Retention and loss of RNA interference pathways in trypanosomatid protozoans

Preassembled complexes of hAgo2 and ssRNA delivered by nanoparticles: a novel silencing gene expression approach overcoming the absence of the canonical pathway of siRNA processing in the apicomplexan parasite Babesia microti, blood parasite of veterinary and zoonotic importance

Due to the lack of efficacy of the currently used chemical drugs, poor tick control, and lack of effective vaccines against Babesia, novel control strategies are urgently needed. In this regard, searching for anti-Babesia gene therapy may facilitate the control of this infection. Following this pattern, small interfering RNAs (siRNAs) are widely used to study gene function and hence open the way to control the parasite. However, the primary constraint of this approach is the lack of Babesia to RNA-induced silencing complex (RISC) enzymes, making siRNA impractical. In this study, we preassembled complexes with the human enzyme argonaute 2 (hAgo2) and a small interfering RNA (siRNA)/single-stranded RNA (ssRNA) against B. gibsoni and B. microti metabolite transporters. The assembled complexes were generated by developing a gene delivery system with chitosan dehydroascorbic acid nanoparticles. The delivery system effectively protected the loaded RNAi and targeted Babesia-infected RBCs with a relatively high internalization rate. The assembled complexes were successfully transfected into live parasites for specific slicing of Babesia targets. We demonstrated a reduction in the expression of target genes at the mRNA level. Furthermore, this silencing inhibited Babesia growth in vitro and in vivo. For the first time, we used this method to confirm the role of the assembled complexes in manipulating the noncanonical pathway of RNAi in Babesia parasites. This novel method provides a means of silencing Babesia genes to study their role in host-parasite interactions and as potential targets for gene therapy and control.

RNA interference in protozoan parasites and its application

RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression, via small RNA triggers derived from double-stranded RNA that can target specific genes; it is a natural process that plays a role in both the regulation of protein synthesis and in immunity. Discovery of RNAi by Fire and Mello in 1998 had a profound impact on unraveling novel aspects of eukaryotic biology. RNA interference (RNAi) has proven to be an immensely useful tool for studying gene function and validation of potential drug targets in almost all organisms. A great advance in parasitic protozoa was achieved by the experimental demonstration of RNAi in Trypanosoma brucei, and in other protists such as Leishmania braziliensis, Entamoeba histolytica and Giardia lamblia/intestinalis. These organisms exhibit numerous differences beyond the core 'dicer' and 'slicer' activities, thereby expanding knowledge of the evolutionary diversification of this pathway in eukaryotes. When present, RNAi has led to new technologies for engineering powerful and facile knockdowns in gene expression, revolutionizing biomedical research and opening clinical potentialities. In this review, we discuss the distribution of RNAi pathways, their biological roles, and experimental applications in protozoan parasites.

Improved base editing and functional screening in Leishmania via co-expression of the AsCas12a ultra variant, a T7 RNA polymerase, and a cytosine base editor

The ability to analyze the function of all genes in a genome is highly desirable, yet challenging in Leishmania due to a repetitive genome, limited DNA repair mechanisms, and lack of RNA interference in most species. While our introduction of a cytosine base editor (CBE) demonstrated potential to overcome these limitations (Engstler and Beneke, 2023), challenges remained, including low transfection efficiency, variable editing rates across species, parasite growth effects, and competition between deleterious and non-deleterious mutations. Here, we present an optimized approach addressing these issues. We identified a T7 RNAP promoter variant ensuring high editing rates across Leishmania species without compromising growth. A revised CBE single-guide RNAs (sgRNAs) scoring system was developed to prioritize STOP codon generation. Additionally, a triple-expression construct was created for stable integration of CBE sgRNA expression cassettes into a Leishmania safe harbor locus using AsCas12a ultra-mediated DNA double-strand breaks, increasing transfection efficiency by ~400-fold to 1 transfectant per 70 transfected cells. Using this improved system for a small-scale proof-of-principle pooled screen, we successfully confirmed the essential and fitness-associated functions of CK1.2, CRK2, CRK3, AUK1/AIRK, TOR1, IFT88, IFT139, IFT140, and RAB5A in Leishmania mexicana, demonstrating a significant improvement over our previous method. Lastly, we show the utility of co-expressing AsCas12a ultra, T7 RNAP, and CBE for hybrid CRISPR gene replacement and base editing within the same cell line. Overall, these improvements will broaden the range of possible gene editing applications in Leishmania species and will enable a variety of loss-of-function screens in the near future.

An evolutionary perspective to innate antiviral immunity in animals

Viruses pose a significant threat to cellular organisms. Innate antiviral immunity encompasses both RNA- and protein-based mechanisms designed to sense and respond to infections, a fundamental aspect present in all living organisms. A potent RNA-based antiviral mechanism is RNA interference, where small RNA-programmed nucleases target viral RNAs. Protein-based mechanisms often rely on the induction of transcriptional responses triggered by the recognition of viral infections through innate immune receptors. These responses involve the upregulation of antiviral genes aimed at countering viral infections. In this review, we delve into recent advances in understanding the diversification of innate antiviral immunity in animals. An evolutionary perspective on the gains and losses of mechanisms in diverse animals coupled to mechanistic studies in model organisms such as the fruit fly Drosophila melanogaster is essential to provide deep understanding of antiviral immunity that can be translated to new strategies in the treatment of viral diseases.

Recent development in CRISPR-Cas systems for human protozoan diseases

Protozoan parasitic diseases pose a substantial global health burden. Understanding the pathogenesis of these diseases is crucial for developing intervention strategies in the form of vaccine and drugs. Manipulating the parasite's genome is essential for gaining insights into its fundamental biology. Traditional genomic manipulation methods rely on stochastic homologous recombination events, which necessitates months of maintaining the cultured parasites under drug pressure to generate desired transgenics. The introduction of mega-nucleases (MNs), zinc-finger nucleases (ZFNs), and transcription activator-like effector nucleases (TALENs) greatly reduced the time required for obtaining a desired modification. However, there is a complexity associated with the design of these nucleases. CRISPR (Clustered regularly interspaced short palindromic repeats)/Cas (CRISPR associated proteins) is the latest gene editing tool that provides an efficient and convenient method for precise genomic manipulations in protozoan parasites. In this chapter, we have elaborated various strategies that have been adopted for the use of CRISPR-Cas9 system in Plasmodium, Leishmania and Trypanosoma. We have also discussed various applications of CRISPR-Cas9 pertaining to understanding of the parasite biology, development of drug resistance mechanism, gene drive and diagnosis of the infection.

CRISPR-Cas9 high-throughput screening to study drug resistance in Leishmania infantum

Parasites of the genus Leishmania pose a global health threat with limited treatment options. New drugs are urgently needed, and genomic screens have the potential to accelerate target discovery, mode of action, and resistance mechanisms against these new drugs. We describe here our effort in developing a genome-wide CRISPR-Cas9 screen in Leishmania, an organism lacking a functional nonhomologous end joining system that must rely on microhomology-mediated end joining, single-strand annealing, or homologous recombination for repairing Cas9-induced double-stranded DNA breaks. A new vector for cloning and expressing single guide RNAs (sgRNAs) was designed and proven to be effective in a small pilot project while enriching specific sgRNAs during drug selection. We then developed a whole-genome library of 49,754 sgRNAs, targeting all the genes of Leishmania infantum. This library was transfected in L. infantum expressing Cas9, and these cells were selected for resistance to two antileishmanials, miltefosine and amphotericin B. The sgRNAs the most enriched in the miltefosine screen targeted the miltefosine transporter gene, but sgRNAs targeting genes coding for a RING-variant protein and a transmembrane protein were also enriched. The sgRNAs the most enriched by amphotericin B targeted the sterol 24 C methyltransferase genes and a hypothetical gene. Through gene disruption experiments, we proved that loss of function of these genes was associated with resistance. This study describes the feasibility of carrying out whole-genome CRISPR-Cas9 screens in Leishmania provided that a strong selective pressure is applied. Such a screen can be used for accelerating the development of urgently needed antileishmanial drugs.IMPORTANCELeishmaniasis, a global health threat, lacks adequate treatment options and drug resistance exacerbates the challenge. This study introduces a CRISPR-Cas9 screening approach in Leishmania infantum, unraveling mechanisms of drug resistance at a genome-wide scale. Our screen was applied against two main antileishmanial drugs, and guides were enriched upon drug selection. These guides targeted known and new targets, hence validating the use of this screen against Leishmania. This strategy provides a powerful tool to expedite drug discovery as well as potential therapeutic targets against this neglected tropical disease.

Molecular tools to regulate gene expression in Trypanosoma cruzi

Developing molecular strategies to manipulate gene expression in trypanosomatids is challenging, particularly with respect to the unique gene expression mechanisms adopted by these unicellular parasites, such as polycistronic mRNA transcription and multi-gene families. In the case of Trypanosoma cruzi (T. cruzi), the causative agent of Chagas Disease, the lack of RNA interference machinery further complicated functional genetic studies important for understanding parasitic biology and developing biomarkers and potential therapeutic targets. Therefore, alternative methods of performing knockout and/or endogenous labelling experiments were developed to identify and understand the function of proteins for survival and interaction with the host. In this review, we present the main tools for the genetic manipulation of T. cruzi, focusing on the Clustered Regularly Interspaced Short Palindromic Repeats Cas9-associated system technique widely used in this organism. Moreover, we highlight the importance of using these tools to elucidate the function of uncharacterized and glycosylated proteins. Further developments of these technologies will allow the identification of new biomarkers, therapeutic targets and potential vaccines against Chagas disease with greater efficiency and speed.

Resolving the polycistronic aftermath: Essential role of topoisomerase IA in preventing R-loops in Leishmania

Kinetoplastid parasites are "living bridges" in the evolution from prokaryotes to higher eukaryotes. The near-intronless genome of the kinetoplastid Leishmania exhibits polycistronic transcription which can facilitate R-loop formation. Therefore, to prevent such DNA-RNA hybrids, Leishmania has retained prokaryotic-like DNA Topoisomerase IA (LdTOPIA) in the course of evolution. LdTOPIA is an essential enzyme that is expressed ubiquitously and is adapted for the compartmentalized eukaryotic form in harboring functional bipartite nuclear localization signals. Although exhibiting greater homology to mycobacterial TOPIA, LdTOPIA could functionally complement the growth lethality of Escherichia coli TOPIA null GyrB ts strain at non-permissive temperatures. Purified LdTOPIA exhibits Mg2+-dependent relaxation of only negatively supercoiled DNA and preference towards single-stranded DNA substrates. LdTOPIA prevents nuclear R-loops as conditional LdTOPIA downregulated parasites exhibit R-loop formation and thereby parasite killing. The clinically used tricyclic antidepressant, norclomipramine could specifically inhibit LdTOPIA and lead to R-loop formation and parasite elimination. This comprehensive study therefore paves an avenue for drug repurposing against Leishmania.

In silico prediction of the metabolism of Blastocrithidia nonstop, a trypanosomatid with non-canonical genetic code

BACKGROUND

Almost all extant organisms use the same, so-called canonical, genetic code with departures from it being very rare. Even more exceptional are the instances when a eukaryote with non-canonical code can be easily cultivated and has its whole genome and transcriptome sequenced. This is the case of Blastocrithidia nonstop, a trypanosomatid flagellate that reassigned all three stop codons to encode amino acids.

RESULTS

We in silico predicted the metabolism of B. nonstop and compared it with that of the well-studied human parasites Trypanosoma brucei and Leishmania major. The mapped mitochondrial, glycosomal and cytosolic metabolism contains all typical features of these diverse and important parasites. We also provided experimental validation for some of the predicted observations, concerning, specifically presence of glycosomes, cellular respiration, and assembly of the respiratory complexes.

CONCLUSIONS

In an unusual comparison of metabolism between a parasitic protist with a massively altered genetic code and its close relatives that rely on a canonical code we showed that the dramatic differences on the level of nucleic acids do not seem to be reflected in the metabolisms. Moreover, although the genome of B. nonstop is extremely AT-rich, we could not find any alterations of its pyrimidine synthesis pathway when compared to other trypanosomatids. Hence, we conclude that the dramatic alteration of the genetic code of B. nonstop has no significant repercussions on the metabolism of this flagellate.

Simplified and effective RNA interference and CRISPR-Cas9 systems for Cryptococcus neoformans

The 3,4-dihydroxyphenylalanine (DOPA) melanin is one of the important virulence factors for Cryptococcus neoformans, which may trigger immune responses in the host. While the production of DOPA melanin is catalyzed by laccase that is predominantly encoded by LAC1 gene. Therefore, regulating the genetic expression of C. neoformans is conducive to exploring the impact of interested molecules on the host. In this work, we established two systems that were constructed quickly and easily for the knock-down/knock-out of LAC1 gene: RNA interference (RNAi) and clustered regularly interspaced short palindromic repeats CRISPR-Cas9. The RNAi system was constructed by pSilencer 4.1-CMV neo plasmid and short hairpin RNA to achieve effective transcriptional suppression. The CRISPR-Cas9 system was used the PNK003 vectors to obtain a stable albino mutant strain. The results of phenotype, quantitative real-time polymerase chain reaction, transmission electron microscope, and spectrophotometry were used to assess the ability of melanin production. As a result, the RNAi system displayed attenuation of transcriptional suppression when the transformants continuously passed on new plates. However, the transcriptional suppression of long loop in short hairpin RNA was more powerful and lasted longer. An albino strain produced by CRISPR-Cas9 was completely unable to synthesize melanin. In conclusion, strains with different capacities of melanin production were obtained by RNAi and CRISPR-Cas9 systems, which might be useful for exploring the linear relation between melanin and immunoreaction of the host. In addition, the two systems in this article might be convenient to quickly screen the possible trait-regulating genes of other serotypes of C. neoformans.

Comparative microRNA profiling of Trypanosoma cruzi infected human cells

Introduction

Trypanosoma cruzi, the causative agent of Chagas disease, can infect almost any nucleated cell in the mammalian host. Although previous studies have described the transcriptomic changes that occur in host cells during parasite infection, the understanding of the role of post-transcriptional regulation in this process is limited. MicroRNAs, a class of short non-coding RNAs, are key players in regulating gene expression at the post-transcriptional level, and their involvement in the host-T. cruzi interplay is a growing area of research. However, to our knowledge, there are no comparative studies on the microRNA changes that occur in different cell types in response to T. cruzi infection.

Methods and results

Here we investigated microRNA changes in epithelial cells, cardiomyocytes and macrophages infected with T. cruzi for 24 hours, using small RNA sequencing followed by careful bioinformatics analysis. We show that, although microRNAs are highly cell type-specific, a signature of three microRNAs -miR-146a, miR-708 and miR-1246, emerges as consistently responsive to T. cruzi infection across representative human cell types. T. cruzi lacks canonical microRNA-induced silencing mechanisms and we confirm that it does not produce any small RNA that mimics known host microRNAs. We found that macrophages show a broad response to parasite infection, while microRNA changes in epithelial and cardiomyocytes are modest. Complementary data indicated that cardiomyocyte response may be greater at early time points of infection.

Conclusions

Our findings emphasize the significance of considering microRNA changes at the cellular level and complement previous studies conducted at higher organizational levels, such as heart samples. While miR-146a has been previously implicated in T. cruzi infection, similarly to its involvement in many other immunological responses, miR-1246 and miR-708 are demonstrated here for the first time. Given their expression in multiple cell types, we anticipate our work as a starting point for future investigations into their role in the post-transcriptional regulation of T. cruzi infected cells and their potential as biomarkers for Chagas disease.

State-of-the-Art in the Drug Discovery Pathway for Chagas Disease: A Framework for Drug Development and Target Validation

Chagas disease is the most important protozoan infection in the Americas, and constitutes a significant public health concern throughout the world. Development of new medications against its etiologic agent, Trypanosoma cruzi, has been traditionally slow and difficult, lagging in comparison with diseases caused by other kinetoplastid parasites. Among the factors that explain this are the incompletely understood mechanisms of pathogenesis of T. cruzi infection and its complex set of interactions with the host in the chronic stage of the disease. These demand the performance of a variety of in vitro and in vivo assays as part of any drug development effort. In this review, we discuss recent breakthroughs in the understanding of the parasite's life cycle and their implications in the search for new chemotherapeutics. For this, we present a framework to guide drug discovery efforts against Chagas disease, considering state-of-the-art preclinical models and recently developed tools for the identification and validation of molecular targets.

The paradigm of intracellular parasite survival and drug resistance in leishmanial parasite through genome plasticity and epigenetics: Perception and future perspective

Leishmania is an intracellular, zoonotic, kinetoplastid eukaryote with more than 1.2 million cases all over the world. The leishmanial chromosomes are divided into polymorphic chromosomal ends, conserved central domains, and antigen-encoding genes found in telomere-proximal regions. The genome flexibility of chromosomal ends of the leishmanial parasite is known to cause drug resistance and intracellular survival through the evasion of host defense mechanisms. Therefore, in this review, we discuss the plasticity of Leishmania genome organization which is the primary cause of drug resistance and parasite survival. Moreover, we have not only elucidated the causes of such genome plasticity which includes aneuploidy, epigenetic factors, copy number variation (CNV), and post-translation modification (PTM) but also highlighted their impact on drug resistance and parasite survival.

piRNA pathway evolution beyond gonad context: Perspectives from apicomplexa and trypanosomatids

piRNAs function as genome defense mechanisms against transposable elements insertions within germ line cells. Recent studies have unraveled that piRNA pathways are not limited to germ cells as initially reckoned, but are instead also found in non-gonadal somatic contexts. Moreover, these pathways have also been reported in bacteria, mollusks and arthropods, associated with safeguard of genomes against transposable elements, regulation of gene expression and with direct consequences in axon regeneration and memory formation. In this Perspective we draw attention to early branching parasitic protozoa, whose genome preservation is an essential function as in late eukaryotes. However, little is known about the defense mechanisms of these genomes. We and others have described the presence of putative PIWI-related machinery members in protozoan parasites. We have described the presence of a PIWI-like protein in Trypanosoma cruzi, bound to small non-coding RNAs (sRNAs) as cargo of secreted extracellular vesicles relevant in intercellular communication and host infection. Herein, we put forward the presence of members related to Argonaute pathways in both Trypanosoma cruzi and Toxoplasma gondii. The presence of PIWI-like machinery in Trypansomatids and Apicomplexa, respectively, could be evidence of an ancestral piRNA machinery that evolved to become more sophisticated and complex in multicellular eukaryotes. We propose a model in which ancient PIWI proteins were expressed broadly and had functions independent of germline maintenance. A better understanding of current and ancestral PIWI/piRNAs will be relevant to better understand key mechanisms of genome integrity conservation during cell cycle progression and modulation of host defense mechanisms by protozoan parasites.

Review on the Drug Intolerance and Vaccine Development for the Leishmaniasis

Leishmaniasis is one of the Neglected Tropical Diseases (NTDs), a zoonotic disease of vector-borne nature that is caused by a protozoan parasite Leishmania. This parasite is transmitted by the vector sandfly into the human via a bite. Visceral leishmaniasis (VL), also called kala-azar, is the most fatal among the types of leishmaniasis, with high mortality mostly spread in the East Africa and South Asia regions. WHO report stated that approximately 3.3 million disabilities occur every year due to the disease along with approximately 50,000 annual deaths. The real matter of concern is that there is no particular effective medicine/vaccine available against leishmaniasis to date except a few approved drugs and chemotherapy for the infected patient. The current selection of small compounds was constrained, and their growing drug resistance had been a major worry. Additionally, the serious side effects on humans of the available therapy or drugs have made it essential to discover efficient and low-cost methods to speed up the development of new drugs against leishmaniasis. Ideally, the vaccine could be a low risk and effective alternative for both CL and VL and elicit long-lasting immunity against the disease. There are a number of vaccine candidates at various stages of clinical development and preclinical stage. However, none has successfully passed all clinical trials. But, the successful development and approval of commercially available vaccines for dogs against canine leishmaniasis (CanL) provides evidence that it can be possible for humans in distant future. In the present article, the approaches used for the development of vaccines for leishmaniasis are discussed and the progress being made is briefly reviewed.

An RNA Interference (RNAi) Toolkit and Its Utility for Functional Genetic Analysis of Leishmania (Viannia)

RNA interference (RNAi) is a powerful tool whose efficacy against a broad range of targets enables functional genetic tests individually or systematically. However, the RNAi pathway has been lost in evolution by a variety of eukaryotes including most Leishmania sp. RNAi was retained in species of the Leishmania subgenus Viannia, and here we describe the development, optimization, and application of RNAi tools to the study of L. (Viannia) braziliensis (Lbr). We developed vectors facilitating generation of long-hairpin or "stem-loop" (StL) RNAi knockdown constructs, using GatewayTM site-specific recombinase technology. A survey of applications of RNAi in L. braziliensis included genes interspersed within multigene tandem arrays such as quinonoid dihydropteridine reductase (QDPR), a potential target or modulator of antifolate sensitivity. Other tests include genes involved in cell differentiation and amastigote proliferation (A600), and essential genes of the intraflagellar transport (IFT) pathway. We tested a range of stem lengths targeting the L. braziliensis hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and reporter firefly luciferase (LUC) genes and found that the efficacy of RNAi increased with stem length, and fell off greatly below about 128 nt. We used the StL length dependency to establish a useful 'hypomorphic' approach not possible with other gene ablation strategies, with shorter IFT140 stems yielding viable cells with compromised flagellar morphology. We showed that co-selection for RNAi against adenine phosphoryl transferase (APRT1) using 4-aminopyrazolpyrimidine (APP) could increase the efficacy of RNAi against reporter constructs, a finding that may facilitate improvements in future work. Thus, for many genes, RNAi provides a useful tool for studying Leishmania gene function with some unique advantages.

CRISPR-Cas9: Taming protozoan parasites with bacterial scissor

The invention of CRISPR-Cas9 technology has opened a new era in which genome manipulation has become precise, faster, cheap and more accurate than previous genome editing strategies. Despite the intricacies of the genomes associated with several protozoan parasites, CRISPR-Cas9 has made a substantial contribution to parasitology. The study of functional genomics through CRISPR-Cas9 mediated gene knockout, insertion, deletion and mutation has helped in understanding intrinsic parasite biology. The invention of CRISPR-dCas9 has helped in the programmable control of protozoan gene expression and epigenetic engineering. CRISPR and CRISPR-based alternatives will continue to thrive and may aid in the development of novel anti-protozoan strategies to tame the protozoan parasites in the imminent future.

Genome deletions to overcome the directed loss of gene function in Leishmania

With the global reach of the Neglected Tropical Disease leishmaniasis increasing, coupled with a tiny armory of therapeutics which all have problems with resistance, cost, toxicity and/or administration, the validation of new drug targets in the causative insect vector borne protozoa Leishmania spp is more important than ever. Before the introduction of CRISPR Cas9 technology in 2015 genetic validation of new targets was carried out largely by targeted gene knockout through homologous recombination, with the majority of genes targeted (~70%) deemed non-essential. In this study we exploit the ready availability of whole genome sequencing technology to reanalyze one of these historic cell lines, a L. major knockout in the catalytic subunit of serine palmitoyltransferase (LCB2), which causes a complete loss of sphingolipid biosynthesis but remains viable and infective. This revealed a number of Single Nucleotide Polymorphisms, but also the complete loss of several coding regions including a gene encoding a putative ABC3A orthologue, a putative sterol transporter. Hypothesizing that the loss of such a transporter may have facilitated the directed knockout of the catalytic subunit of LCB2 and the complete loss of de novo sphingolipid biosynthesis, we re-examined LCB2 in a L. mexicana line engineered for straightforward CRISPR Cas9 directed manipulation. Strikingly, LCB2 could not be knocked out indicating essentiality. However, simultaneous deletion of LCB2 and the putative ABC3A was possible. This indicated that the loss of the putative ABC3A facilitated the loss of sphingolipid biosynthesis in Leishmania, and suggested that we should re-examine the many other Leishmania knockout lines where genes were deemed non-essential.

Differential microRNAs expression during ex vivo infection of canine and ovine placental explants with Trypanosoma cruzi and Toxoplasma gondii

Trypanosoma cruzi and Toxoplasma gondii are two zoonotic parasites that constitute significant human and animal health threats, causing a significant economic burden worldwide. Both parasites can be transmitted congenitally, but transmission rates for T. gondii are high, contrary to what has been observed for T. cruzi. The probability of congenital transmission depends on complex interactions between the pathogen and the host, including the modulation of host cell gene expression by miRNAs. During ex vivo infection of canine and ovine placental explants, we evaluated the expression of 3 miRNAs (miR-30e-3p, miR-3074-5p, and miR-127-3p) previously associated with parasitic and placental diseases and modulated by both parasites. In addition, we identified the possible target genes of the miRNAs by using computational prediction tools and performed GO and KEGG enrichment analyses to identify the biological functions and associated pathologies. The three miRNAs are differentially expressed in the canine and ovine placenta in response to T. cruzi and T. gondii. We conclude that the observed differential expression and associated functions might explain, at least partially, the differences in transmission rates and susceptibility to parasite infection in different species.

Elimination of LRVs Elicits Different Responses in Leishmania spp

Leishmaniaviruses (LRVs) have been demonstrated to enhance progression of leishmaniasis, a vector-transmitted disease with a wide range of clinical manifestations that is caused by flagellates of the genus Leishmania. Here, we used two previously proposed strategies of the LRV ablation to shed light on the relationships of two Leishmania spp. with their respective viral species (L. guyanensis, LRV1 and L. major, LRV2) and demonstrated considerable difference between two studied systems. LRV1 could be easily eliminated by the expression of exogenous capsids regardless of their origin (the same or distantly related LRV1 strains, or even LRV2), while LRV2 was only partially depleted in the case of the native capsid overexpression. The striking differences were also observed in the effects of complete viral elimination with 2'C-methyladenosine (2-CMA) on the transcriptional profiles of these two Leishmania spp. While virtually no differentially expressed genes were detected after the LRV1 removal from L. guyanensis, the response of L. major after ablation of LRV2 involved 87 genes, the analysis of which suggested a considerable stress experienced even after several passages following the treatment. This effect on L. major was also reflected in a significant decrease of the proliferation rate, not documented in L. guyanensis and naturally virus-free strain of L. major. Our findings suggest that integration of L. major with LRV2 is deeper compared with that of L. guyanensis with LRV1. We presume this determines different effects of the viral presence on the Leishmania spp. infections.

IMPORTANCELeishmania spp. represent human pathogens that cause leishmaniasis, a widespread parasitic disease with mild to fatal clinical manifestations. Some strains of leishmaniae bear leishmaniaviruses (LRVs), and this has been shown to aggravate disease course. We investigated the relationships of two distally related Leishmania spp. with their respective LRVs using different strategies of virus removal. Our results suggest the South American L. guyanensis easily loses its virus with no important consequences for the parasite in the laboratory culture. Conversely, the Old-World L. major is refractory to virus removal and experiences a prominent stress if this removal is nonetheless completed. The drastically different levels of integration between the studied Leishmania spp. and their viruses suggest distinct effects of the viral presence on infections in these species of parasites.

Current Status of Regulatory Non-Coding RNAs Research in the Tritryp

Trypanosomatids are protozoan parasites that cause devastating vector-borne human diseases. Gene expression regulation of these organisms depends on post-transcriptional control in responding to diverse environments while going through multiple developmental stages of their complex life cycles. In this scenario, non-coding RNAs (ncRNAs) are excellent candidates for a very efficient, quick, and economic strategy to regulate gene expression. The advent of high throughput RNA sequencing technologies show the presence and deregulation of small RNA fragments derived from canonical ncRNAs. This review seeks to depict the ncRNA landscape in trypanosomatids, focusing on the small RNA fragments derived from functional RNA molecules observed in RNA sequencing studies. Small RNA fragments derived from canonical ncRNAs (tsRNAs, snsRNAs, sdRNAs, and sdrRNAs) were identified in trypanosomatids. Some of these RNAs display changes in their levels associated with different environments and developmental stages, demanding further studies to determine their functional characterization and potential roles. Nevertheless, a comprehensive and detailed ncRNA annotation for most trypanosomatid genomes is still needed, allowing better and more extensive comparative and functional studies.

Leishmania guyanensis M4147 as a new LRV1-bearing model parasite: Phosphatidate phosphatase 2-like protein controls cell cycle progression and intracellular lipid content

Leishmaniasis is a parasitic vector-borne disease caused by the protistan flagellates of the genus Leishmania. Leishmania (Viannia) guyanensis is one of the most common causative agents of the American tegumentary leishmaniasis. It has previously been shown that L. guyanensis strains that carry the endosymbiotic Leishmania RNA virus 1 (LRV1) cause more severe form of the disease in a mouse model than those that do not. The presence of the virus was implicated into the parasite's replication and spreading. In this respect, studying the molecular mechanisms of cellular control of viral infection is of great medical importance. Here, we report ~30.5 Mb high-quality genome assembly of the LRV1-positive L. guyanensis M4147. This strain was turned into a model by establishing the CRISPR-Cas9 system and ablating the gene encoding phosphatidate phosphatase 2-like (PAP2L) protein. The orthologue of this gene is conspicuously absent from the genome of an unusual member of the family Trypanosomatidae, Vickermania ingenoplastis, a species with mostly bi-flagellated cells. Our analysis of the PAP2L-null L. guyanensis showed an increase in the number of cells strikingly resembling the bi-flagellated V. ingenoplastis, likely as a result of the disruption of the cell cycle, significant accumulation of phosphatidic acid, and increased virulence compared to the wild type cells.

MicroRNAs: master regulators in host-parasitic protist interactions

MicroRNAs (miRNAs) are a group of small non-coding RNAs present in a wide diversity of organisms. MiRNAs regulate gene expression at a post-transcriptional level through their interaction with the 3' untranslated regions of target mRNAs, inducing translational inhibition or mRNA destabilization and degradation. Thus, miRNAs regulate key biological processes, such as cell death, signal transduction, development, cellular proliferation and differentiation. The dysregulation of miRNAs biogenesis and function is related to the pathogenesis of diseases, including parasite infection. Moreover, during host-parasite interactions, parasites and host miRNAs determine the probability of infection and progression of the disease. The present review is focused on the possible role of miRNAs in the pathogenesis of diseases of clinical interest caused by parasitic protists. In addition, the potential role of miRNAs as targets for the design of drugs and diagnostic and prognostic markers of parasitic diseases is also discussed.

Targeted Deletion of Centrin in Leishmania braziliensis Using CRISPR-Cas9-Based Editing

Leishmania braziliensis is the main causative agent of Tegumentary Leishmaniasis in the Americas. However, difficulties related to genome manipulation, experimental infection, and parasite growth have so far limited studies with this species. CRISPR-Cas9-based technology has made genome editing more accessible, and here we have successfully employed the LeishGEdit approach to attenuate L. braziliensis. We generated a transgenic cell line expressing Cas9 and T7 RNA polymerase, which was employed for the targeted deletion of centrin, a calcium-binding cytoskeletal protein involved in the centrosome duplication in eukaryotes. Centrin-deficient Leishmania exhibit growth arrest at the amastigote stage. Whole-genome sequencing of centrin-deficient L. braziliensis (LbCen^−/−) did not indicate the presence of off-target mutations. In vitro, the growth rates of LbCen^−/− and wild-type promastigotes were similar, but axenic and intracellular LbCen^−/− amastigotes showed a multinucleated phenotype with impaired survival following macrophage infection. Upon inoculation into BALB/c mice, LbCen^−/− were detected at an early time point but failed to induce lesion formation, contrary to control animals, infected with wild-type L. braziliensis. A significantly lower parasite burden was also observed in mice inoculated with LbCen^−/−, differently from control mice. Given that centrin-deficient Leishmania sp. have become candidates for vaccine development, we propose that LbCen^−/− can be further explored for the purposes of immunoprophylaxis against American Tegumentary Leishmaniasis.

Impact of Genetic Diversity and Genome Plasticity of Leishmania spp. in Treatment and the Search for Novel Chemotherapeutic Targets

Leishmaniasis is one of the major public health concerns in Latin America, Africa, Asia, and Europe. The absence of vaccines for human use and the lack of effective vector control programs make chemotherapy the main strategy to control all forms of the disease. However, the high toxicity of available drugs, limited choice of therapeutic agents, and occurrence of drug-resistant parasite strains are the main challenges related to chemotherapy. Currently, only a small number of drugs are available for leishmaniasis treatment, including pentavalent antimonials (Sb^V), amphotericin B and its formulations, miltefosine, paromomycin sulphate, and pentamidine isethionate. In addition to drug toxicity, therapeutic failure of leishmaniasis is a serious concern. The occurrence of drug-resistant parasites is one of the causes of therapeutic failure and is closely related to the diversity of parasites in this genus. Owing to the enormous plasticity of the genome, resistance can occur by altering different metabolic pathways, demonstrating that resistance mechanisms are multifactorial and extremely complex. Genetic variability and genome plasticity cause not only the available drugs to have limitations, but also make the search for new drugs challenging. Here, we examined the biological characteristics of parasites that hinder drug discovery.

Laboratory Selection of Trypanosomatid Pathogens for Drug Resistance

The selection of parasites for drug resistance in the laboratory is an approach frequently used to investigate the mode of drug action, estimate the risk of emergence of drug resistance, or develop molecular markers for drug resistance. Here, we focused on the How rather than the Why of laboratory selection, discussing different experimental set-ups based on research examples with Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. The trypanosomatids are particularly well-suited to illustrate different strategies of selecting for drug resistance, since it was with African trypanosomes that Paul Ehrlich performed such an experiment for the first time, more than a century ago. While breakthroughs in reverse genetics and genome editing have greatly facilitated the identification and validation of candidate resistance mutations in the trypanosomatids, the forward selection of drug-resistant mutants still relies on standard in vivo models and in vitro culture systems. Critical questions are: is selection for drug resistance performed in vivo or in vitro? With the mammalian or with the insect stages of the parasites? Under steady pressure or by sudden shock? Is a mutagen used? While there is no bona fide best approach, we think that a methodical consideration of these questions provides a helpful framework for selection of parasites for drug resistance in the laboratory.

Analyses of Leishmania-LRV Co-Phylogenetic Patterns and Evolutionary Variability of Viral Proteins

Leishmania spp. are important pathogens causing a vector-borne disease with a broad range of clinical manifestations from self-healing ulcers to the life-threatening visceral forms. Presence of Leishmania RNA virus (LRV) confers survival advantage to these parasites by suppressing anti-leishmanial immunity in the vertebrate host. The two viral species, LRV1 and LRV2 infect species of the subgenera Viannia and Leishmania, respectively. In this work we investigated co-phylogenetic patterns of leishmaniae and their viruses on a small scale (LRV2 in L. major) and demonstrated their predominant coevolution, occasionally broken by intraspecific host switches. Our analysis of the two viral genes, encoding the capsid and RNA-dependent RNA polymerase (RDRP), revealed them to be under the pressure of purifying selection, which was considerably stronger for the former gene across the whole tree. The selective pressure also differs between the LRV clades and correlates with the frequency of interspecific host switches. In addition, using experimental (capsid) and predicted (RDRP) models we demonstrated that the evolutionary variability across the structure is strikingly different in these two viral proteins.

Effective Genome Editing in Leishmania (Viannia) braziliensis Stably Expressing Cas9 and T7 RNA Polymerase

Until 2015, loss-of-function studies to elucidate protein function in Leishmania relied on gene disruption through homologous recombination. Then, the CRISPR/Cas9 revolution reached these protozoan parasites allowing efficient genome editing with one round of transfection. In addition, the development of LeishGEdit, a PCR-based toolkit for generating knockouts and tagged lines using CRISPR/Cas9, allowed a more straightforward and effective genome editing. In this system, the plasmid pTB007 is delivered to Leishmania for episomal expression or integration in the β-tubulin locus and for the stable expression of T7 RNA polymerase and Cas9. In South America, and especially in Brazil, Leishmania (Viannia) braziliensis is the most frequent etiological agent of tegumentary leishmaniasis. The L. braziliensis β-tubulin locus presents significant sequence divergence in comparison with Leishmania major, which precludes the efficient integration of pTB007 and the stable expression of Cas9. To overcome this limitation, the L. major β-tubulin sequences, present in the pTB007, were replaced by a Leishmania (Viannia) β-tubulin conserved sequence generating the pTB007_Viannia plasmid. This modification allowed the successful integration of the pTB007_Viannia cassette in the L. braziliensis M2903 genome, and in silico predictions suggest that this can also be achieved in other Viannia species. The activity of Cas9 was evaluated by knocking out the flagellar protein PF16, which caused a phenotype of immobility in these transfectants. Endogenous PF16 was also successfully tagged with mNeonGreen, and an in-locus complementation strategy was employed to return a C-terminally tagged copy of the PF16 gene to the original locus, which resulted in the recovery of swimming capacity. The modified plasmid pTB007_Viannia allowed the integration and stable expression of both T7 RNA polymerase and Cas9 in L. braziliensis and provided an important tool for the study of the biology of this parasite.

Actin-related protein 4: An unconventional negative regulator of mitochondrial calcium in protozoan parasite Leishmania

Regulation of mitochondrial calcium import is less understood in evolutionarily distinct protozoan parasites, such as Leishmania, as some of the mitochondrial calcium uniporter complex proteins are either missing or functionally diverged. Here, we show that Actin-related protein4 (ARP4), localizes exclusively into the Leishmania mitochondrion and depletion of this protein causes cells to accumulate calcium in the mitochondrion. The ARP4 depleted cells show increased activation of pyruvate dehydrogenase and production of ATP. Overall, our results indicate that ARP4 negatively regulates calcium uptake in the Leishmania mitochondrion.

Genome-scale RNAi screens in African trypanosomes

Genome-scale genetic screens allow researchers to rapidly identify the genes and proteins that impact a particular phenotype of interest. In African trypanosomes, RNA interference (RNAi) knockdown screens have revealed mechanisms underpinning drug resistance, drug transport, prodrug metabolism, quorum sensing, genome replication, and gene expression control. RNAi screening has also been remarkably effective at highlighting promising potential antitrypanosomal drug targets. The first ever RNAi library screen was implemented in African trypanosomes, and genome-scale RNAi screens and other related approaches continue to have a major impact on trypanosomatid research. Here, I review those impacts in terms of both discovery and translation.

Genome Assemblies across the Diverse Evolutionary Spectrum of Leishmania Protozoan Parasites

We report the high-quality draft assemblies and gene annotations for 13 species and/or strains of the protozoan parasite genera Leishmania, Endotrypanum, and Crithidia, which span the phylogenetic diversity of the subfamily Leishmaniinae within the kinetoplastid order of the phylum Euglenazoa. These resources will support studies on the origins of parasitism.

Genomics and functional genomics in Leishmania and Trypanosoma cruzi: statuses, challenges and perspectives

The availability of Trypanosomatid genomic data in public databases has opened myriad experimental possibilities that have contributed to a more comprehensive understanding of the biology of these parasites and their interactions with hosts. In this review, after brief remarks on the history of the Trypanosoma cruzi and Leishmania genome initiatives, we present an overview of the relevant contributions of genomics, transcriptomics and functional genomics, discussing the primary obstacles, challenges, relevant achievements and future perspectives of these technologies.

Genome Sequencing of Leishmania infantum Causing Cutaneous Leishmaniosis from a Turkish Isolate with Next-Generation Sequencing Technology

PURPOSE

Leishmania subgenus Leishmania causes leishmaniosis, which is a chronic systemic disease in humans and animals, in which the skin and visceral organs can be affected. The disease generally consists of three different clinical types in humans: visceral (kala-azar, VL), cutaneous (CL) and mucocutaneous leishmaniosis (MCL). According to the World Health Organization (WHO), leishmaniosis is still one of the world's most neglected diseases. It has been nearly 13-14 years since the completion of the first complete genome sequence of a Leishmania parasite. However, much information about these parasites remains to be elucidated, such as the causes of differences in tissue tropism. The aim of this study is to perform the whole-genome sequencing of Leishmania infantum causing cutaneous leishmaniosis from a Turkish isolate with next-generation sequencing technology.

METHODS

Genomic sequencing was performed on the Illumina HiSeq 2500 platform. The TruSeq Nano DNA Low Throughput Library Prep Kit, compatible with the Illumina HiSeq 2500 platform, was used to generate the library. Synthesis sequencing (SBS) was performed with a HiSeq Rapid SBS Kit v2 to generate single-fragment reads (2 × 150 bp; PE) with two fragment end-to-end assemblies. Bioinformatics analyses were performed on the Geneious 11.0.5. ( www.genius.com ) platform.

RESULTS

In our study, a high-quality whole-genome sequence (WGS) of L. infantum was successfully generated, and a total of 32,009,137 base pairs of genomic DNA from 36 chromosomes were obtained. The resulting genomic DNA sequence was submitted to the US National Center for Biotechnology Information (NCBI) GenBank ( www.ncbi.nlm.nih.gov ) database and registered under the name Leishmania infantum_TR01 (Lin_TR01). The following accession numbers were assigned by NCBI to the 36 chromosomes of the Lin_TR01 genome: CP027807, CP027810, CP027808, CP027811, CP027809, CP027812, CP027813, CP027814, CP027817, CP027818, CP027819, CP027815, CP027821, CP027816, CP027823, CP027820, CP027822, CP027824, CP027825, CP027826, CP027827, CP027828, CP027829, CP027830, CP027831, CP027832, CP027833, CP027834, CP027835, CP027836, CP027837, CP027838, CP027839, CP027840, CP027841, CP027842. As a result of the annotation of the Lin_TR01 genome, 3153 polymorphisms, 8324 genes, 8199 CDSs, 8109 mRNAs, 67 tRNAs, 11 rRNAs and 58 ncRNA were identified. Among the 8199 CDS obtained, 5278 encode hypothetical proteins.

CONCLUSION

In this study, a high-quality WGS of Leishmania infantum was successfully obtained for the first time in Turkey. According to a review of WGS studies on this subject, the Lin_TR01 strain is the first strain to be isolated from cutaneous leishmaniosis. The reference genome of L. infantum JPCM5 (Peacock et al., 2007) was obtained from a visceral leishmaniosis case, in accordance with the classical tissue and organ tropism of the species. Lin_TR01 is the second whole-genome-sequenced strain in the world after the JPCM5 strain. The Lin_TR01 genome is the only L. infantum whole-genome sequence that is completed assembly level from 36 chromosomes among the genomes obtained thus far ( https://www.ncbi.nlm.nih.gov/genome/genomes/249 ).

Shotgun Lipidomic Analysis of Leishmania Cells

To understand the interplay of lipids between Leishmania promastigotes, amastigotes, and vertebrate host cells, a robust method for cultivating Leishmania parasites, lipid extraction, and shotgun lipidomic analysis with loop injection is described. This book chapter provides the step-by-step workflow to guide readers from sample preparation to the global lipid analysis by multiple stage mass spectrometry with high resolution and tandem quadrupole mass spectrometric approaches toward studying the metabolomic roles that lipids may play in Leishmania parasite infections.

Testing the CRISPR-Cas9 and glmS ribozyme systems in Leishmania tarentolae

Leishmania parasites include important pathogens and model organisms and are even used for the production of recombinant proteins. However, functional genomics and the characterization of essential genes are often limited in Leishmania because of low-throughput technologies for gene disruption or tagging and the absence of components for RNA interference. Here, we tested the T7 RNA polymerase-dependent CRISPR-Cas9 system by Beneke et al. and the glmS ribozyme-based knock-down system in the model parasite Leishmania tarentolae. We successfully deleted two reference genes encoding the flagellar motility factor Pf16 and the salvage-pathway enzyme adenine phosphoribosyltransferase, resulting in immotile and drug-resistant parasites, respectively. In contrast, we were unable to disrupt the gene encoding the mitochondrial flavoprotein Erv. Cultivation of L. tarentolae in standard BHI medium resulted in a constitutive down-regulation of an episomal mCherry-glmS reporter by 40 to 60%. For inducible knock-downs, we evaluated the growth of L. tarentolae in alternative media and identified supplemented MEM, IMDM and McCoy's 5A medium as candidates. Cultivation in supplemented MEM allowed an inducible, glucosamine concentration-dependent down-regulation of the episomal mCherry-glmS reporter by more than 70%. However, chromosomal glmS-tagging of the genes encoding Pf16, adenine phosphoribosyltransferase or Erv did not reveal a knock-down phenotype. Our data demonstrate the suitability of the CRISPR-Cas9 system for the disruption and tagging of genes in L. tarentolae as well as the limitations of the glmS system, which was restricted to moderate efficiencies for episomal knock-downs and caused no detectable phenotype for chromosomal knock-downs.

Reductionist Pathways for Parasitism in Euglenozoans? Expanded Datasets Provide New Insights

The unicellular trypanosomatids belong to the phylum Euglenozoa and all known species are obligate parasites. Distinct lineages infect plants, invertebrates, and vertebrates, including humans. Genome data for marine diplonemids, together with freshwater euglenids and free-living kinetoplastids, the closest known nonparasitic relatives to trypanosomatids, recently became available. Robust phylogenetic reconstructions across Euglenozoa are now possible and place the results of parasite-focused studies into an evolutionary context. Here we discuss recent advances in identifying the factors shaping the evolution of Euglenozoa, focusing on ancestral features generally considered parasite-specific. Remarkably, most of these predate the transition(s) to parasitism, suggesting that the presence of certain preconditions makes a significant lifestyle change more likely.

Application of CRISPR/Cas9-Based Reverse Genetics in Leishmania braziliensis: Conserved Roles for HSP100 and HSP23

The protozoan parasite Leishmania (Viannia) braziliensis (L. braziliensis) is the main cause of human tegumentary leishmaniasis in the New World, a disease affecting the skin and/or mucosal tissues. Despite its importance, the study of the unique biology of L. braziliensis through reverse genetics analyses has so far lagged behind in comparison with Old World Leishmania spp. In this study, we successfully applied a cloning-free, PCR-based CRISPR–Cas9 technology in L. braziliensis that was previously developed for Old World Leishmania major and New World L. mexicana species. As proof of principle, we demonstrate the targeted replacement of a transgene (eGFP) and two L. braziliensis single-copy genes (HSP23 and HSP100). We obtained homozygous Cas9-free HSP23- and HSP100-null mutants in L. braziliensis that matched the phenotypes reported previously for the respective L. donovani null mutants. The function of HSP23 is indeed conserved throughout the Trypanosomatida as L. majorHSP23 null mutants could be complemented phenotypically with transgenes from a range of trypanosomatids. In summary, the feasibility of genetic manipulation of L. braziliensis by CRISPR–Cas9-mediated gene editing sets the stage for testing the role of specific genes in that parasite’s biology, including functional studies of virulence factors in relevant animal models to reveal novel therapeutic targets to combat American tegumentary leishmaniasis.

Experimental Strategies to Explore Drug Action and Resistance in Kinetoplastid Parasites

Kinetoplastids are the causative agents of leishmaniasis, human African trypanosomiasis, and American trypanosomiasis. They are responsible for high mortality and morbidity in (sub)tropical regions. Adequate treatment options are limited and have several drawbacks, such as toxicity, need for parenteral administration, and occurrence of treatment failure and drug resistance. Therefore, there is an urgency for the development of new drugs. Phenotypic screening already allowed the identification of promising new chemical entities with anti-kinetoplastid activity potential, but knowledge on their mode-of-action (MoA) is lacking due to the generally applied whole-cell based approach. However, identification of the drug target is essential to steer further drug discovery and development. Multiple complementary techniques have indeed been used for MoA elucidation. In this review, the different 'omics' approaches employed to define the MoA or mode-of-resistance of current reference drugs and some new anti-kinetoplastid compounds are discussed.

The current drug discovery landscape for trypanosomiasis and leishmaniasis: Challenges and strategies to identify drug targets

Human trypanosomiasis and leishmaniasis are vector-borne neglected tropical diseases caused by infection with the protozoan parasites Trypanosoma spp. and Leishmania spp., respectively. Once restricted to endemic areas, these diseases are now distributed worldwide due to human migration, climate change, and anthropogenic disturbance, causing significant health and economic burden globally. The current chemotherapy used to treat these diseases has limited efficacy, and drug resistance is spreading. Hence, new drugs are urgently needed. Phenotypic compound screenings have prevailed as the leading method to discover new drug candidates against these diseases. However, the publication of the complete genome sequences of multiple strains, advances in the application of CRISPR/Cas9 technology, and in vivo bioluminescence-based imaging have set the stage for advancing target-based drug discovery. This review analyses the limitations of the narrow pool of available drugs presently used for treating these diseases. It describes the current drug-based clinical trials highlighting the most promising leads. Furthermore, the review presents a focused discussion on the most important biological and pharmacological challenges that target-based drug discovery programs must overcome to advance drug candidates. Finally, it examines the advantages and limitations of modern research tools designed to identify and validate essential genes as drug targets, including genomic editing applications and in vivo imaging.

Genetic validation of Leishmania genes essential for amastigote survival in vivo using N-myristoyltransferase as a model

BACKGROUND

Proving that specific genes are essential for the intracellular viability of Leishmania parasites within macrophages remains a challenge for the identification of suitable targets for drug development. This is especially evident in the absence of a robust inducible expression system or functioning RNAi machinery that works in all Leishmania species. Currently, if a target gene of interest in extracellular parasites can only be deleted from its genomic locus in the presence of ectopic expression from a wild type copy, it is assumed that this gene will also be essential for viability in disease-promoting intracellular parasites. However, functional essentiality must be proven independently in both life-cycle stages for robust validation of the gene of interest as a putative target for chemical intervention.

METHODS

Here, we have used plasmid shuffle methods in vivo to provide supportive genetic evidence that N-myristoyltransferase (NMT) is essential for Leishmania viability throughout the parasite life-cycle. Following confirmation of NMT essentiality in vector-transmitted promastigotes, a range of mutant parasites were used to infect mice prior to negative selection pressure to test the hypothesis that NMT is also essential for parasite viability in an established infection.

RESULTS

Ectopically-expressed NMT was only dispensable under negative selection in the presence of another copy. Total parasite burdens in animals subjected to negative selection were comparable to control groups only if an additional NMT copy, not affected by the negative selection, was expressed.

CONCLUSIONS

NMT is an essential gene in all parasite life-cycle stages, confirming its role as a genetically-validated target for drug development.

Generation and Characterization of a Dual-Reporter Transgenic Leishmania braziliensis Line Expressing eGFP and Luciferase

In this study, we generated a transgenic strain of Leishmania braziliensis, an etiological agent associated with a diversity of clinical manifestations of leishmaniasis ranging from localized cutaneous to mucocutaneous to disseminated disease. Transgenic parasites expressing reporter proteins are valuable tools for studies of parasite biology, host-pathogen interactions, and anti-parasitic drug development. To this end, we constructed an L. braziliensis line stably expressing the reporters eGFP and luciferase (eGFP-LUC L. braziliensis). The integration cassette co-expressing the two reporters was targeted to the ribosomal locus (SSU) of the parasite genome. Transgenic parasites were characterized for their infectivity and stability both in vitro and in vivo. Parasite maintenance in axenic long-term culture in the absence of selective drugs did not alter expression of the two reporters or infection of BALB/c mice, indicating stability of the integrated cassette. Infectivity of eGFP-LUC, L. braziliensis, both in vivo and in vitro was similar to that obtained with the parental wild type strain. The possibility of L. braziliensis tracking and quantification using fluorescence and luminescence broadens the scope of research involving this neglected species, despite its importance in terms of public health concerning the leishmaniasis burden.

Application of CRISPR/Cas9-Mediated Genome Editing in Leishmania

CRISPR-Cas9 is an RNA guided endonuclease derived from the bacterium Streptococcus pyogenes. Due to its simplicity, versatility, and high efficiency, it has been widely used for genome editing in a variety of organisms including the protozoan parasite Leishmania, the causative agent of human leishmaniasis. Compared to the traditional homologous recombination gene targeting method, CRISPR-Cas9 has been shown to be a more efficient method to delete or disrupt Leishmania genes, generate point mutations, and add tags to endogenous genes. Notably, the stable CRISPR expression systems were shown to delete multicopy family Leishmania genes and genes present in multiploid chromosomes, identify essential Leishmania genes, and create specific chromosome translocations. In this chapter, we describe detailed procedures on using the stable CRISPR expression system for genome editing in Leishmania. These procedures include CRISPR targeting site selection, gRNA design, cloning single and double gRNA coding sequences into the Leishmania CRISPR vector pLdCN, oligonucleotide donor and drug resistance selection donor design, Leishmania cell transfection, screening, and isolation of CRISPR-edited mutants. As the principles of gene editing are generally similar, many of these procedures could also apply to the transient Leishmania CRISPR systems described by other labs.

A Novel Method to Silence Genes in Cryptosporidium

The parasites of the genus Cryptosporidium are important causes of diarrheal diseases, specifically cryptosporidiosis, worldwide. A major bottleneck for developing drugs and vaccines against cryptosporidiosis is the lack of methods to study gene function in this parasite. Silencing of genes by RNA interference (RNAi) is a powerful method to investigate gene function that has been widely used in the identification of targets for several pathogens. Unfortunately, as Cryptosporidium does not possess the enzymes of the RNAi pathway, its genes cannot be silenced by standard siRNA technology. To circumvent that problem, we have developed a novel strategy to knock down Cryptosporidium genes by reconstituting the effector arm of the siRNA pathway. We have induced silencing of several genes in Cryptosporidium by transfecting parasites with hybrid complexes formed between recombinant human Argonaute (hAgo2) and Cryptosporidium single-stranded RNA (ssRNA). This novel methodology provides an effective strategy to study the role of selected genes in host-parasite interactions, and also can be used to identify potential targets for chemotherapy. The standardized methodology based on this strategy is described in this chapter.

Protective or Detrimental? Understanding the Role of Host Immunity in Leishmaniasis

The intracellular protozoan parasites of the genus Leishmania are the causative agents of leishmaniasis, a vector-borne disease of major public health concern, estimated to affect 12 million people worldwide. The clinical manifestations of leishmaniasis are highly variable and can range from self-healing localized cutaneous lesions to life-threatening disseminated visceral disease. Once introduced into the skin by infected sandflies, Leishmania parasites interact with a variety of immune cells, such as neutrophils, monocytes, dendritic cells (DCs), and macrophages. The resolution of infection requires a finely tuned interplay between innate and adaptive immune cells, culminating with the activation of microbicidal functions and parasite clearance within host cells. However, several factors derived from the host, insect vector, and Leishmania spp., including the presence of a double-stranded RNA virus (LRV), can modulate the host immunity and influence the disease outcome. In this review, we discuss the immune mechanisms underlying the main forms of leishmaniasis, some of the factors involved with the establishment of infection and disease severity, and potential approaches for vaccine and drug development focused on host immunity.

Coupling chemical mutagenesis to next generation sequencing for the identification of drug resistance mutations in Leishmania

Current genome-wide screens allow system-wide study of drug resistance but detecting small nucleotide variants (SNVs) is challenging. Here, we use chemical mutagenesis, drug selection and next generation sequencing to characterize miltefosine and paromomycin resistant clones of the parasite Leishmania. We highlight several genes involved in drug resistance by sequencing the genomes of 41 resistant clones and by concentrating on recurrent SNVs. We associate genes linked to lipid metabolism or to ribosome/translation functions with miltefosine or paromomycin resistance, respectively. We prove by allelic replacement and CRISPR-Cas9 gene-editing that the essential protein kinase CDPK1 is crucial for paromomycin resistance. We have linked CDPK1 in translation by functional interactome analysis, and provide evidence that CDPK1 contributes to antimonial resistance in the parasite. This screen is powerful in exploring networks of drug resistance in an organism with diploid to mosaic aneuploid genome, hence widening the scope of its applicability.

Here, Bhattacharya et al. chemically mutagenize Leishmania and identify genes associated with resistance to miltefosine and paromomycin by next generation sequencing. The study shows that a protein kinase (CDPK1) can mediate resistance to paromomycin by affecting translation.

Leishmania Parasite Quantification by Bioluminescence in Murine Models

Leishmaniasis remains a major public health problem worldwide with a prevalence of 12 million, an incidence of 1 million persons, and 350 million people being at risk. Murine models have been largely used for studying the host-pathogen relationship and developing effective chemotherapies against Leishmania parasites. Thus, preclinical imaging is crucial for monitoring the disease outcome. The aim of this protocol is to quantify parasite burden using bioluminescence in vivo imaging. Here, we describe a high-throughput imaging workflow, together with data acquisition and analysis ideal to assess in vivo parasite load in mouse models.

Transcriptional and genomic parallels between the monoxenous parasite Herpetomonas muscarum and Leishmania

Trypanosomatid parasites are causative agents of important human and animal diseases such as sleeping sickness and leishmaniasis. Most trypanosomatids are transmitted to their mammalian hosts by insects, often belonging to Diptera (or true flies). These are called dixenous trypanosomatids since they infect two different hosts, in contrast to those that infect just insects (monoxenous). However, it is still unclear whether dixenous and monoxenous trypanosomatids interact similarly with their insect host, as fly-monoxenous trypanosomatid interaction systems are rarely reported and under-studied-despite being common in nature. Here we present the genome of monoxenous trypanosomatid Herpetomonas muscarum and discuss its transcriptome during in vitro culture and during infection of its natural insect host Drosophila melanogaster. The H. muscarum genome is broadly syntenic with that of human parasite Leishmania major. We also found strong similarities between the H. muscarum transcriptome during fruit fly infection, and those of Leishmania during sand fly infections. Overall this suggests Drosophila-Herpetomonas is a suitable model for less accessible insect-trypanosomatid host-parasite systems such as sand fly-Leishmania.

Molecular Characterization of Leishmania RNA virus 2 in Leishmania major from Uzbekistan

Here we report sequence and phylogenetic analysis of two new isolates of Leishmania RNA virus 2 (LRV2) found in Leishmania major isolated from human patients with cutaneous leishmaniasis in south Uzbekistan. These new virus-infected flagellates were isolated in the same region of Uzbekistan and the viral sequences differed by only nineteen SNPs, all except one being silent mutations. Therefore, we concluded that they belong to a single LRV2 species. New viruses are closely related to the LRV2-Lmj-ASKH documented in Turkmenistan in 1995, which is congruent with their shared host (L. major) and common geographical origin.

Deletion of a Single LeishIF4E-3 Allele by the CRISPR-Cas9 System Alters Cell Morphology and Infectivity of Leishmania

Leishmania species are the causative agents of a spectrum of diseases. Available drug treatment is toxic and expensive, with drug resistance a growing concern. Leishmania parasites migrate between transmitting sand flies and mammalian hosts, experiencing unfavorable extreme conditions. The parasites therefore developed unique mechanisms for promoting a stage-specific program for gene expression, with translation playing a central role. There are six paralogs of the cap-binding protein eIF4E, which vary in their function, expression profiles, and assemblages. Using the CRISPR-Cas9 system for Leishmania, we deleted one of the two LeishIF4E-3 alleles. Expression of LeishIF4E-3 in the deletion mutant was low, leading to reduction in global translation and growth of the mutant cells. Cell morphology also changed, affecting flagellum growth, cell shape, and infectivity. The importance of this study is in highlighting that LeishIF4E-3 is essential for completion of the parasite life cycle. Our study gives new insight into how parasite virulence is determined.

The genomes of Leishmania and trypanosomes encode six paralogs of the eIF4E cap-binding protein, known in other eukaryotes to anchor the translation initiation complex. In line with the heteroxenous nature of these parasites, the different LeishIF4E paralogs vary in their biophysical features and their biological behavior. We therefore hypothesize that each has a specialized function, not limited to protein synthesis. Of the six paralogs, LeishIF4E-3 has a weak cap-binding activity. It participates in the assembly of granules that store inactive transcripts and ribosomal proteins during nutritional stress that is experienced in the sand fly. We investigated the role of LeishIF4E-3 in Leishmania mexicana promastigotes using the CRISPR-Cas9 system. We deleted one of the two LeishIF4E-3 alleles, generating a heterologous deletion mutant with reduced LeishIF4E-3 expression. The mutant showed a decline in de novo protein synthesis and growth kinetics, altered morphology, and impaired infectivity. The mutant cells were rounded and failed to transform into the nectomonad-like form, in response to purine starvation. Furthermore, the infectivity of macrophage cells by the LeishIF4E-3(+/−) mutant was severely reduced. These phenotypic features were not observed in the addback cells, in which expression of LeishIF4E-3 was restored. The observed phenotypic changes correlated with the profile of transcripts associated with LeishIF4E-3. These were enriched for cytoskeleton- and flagellum-encoding genes, along with genes for RNA binding proteins. Our data illustrate the importance of LeishIF4E-3 in translation and in the parasite virulence.

IMPORTANCELeishmania species are the causative agents of a spectrum of diseases. Available drug treatment is toxic and expensive, with drug resistance a growing concern. Leishmania parasites migrate between transmitting sand flies and mammalian hosts, experiencing unfavorable extreme conditions. The parasites therefore developed unique mechanisms for promoting a stage-specific program for gene expression, with translation playing a central role. There are six paralogs of the cap-binding protein eIF4E, which vary in their function, expression profiles, and assemblages. Using the CRISPR-Cas9 system for Leishmania, we deleted one of the two LeishIF4E-3 alleles. Expression of LeishIF4E-3 in the deletion mutant was low, leading to reduction in global translation and growth of the mutant cells. Cell morphology also changed, affecting flagellum growth, cell shape, and infectivity. The importance of this study is in highlighting that LeishIF4E-3 is essential for completion of the parasite life cycle. Our study gives new insight into how parasite virulence is determined.