Molecular and functional analyses of a novel class I secretory nuclease from the human pathogen, Leishmania donovani

Leishmania species: A narrative review on surface proteins with structural aspects involved in host-pathogen interaction

In tropical and subtropical regions of the world, leishmaniasis is endemic and causes a range of clinical symptoms in people, from severe tegumentary forms (such as cutaneous, mucocutaneous, and diffuse leishmaniasis) to lethal visceral forms. The protozoan parasite of the genus Leishmania causes leishmaniasis, which is still a significant public health issue, according to the World Health Organization 2022. The public's worry about the neglected tropical disease is growing as new foci of the illness arise, which are exacerbated by alterations in behavior, changes in the environment, and an enlarged range of sand fly vectors. Leishmania research has advanced significantly during the past three decades in a few different avenues. Despite several studies on Leishmania, many issues, such as illness control, parasite resistance, parasite clearance, etc., remain unresolved. The key virulence variables that play a role in the pathogenicity-host-pathogen relationship of the parasite are comprehensively discussed in this paper. The important Leishmania virulence factors, such as Kinetoplastid Membrane Protein-11 (KMP-11), Leishmanolysin (GP63), Proteophosphoglycan (PPG), Lipophosphoglycan (LPG), Glycosylinositol Phospholipids (GIPL), and others, have an impact on the pathophysiology of the disease and enable the parasite to spread the infection. Leishmania infection may arise from virulence factors; they are treatable with medications or vaccinations more promptly and might greatly shorten the duration of treatment. Additionally, our research sought to present a modeled structure of a few putative virulence factors that might aid in the development of new chemotherapeutic approaches for the treatment of leishmaniasis. The predicted virulence protein's structure is utilized to design novel drugs, therapeutic targets, and immunizations for considerable advantage from a higher understanding of the host immune response.

Non-Vesicular Lipid Transport Machinery in Leishmania donovani: Functional Implications in Host-Parasite Interaction

Eukaryotic cells have distinct membrane-enclosed organelles, each with a unique biochemical signature and specialized function. The unique identity of each organelle is greatly governed by the asymmetric distribution and regulated intracellular movement of two important biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid-transfer proteins (LTPs) plays essential roles in intra-cellular lipid trafficking and cellular lipid homeostasis, while vesicular transport regulates protein trafficking. A comparative analysis of non-vesicular lipid transport machinery in protists could enhance our understanding of parasitism and basis of eukaryotic evolution. Leishmania donovani, the trypanosomatid parasite, greatly depends on receptor-ligand mediated signalling pathways for cellular differentiation, nutrient uptake, secretion of virulence factors, and pathogenesis. Lipids, despite being important signalling molecules, have intracellular transport mechanisms that are largely unexplored in L. donovani. We have identified a repertoire of sixteen (16) potential lipid transfer protein (LTP) homologs based on a domain-based search on TriTrypDB coupled with bioinformatics analyses, which signifies the presence of well-organized lipid transport machinery in this parasite. We emphasized here their evolutionary uniqueness and conservation and discussed their potential implications for parasite biology with regards to future therapeutic targets against visceral leishmaniasis.

Stage-Specific Class I Nucleases of Leishmania Play Important Roles in Parasite Infection and Survival

Protozoans of the genus Leishmania are the causative agents of an important neglected tropical disease referred to as leishmaniasis. During their lifecycle, the parasites can colonize the alimentary tract of the sand fly vector and the parasitophorous vacuole of the mammalian host, differentiating into distinct stages. Motile promastigotes are found in the sand fly vector and are transmitted to the mammalian host during the insect blood meal. Once in the vertebrate host, the parasites differentiate into amastigotes and multiply inside macrophages. To successfully establish infection in mammalian hosts, Leishmania parasites exhibit various strategies to impair the microbicidal power of the host immune system. In this context, stage-specific class I nucleases play different and important roles related to parasite growth, survival and development. Promastigotes express 3’-nucleotidase/nuclease (3’-NT/NU), an ectoenzyme that can promote parasite escape from neutrophil extracellular traps (NET)-mediated death through extracellular DNA hydrolysis and increase Leishmania-macrophage interactions due to extracellular adenosine generation. Amastigotes express secreted nuclease activity during the course of human infection that may be involved in the purine salvage pathway and can mobilize extracellular nucleic acids available far from the parasite. Another nuclease expressed in amastigotes (P4/LmC1N) is located in the endoplasmic reticulum of the parasite and may be involved in mRNA stability and DNA repair. Homologs of this class I nuclease can induce protection against infection by eliciting a T helper 1-like immune response. These immunogenic properties render these nucleases good targets for the development of vaccines against leishmaniasis, mainly because amastigotes are the form responsible for the development and progression of the disease. The present review aims to present and discuss the roles played by different class I nucleases during the Leishmania lifecycle, especially regarding the establishment of mammalian host infection.

Insights into Leishmania Molecules and Their Potential Contribution to the Virulence of the Parasite

Neglected parasitic diseases affect millions of people worldwide, resulting in high morbidity and mortality. Among other parasitic diseases, leishmaniasis remains an important public health problem caused by the protozoa of the genus Leishmania, transmitted by the bite of the female sand fly. The disease has also been linked to tropical and subtropical regions, in addition to being an endemic disease in many areas around the world, including the Mediterranean basin and South America. Although recent years have witnessed marked advances in Leishmania-related research in various directions, many issues have yet to be elucidated. The intention of the present review is to give an overview of the major virulence factors contributing to the pathogenicity of the parasite. We aimed to provide a concise picture of the factors influencing the reaction of the parasite in its host that might help to develop novel chemotherapeutic and vaccine strategies.

Role of the lipoxin A4 receptor in the development of neutrophil extracellular traps in Leishmania infantum infection

BACKGROUND

Neutrophils play an immunomodulatory role through the release of neutrophil extracellular traps (NETs). NETs are released in response to Leishmania infection, but the mechanism of NET extrusion has not been elucidated. The lipoxin A4 receptor on neutrophils is crucial for the inflammatory response and immune regulation of many diseases, including Leishmania infection. Therefore, in the present study, we tried to explore whether Leishmania infantum promastigotes stimulate neutrophil activation and NET release via activating the lipoxin A4 receptor.

RESULTS

Leishmania infantum promastigotes stimulated neutrophil activity, but blocking of the lipoxin A4 receptor with its antagonist Boc prior to L. infantum stimulation abrogated these effects. Neutrophils showed citrullinated histone H3 expression and simultaneous NET extrusion on L. infantum stimulation, but a decline in both was observed on blocking of the lipoxin A4 receptor. Moreover, differentiated HL-60 cells with lipoxin A4 receptor silencing showed a decrease in citrullinated histone H3 expression as compared to the unsilenced HL-60 samples on stimulation with promastigotes.

CONCLUSIONS

Leishmania infantum promastigotes altered the characteristics of neutrophils and induced NET extrusion by activating the lipoxin A4 receptor. The lipoxin A4 receptor may have potential as a therapeutic target in relation to NET extrusion in the treatment of leishmaniasis, but its mechanisms of action need to be explored in more depth.

Tinkering with targeting nucleotide signaling for control of intracellular Leishmania parasites

Nucleotides are one of the most primitive extracellular signalling molecules across all phyla and regulate a multitude of responses. The biological effects of extracellular nucleotides/sides are mediated via the specific purinergic receptors present on the cell surface. In mammalian system, adenine nucleotides are the predominant nucleotides found in the extracellular milieu and mediate a constellation of physiological functions. In the context of host-pathogen interaction, extracellular ATP is recognized as a danger signal and potentiates the release of pro-inflammatory mediators from activated immune cells, on the other hand, its breakdown product adenosine exerts potential anti-inflammatory and immunosuppressive actions. Therefore, it is increasingly apparent that the interplay between extracellular ATP/adenosine ratios has a significant role in coordinating the regulation of the immune system in health and diseases. Several pathogens express ectonucleotidases on their surface and exploit the purinergic signalling as one of the mechanisms to modulate the host immune response. Leishmania pathogens are one of the most successful intracellular pathogens which survive within host macrophages and manipulate protective Th1 response into disease promoting Th2 response. In this review, we discuss the regulation of extracellular ATP and adenosine levels, the role of ATP/adenosine counter signalling in regulating the inflammation and immune responses during infection and how Leishmania parasites exploit the purinergic signalling to manipulate host response. We also discuss the challenges and opportunities in targeting purinergic signalling and the future prospects.

Highly stable single-strand-specific 3'-nuclease/nucleotidase from Legionella pneumophila

The Gram-negative bacterium Legionella pneumophila is one of the known opportunistic human pathogens with a gene coding for a zinc-dependent S1-P1 type nuclease. Bacterial zinc-dependent 3'-nucleases/nucleotidases are little characterized and not fully understood, including L. pneumophila nuclease 1 (Lpn1), in contrast to many eukaryotic representatives with in-depth studies available. To help explain the principle properties and role of these enzymes in intracellular prokaryotic pathogens we have designed and optimized a heterologous expression protocol utilizing E. coli together with an efficient purification procedure, and performed detailed characterization of the enzyme. Replacement of Ni²⁺ ions by Zn²⁺ ions in affinity purification proved to be a crucial step in the production of pure and stable protein. The production protocol provides protein with high yield, purity, stability, and solubility for structure-function studies. We show that highly thermostable Lpn1 is active mainly towards RNA and ssDNA, with pH optima 7.0 and 6.0, respectively, with low activity towards dsDNA; the enzyme features pronounced substrate inhibition. Bioinformatic and experimental analysis, together with computer modeling and electrostatics calculations point to an unusually high positive charge on the enzyme surface under optimal conditions for catalysis. The results help explain the catalytic properties of Lpn1 and its substrate inhibition.

Secretory lipase from the human pathogen Leishmania major: Heterologous expression in the yeast Pichia pastoris and biochemical characterization

Leishmaniasis is a parasitic reticuloendotheliosis whose pathogen is a zooflagellate belonging to the genus Leishmania transmitted by the bite of an infected phlebotome. Recently, a unique secretory lipase from the human pathogen Leishmania donovani Ldlip3 has been identified and characterized. This lipase has a high identity with a putative triacylglycerol lipase of Leishmania major (Lmlip2). In the present study, Lmlip2 was expressed in the eukaryotic heterologous expression system Pichia pastoris as tagged enzyme of 308 amino acids. Maximal protein production was reached after 2 days of fermentation. Optimal Lmlip2 lipase activity was measured using the pH stat technique at pH 8 at 26 °C using vinyl esters and triacylglycerols (true lipids) as substrates. Moreover, biochemical characterization of Lmlip2 contained in culture supernatant, illustrates that L. major secreted lipase is active and stable at low temperatures especially 26°and prefer neutral pH; concerning substrate specificityLmlip2 presents a preference for short chains lipid substrates vinyl esters such as VC2, VC3 and VC4 likewise, it is capable to hydrolyze long chain triacylglycerols like olive oil. Metal ions and surfactants tested in this study decrease Lmlip2 activity. Further studies are needed to clarify the relation between the lipase activity and the virulence. Thus, it could lead to the identification of novel targets to block cutaneous Leishmaniasis in human hosts.

Role of Mast Cells in clearance of Leishmania through extracellular trap formation

Mast Cells (MCs) are one of the first immune cells encountered by invading pathogens. Their presence in large numbers in the superficial dermis, where Leishmania is encountered, suggests that they may play a critical role in immune responses to Leishmania. In this study the interactions of Leishmania donovani, the causative agent of visceral Leishmaniasis, and Leishmania tropica, the causative agent of cutaneous Leishmaniasis with MCs were studied. Co-culture of Leishmania with Peritoneal Mast Cells (PMCs) from BALB/c mice and Rat Basophilic Leukaemia (RBL-2H3) MCs led to significant killing of L. tropica and to a lesser extent of L. donovani. Also, while there was significant uptake of L. tropica by MCs, L. donovani was not phagocytosed. There was significant generation of Reactive Oxygen Species (ROS) by MCs on co-culture with these species of Leishmania which may contribute to their clearance. Interactions of MCs with Leishmania led to generation of MC extracellular traps comprising of DNA, histones and tryptase probably to ensnare these pathogens. These results clearly establish that MCs may contribute to host defences to Leishmania in a differential manner, by actively taking up these pathogens, and also by mounting effector responses for their clearance by extracellular means.

A unique, highly conserved secretory invertase is differentially expressed by promastigote developmental forms of all species of the human pathogen, Leishmania

Leishmania are protozoan pathogens of humans that exist as extracellular promastigotes in the gut of their sand fly vectors and as obligate intracellular amastigotes within phagolysosomes of infected macrophages. Between infectious blood meal feeds, sand flies take plant juice meals that contain sucrose and store these sugars in their crop. Such sugars are regurgitated into the sand fly anterior midgut where they impact the developing promastigote parasite population. In this report we showed that promastigotes of all Leishmania species secreted an invertase/sucrase enzyme during their growth in vitro. In contrast, neither L. donovani nor L. mexicana amastigotes possessed any detectable invertase activity. Importantly, no released/secreted invertase activity was detected in culture supernatants from either Trypanosoma brucei or Trypanosoma cruzi. Using HPLC, the L. donovani secretory invertase was isolated and subjected to amino acid sequencing. Subsequently, we used a molecular approach to identify the LdINV and LmexINV genes encoding the ~72 kDa invertases produced by these organisms. Interestingly, we identified high fidelity LdINV-like homologs in the genomes of all Leishmania sp. but none were present in either T. brucei or T. cruzi. Northern blot and RT-PCR analyses showed that these genes were developmentally/differentially expressed in promastigotes but not amastigotes of these parasites. Homologous transfection studies demonstrated that these genes in fact encoded the functional secretory invertases produced by these parasites. Cumulatively, our results suggest that these secretory enzymes play critical roles in the survival/growth/development and transmission of all Leishmania parasites within their sand fly vector hosts.

3'-nucleotidase/nuclease activity allows Leishmania parasites to escape killing by neutrophil extracellular traps

Leishmaniasis is a widespread neglected tropical disease caused by parasites of the Leishmania genus. These parasites express the enzyme 3'-nucleotidase/nuclease (3'NT/NU), which has been described to be involved in parasite nutrition and infection. Bacteria that express nucleases escape the toxic effects of neutrophil extracellular traps (NETs). Hence, we investigated the role of 3'NT/NU in Leishmania survival of NET-mediated killing. Promastigotes of Leishmania infantum were cultured in high-phosphate (HP) or low-phosphate (LP) medium to modulate nuclease activity. We compared the survival of the two different groups of Leishmania during interaction with human neutrophils, assessing the role of neutrophil extracellular traps. As previously reported, we detected higher nuclease activity in parasites cultured in LP medium. Both LP and HP promastigotes were capable of inducing the release of neutrophil extracellular traps from human neutrophils in a dose- and time-dependent manner. LP parasites had 2.4 times more survival than HP promastigotes. NET disruption was prevented by the treatment of the parasites with ammonium tetrathiomolybdate (TTM), a 3'NT/NU inhibitor. Inhibition of 3'NT/NU by 3'-AMP, 5'-GMP, or TTM decreased promastigote survival upon interaction with neutrophils. Our results show that Leishmania infantum induces NET release and that promastigotes can escape NET-mediated killing by 3'-nucleotidase/nuclease activity, thus ascribing a new function to this enzyme.

Diverse viscerotropic isolates of Leishmania all express a highly conserved secretory nuclease during human infections

Previously, we characterized a gene encoding the unique nuclease (LdNuc(s)) from a Sudanese isolate of the human pathogen Leishmania donovani. This parasite secretory enzyme is involved in the salvage of host-derived purines and is constitutively expressed by both developmental forms of the parasite. Currently, we assessed whether an LdNuc(s)-like nuclease was conserved among other geographically disparate isolates of L. donovani and whether this enzyme was produced by intracellular amastigotes during human infections. Using RT-PCR and Southern blotting, we showed that LdNuc(s) gene homologs were present in each of the viscerotropic Leishmania tested (i.e., L. donovani isolates from the Sudan, Ethiopia and India as well as L. infantum). Further results of in situ enzyme activity gel analyses showed that each of these parasite isolates also expressed a released/secreted LdNuc(s)-like nuclease activity. In Western blots, our anti-LdNuc(s) (Sudan) peptide-specific antibody reacted with only a single ~35 kDa protein in each of the viscerotropic Leishmania isolates. Further, the ~35 kDa nuclease secreted by each of these isolates was specifically immunoprecipitated by the anti-LdNuc(s) antibody above. In situ gel analyses showed that each of these immunoprecipitates had LdNuc(s)-like nuclease activity. Moreover, sera from acute visceral leishmaniasis patients from India, Sudan and Brazil all immunoprecipitated an LdNuc(s)-HA expressed nuclease demonstrating, that these patients possessed antibodies against this parasite secretory enzyme. Cumulatively, these results showed that the LdNuc(s) homologs were functionally conserved among geographically disparate visceral Leishmania spp. and that amastigotes of these parasites must produce this nuclease enzyme during the course of human disease.

Identification, characterization, and expression of a unique secretory lipase from the human pathogen Leishmania donovani

Lipases have been implicated to be of importance in the life cycle development, virulence, and transmission of a variety of parasitic organisms. Potential functions include the acquisition of host resources for energy metabolism and as simple building blocks for the synthesis of complex parasite lipids important for membrane remodeling and structural purposes. Using a molecular approach, we identified and characterized the structure of an LdLip3-lipase gene from the primitive trypanosomatid pathogen of humans, Leishmania donovani. The LdLip3 encodes a approximately 33 kDa protein, with a well-conserved substrate-binding and catalytic domains characteristic of members of the serine lipase-protein family. Further, we showed that LdLip3 mRNA is constitutively expressed by both the insect vector (i.e., promastigote) and mammalian (i.e., amastigote) life cycle developmental forms of this protozoan parasite. Moreover, a homologous episomal expression system was used to express an HA epitope-tagged LdLip3 chimeric construct (LdLip3::HA) in these parasites. Expression of the LdLip3 chimera was verified in these transfectants by Western blots and indirect immuno-fluorescence analyses. Results of coupled immuno-affinity purification and enzyme activity experiments demonstrated that the LdLip3::HA chimeric protein was secreted/released by transfected L. donovani parasites and that it possessed functional lipase enzyme activity. Taken together these observations suggest that this novel secretory lipase might play essential role(s) in the survival, growth, and development of this important group of human pathogens.

Leishmania infantum chagasi: a genome-based approach to identification of excreted/secreted proteins

The parasitic protozoan, Leishmania, survives in harsh environments within its mammalian and sand fly hosts. Secreted proteins likely play critical roles in the parasite's interactions with its environment. As a preliminary identification of the spectrum of potential excreted/secreted (ES) proteins of Leishmania infantum chagasi (Lic), a causative agent of visceral leishmaniasis, we used standard algorithms to screen the annotated L. infantum genome for genes whose predicted protein products have an N-terminal signal peptide and lack transmembrane domains and membrane anchors. A suite of 181 candidate ES proteins were identified. These included several that were documented in the literature to be released by other Leishmania spp. Six candidate ES proteins were selected for further validation of their expression and release by different parasite stages. We found both amastigote-specific and promastigote-specific released proteins. The ES proteins of Lic are candidates for future studies of parasite virulence determinants and host protective immunity.

The major component in schistosome eggs responsible for conditioning dendritic cells for Th2 polarization is a T2 ribonuclease (omega-1)

Schistosoma mansoni eggs contain factors that trigger potent Th2 responses in vivo and condition mouse dendritic cells (DCs) to promote Th2 lymphocyte differentiation. Using an in vitro bystander polarization assay as the readout, we purified and identified the major Th2-inducing component from soluble egg extract (SEA) as the secreted T2 ribonuclease, omega-1. The Th2-promoting activity of omega-1 was found to be sensitive to ribonuclease inhibition and did not require MyD88/TRIF signaling in DCs. In common with unfractioned SEA, the purified native protein suppresses lipopolysaccharide-induced DC activation, but unlike SEA, it fails to trigger interleukin 4 production from basophils. Importantly, omega-1-exposed DCs displayed pronounced cytoskeletal changes and exhibited decreased antigen-dependent conjugate formation with CD4(+) T cells. Based on this evidence, we hypothesize that S. mansoni omega-1 acts by limiting the interaction of DCs with CD4(+) T lymphocytes, thereby lowering the strength of the activation signal delivered.

Identification and biochemical characterization of unique secretory nucleases of the human enteric pathogen, Entamoeba histolytica

The ancient eukaryotic human pathogen, Entamoeba histolytica, is a nucleo-base auxotroph (i.e. lacks the ability to synthesize purines or pyrimidines de novo) and therefore is totally dependent upon its host for the supply of these essential nutrients. In this study, we identified two unique 28-kDa, dithiothreitol-sensitive nucleases and showed that they are constitutively released/secreted by parasites during axenic culture. Using several different molecular approaches, we identified and characterized the structure of EhNucI and EhNucII, genes that encode ribonuclease T2 family proteins. Homologous episomal expression of epitope-tagged EhNucI and EhNucII chimeric constructs was used to define the functional and biochemical properties of these released/secreted enzymes. Results of coupled immunoprecipitation-enzyme activity analyses demonstrated that these "secretory" enzymes could hydrolyze a variety of synthetic polynucleotides, as well as the natural nucleic acid substrate RNA. Furthermore, our results demonstrated that sera from acutely infected amebiasis patients recognized and immunoprecipitated these parasite secretory enzymes. Based on these observations, we hypothesize that within its host, these secretory nucleases could function, at a distance away from the parasite, to harness (i.e. hydrolyze/access) host-derived nucleic acids to satisfy the essential purine and pyrimidine requirements of these organisms. Thus, these enzymes might play an important role in facilitating the survival, growth, and development of this important human pathogen.

Episomally driven antisense mRNA abrogates the hyperinducible expression and function of a unique cell surface class I nuclease in the primitive trypanosomatid parasite, Crithidia luciliae

Here, we show that Crithidia luciliae, a primitive trypanosomatid, purine auxotroph, up-expressed its unique, bi-functional, surface membrane 3'-nucleotidase/nuclease (Cl 3'NT/NU) activity by approximately 1000-fold in response to purine starvation. A second surface membrane phospho-monoesterase, i.e. a tartrate-resistant acid phosphatase (Cl MAcP) was also found to be up-expressed in such purine-starved cells. Here, we used homologous episomal-expression of an antisense construct of the Cl3'NT/NU to dissect the functional expression of these two surface membrane enzymes. In antisense transfected cells, a large excess of the antisense transcript was produced and no trace of any endogenous Cl3'NT/NU sense message was detected. Further, the purine-starvation hyper-induced levels of 3'NT/NU enzyme activity were completely abrogated in these transfected cells versus controls. Moreover, such antisense transcription completely abolished the ability of these transfectants to grow in poly(A)-containing medium demonstrating the essential nature of the 3'NT/NU for the growth/survival of this parasite. In contrast, antisense transcription had no apparent deleterious effects on either endogenous or purine-starvation-induced levels of MAcP enzyme activity, its steady-state mRNA levels, or the constitutive expression of house-keeping genes (e.g. Cl alpha-tubulin) in these transfectants. Cumulatively, results of our antisense experiments demonstrated that the functional nuclease activity of the surface membrane Cl 3'NT/NU was, in fact, critical/essential for the growth and development of these primitive parasites.