ 75% inhibition of parasite growth in the initial screen (16 μM), with fifteen showing ≥ 95% inhibition. The Pathogen Box was also screened against Nanoluciferase expressing (Nluc) C. parvum, yielding 85 compounds with ≥ 75% parasite growth inhibition at 10 μM, with six showing ≥ 95% inhibition. A representative set of seven compounds with activity against both parasites were further analyzed to determine the effective concentration that causes 50% growth inhibition (EC50) and cytotoxicity against mammalian HepG2 cells. Four of the seven compounds were previously known to be effective in treating either Giardia or Cryptosporidium. The remaining three shared no obvious chemical similarity with any previously characterized anti-parasite diarrheal drugs and offer new medicinal chemistry opportunities for therapeutic development. These results suggest that the bioluminescent assays are suitable for large-scale screening of chemical libraries against both C. parvum and G. lamblia.

Myosin-independent cytokinesis in Giardia utilizes flagella to coordinate force generation and direct membrane trafficking

Devoid of all known canonical actin-binding proteins, the prevalent parasite Giardia lamblia uses an alternative mechanism for cytokinesis. Unique aspects of this mechanism can potentially be leveraged for therapeutic development. Here, live-cell imaging methods were developed for Giardia to establish division kinetics and the core division machinery. Surprisingly, Giardia cytokinesis occurred with a median time that is ∼60 times faster than mammalian cells. In contrast to cells that use a contractile ring, actin was not concentrated in the furrow and was not directly required for furrow progression. Live-cell imaging and morpholino depletion of axonemal Paralyzed Flagella 16 indicated that flagella-based forces initiated daughter cell separation and provided a source for membrane tension. Inhibition of membrane partitioning blocked furrow progression, indicating a requirement for membrane trafficking to support furrow advancement. Rab11 was found to load onto the intracytoplasmic axonemes late in mitosis and to accumulate near the ends of nascent axonemes. These developing axonemes were positioned to coordinate trafficking into the furrow and mark the center of the cell in lieu of a midbody/phragmoplast. We show that flagella motility, Rab11, and actin coordination are necessary for proper abscission. Organisms representing three of the five eukaryotic supergroups lack myosin II of the actomyosin contractile ring. These results support an emerging view that flagella play a central role in cell division among protists that lack myosin II and additionally implicate the broad use of membrane tension as a mechanism to drive abscission.

Use of Translation Blocking Morpholinos for Gene Knockdown in Giardia lamblia

Giardia lamblia, a major parasite, is an emerging model organism due to its compact genomic arrangement and composition. The most popular reverse genetic technique, RNAi, is ineffective in Giardia. In contrast, protein depletion by translation blocking morpholinos is suitable for most gene targets and provides up to 80% depletion of the target protein. The method is fast, reliable, and specific. After antisense morpholino oligomer delivery into Giardia trophozoites by electroporation, the cells can be used for many subsequent analyses 8-48 h after treatment. In this chapter, suitable gene tags, plasmids, and techniques necessary for proper morpholino targeting are described.