Parasite Cathepsin D-Like Peptidases and Their Relevance as Therapeutic Targets

Advances in protease inhibition-based chemotherapy: A decade of insights from Malaria research

Over the last decade, research on the most studied parasite, Plasmodium falciparum, has disclosed significant findings in protease research. Detailed descriptions of the individual roles of protease isoenzymes from various protease classes encoded by the parasite genome have been elucidated, along with their functional and biochemical characterizations. These insights have enabled the development of innovative chemotherapy using low molecular weight inhibitors targeting specific molecular sites. Progress has been made in understanding the proteolytic cascade associated with the apical complex, particularly the roles of aspartyl proteases plasmepsins IX and X as master regulators. Additionally, advancements in direct and alternative methods of proteasome inhibition and expression regulation have been achieved. Research on digestive/food vacuole-associated proteases, with a focus on essential metalloproteases, has also seen significant developments. The rise of extensive genomic datasets and functional genomic tools for other parasitic organisms now allows these approaches to be applied to the study and treatment of other, less known parasitic diseases, aiming to uncover specific biological mechanisms and develop innovative, less toxic chemotherapies.

Unveiling the peptidases of parasites from the office chair - The endothelin-converting enzyme case study

The emergence of high-throughput methodologies such as next-generation sequencing and proteomics has necessitated significant advancements in biological databases and bioinformatic tools, therefore reshaping the landscape of research into parasitic peptidases. In this review we outline the development of these resources along the -omics technologies and their transformative impact on the field. Apart from extensive summary of general and specific databases and tools, we provide a general pipeline on how to use these resources effectively to identify candidate peptidases from these large datasets and how to gain as much information about them as possible without leaving the office chair. This pipeline is then applied in an illustrative case study on the endothelin-converting enzyme 1 homologue from Schistosoma mansoni and attempts to highlight the contemporary capabilities of bioinformatics. The case study demonstrate how such approach can aid to hypothesize enzyme functions and interactions through computational analysis alone effectively and emphasizes how such virtual investigations can guide and optimize subsequent wet lab experiments therefore potentially saving precious time and resources. Finally, by showing what can be achieved without traditional wet laboratory methods, this review provides a compelling narrative on the use of bioinformatics to bridge the gap between big data and practical research applications, highlighting the key role of these technologies in furthering our understanding of parasitic diseases.

The effects of maternal care on the developmental transcriptome and metatranscriptome of a wild bee

Maternal care acts as a strong environmental stimulus that can induce phenotypic plasticity in animals and may also alter their microbial communities through development. Here, we characterize the developmental metatranscriptome of the small carpenter bee, Ceratina calcarata, across developmental stages and in the presence or absence of mothers. Maternal care had the most influence during early development, with the greatest number and magnitude of differentially expressed genes between maternal care treatments, and enrichment for transcription factors regulating immune response in motherless early larvae. Metatranscriptomic data revealed fungi to be the most abundant group in the microbiome, with Aspergillus the most abundant in early larvae raised without mothers. Finally, integrative analysis between host transcriptome and metatranscriptome highlights several fungi correlating with developmental and immunity genes. Our results provide characterizations of the influence of maternal care on gene expression and the microbiome through development in a wild bee.

Degrade to survive: the intricate world of piroplasmid proteases

Piroplasmids of the genera Babesia, Theileria, and Cytauxzoon are tick-transmitted parasites with a high impact on animals and humans. They have complex life cycles in their definitive arthropod and intermediate vertebrate hosts involving numerous processes, including invasion of, and egress from, host cells, parasite growth, transformation, and migration. Like other parasitic protozoa, piroplasmids are equipped with different types of protease to fulfill many of such essential processes. Blockade of some key proteases, using inhibitors or antibodies, hinders piroplasmid growth, highlighting their potential usefulness in drug therapies and vaccine development. A better understanding of the functional significance of these enzymes will contribute to the development of improved control measures for the devastating animal and human diseases caused by these pathogens.

Proteases and their inhibitors involved in Schistosoma mansoni egg-host interaction revealed by comparative transcriptomics with Fasciola hepatica eggs

Schistosoma mansoni eggs are the main causative agents of the pathological manifestations of schistosomiasis. The eggs are laid in the host bloodstream, then they migrate through the intestinal wall into the lumen. However, a significant proportion of the eggs become lodged in the liver, where they cause inflammation and fibrosis. In this study, we focus on a specific group of proteins expressed by the egg, namely proteases and their inhibitors. These molecules are often involved in schistosome-host interactions, but are still unexplored in the egg stage. Using RNA-seq and comparative transcriptomics of immature and mature S. mansoni eggs, we mapped the portfolio of proteases and their inhibitors, and determined their gene expression levels. In addition, we compared these data with gene expression of proteases and their inhibitors in Fasciola hepatica eggs. Fasciola hepatica eggs served as a useful comparative model, as they do not migrate through tissues and inflict pathology. We detected transcription of 135 and 117 proteases in S. mansoni and F. hepatica eggs, respectively, with 87 identified as orthologous between the two species. In contrast, we observed only four orthologous inhibitors out of 21 and 16 identified in S. mansoni and F. hepatica eggs, respectively. Among others, we measured high and developmentally regulated levels of expression of metalloproteases in S. mansoni eggs, specifically aminopeptidase N1, endothelin-converting enzyme 1, and several leishmanolysin-like peptidases. We identified highly transcribed protease inhibitors serpin and alpha-2-macroglobulin that are unique to S. mansoni eggs, and antistasin-like inhibitor in F. hepatica eggs. This study provides new insights into the portfolio of proteases and inhibitors expressed by S. mansoni with potential roles in egg tissue migration, stimulation of angiogenesis, and interaction with host blood and immunity.

Functional Role of AsAP in the Reproduction of Adelphocoris suturalis (Hemiptera: Miridae)

Adelphocoris suturalis Jakovlev (Hemiptera: Miridae) is an omnivorous agricultural pest that has severe economic impacts on a diverse range of agricultural crops. Although the targeted disruption of reproductive development among insects has been proposed as a novel control strategy for pest species, the current understanding of the physiology and molecular mechanisms of A. suturalis reproduction is very limited. In this study, we isolated a putative A. suturalisaspartic protease (AsAP) gene that is highly expressed in the fat body and ovaries of sexually mature females. The double-stranded RNA (dsRNA)-mediated knockdown of AsAP suppressed ovarian development and negatively impacted female fertility, which suggested that it plays an essential role in A. suturalis reproduction. The results of this study could help to expand our understanding of A. suturalis reproductive development and have the potential to facilitate the development of effective strategies for the better control of this pest species.

Comparative transcriptome profiling of virulent and avirulent isolates of Neoparamoeba perurans

Neoparamoeba perurans, the aetiological agent of amoebic gill disease, remains a persistent threat to Atlantic salmon mariculture operations worldwide. Innovation in methods of AGD control is required yet constrained by a limited understanding of the mechanisms of amoebic gill disease pathogenesis. In the current study, a comparative transcriptome analysis of two N. perurans isolates of contrasting virulence phenotypes is presented using gill-associated, virulent (wild type) isolates, and in vitro cultured, avirulent (clonal) isolates. Differential gene expression analysis identified a total of 21,198 differentially expressed genes between the wild type and clonal isolates, with 5674 of these genes upregulated in wild type N. perurans. Gene set enrichment analysis predicted gene sets enriched in the wild type isolates including, although not limited to, cortical actin cytoskeleton, pseudopodia, phagocytosis, macropinocytic cup, and fatty acid beta-oxidation. Combined, the results from these analyses suggest that upregulated gene expression associated with lipid metabolism, oxidative stress response, protease activity, and cytoskeleton reorganisation is linked to pathogenicity in wild type N. perurans. These findings provide a foundation for future AGD research and the development of novel therapeutic and prophylactic AGD control measures for commercial aquaculture.

Proteomic profiling of hydatid fluid from pulmonary cystic echinococcosis

BACKGROUND

Most cystic echinococcosis cases in Southern Brazil are caused by Echinococcus granulosus and Echinococcus ortleppi. Proteomic studies of helminths have increased our knowledge about the molecular survival strategies that are used by parasites. Here, we surveyed the protein content of the hydatid fluid compartment in E. granulosus and E. ortleppi pulmonary bovine cysts to better describe and compare their molecular arsenal at the host-parasite interface.

METHODS

Hydatid fluid samples from three isolates of each species were analyzed using mass spectrometry-based proteomics (LC-MS/MS). In silico functional analyses of the identified proteins were performed to examine parasite survival strategies.

RESULTS

The identified hydatid fluid protein profiles showed a predominance of parasite proteins compared to host proteins that infiltrate the cysts. We identified 280 parasitic proteins from E. granulosus and 251 from E. ortleppi, including 52 parasitic proteins that were common to all hydatid fluid samples. The in silico functional analysis revealed important molecular functions and processes that are active in pulmonary cystic echinococcosis, such as adhesion, extracellular structures organization, development regulation, signaling transduction, and enzyme activity.

CONCLUSIONS

The protein profiles described here provide evidence of important mechanisms related to basic cellular processes and functions that act at the host-parasite interface in cystic echinococcosis. The molecular tools used by E. granulosus and E. ortleppi for survival within the host are potential targets for new therapeutic approaches to treat cystic echinococcosis and other larval cestodiases.

Proteases as Therapeutic Targets Against the Parasitic Cnidarian Ceratonova shasta: Characterization of Molecules Key to Parasite Virulence In Salmonid Hosts

Proteases and their inhibitors play critical roles in host-parasite interactions and in the outcomes of infections. Ceratonova shasta is a myxozoan pathogen that causes enteronecrosis in economically important salmonids from the Pacific Northwest of North America. This cnidarian parasite has host-specific genotypes with varying virulence, making it a powerful system to decipher virulence mechanisms in myxozoans. Using C. shasta genome and transcriptome, we identified four proteases of different catalytic types: cathepsin D (aspartic), cathepsin L and Z-like (cysteine) and aminopeptidase-N (metallo); and a stefin (cysteine protease inhibitor), which implied involvement in virulence and hence represent target molecules for the development of therapeutic strategies. We characterized, annotated and modelled their 3D protein structure using bioinformatics and computational tools. We quantified their expression in C. shasta genotype 0 (low virulence, no mortality) and IIR (high virulence and mortality) in rainbow trout Oncorhynchus mykiss, to demonstrate that there are major differences between the genotypes during infection and parasite development. High proliferation of genotype IIR was associated with high expression of the cathepsin D and the stefin, likely correlated with high nutrient demands and to regulate cell metabolism, with upregulation preceding massive proliferation and systemic dispersion. In contrast, upregulation of the cathepsin L and Z-like cysteine proteases may have roles in host immune evasion in genotype 0 infections, which are associated with low proliferation, low inflammation and non-destructive development. In contrast to the other proteases, C. shasta aminopeptidase-N appears to have a prominent role in nematocyst formation in both genotypes, but only during sporogenesis. Homology searches of C. shasta proteases against other myxozoan transcriptomes revealed a high abundance of cathepsin L and aminopeptidase homologs suggesting common gene requirements across species. Our study identified molecules of potential therapeutic significance for aquaculture and serves as a baseline for future research aimed at functional characterisation of these targets.

Comparative transcriptome profiles of Schistosoma japonicum larval stages: Implications for parasite biology and host invasion

Schistosoma japonicum is prevalent in Asia with a wide mammalian host range, which leads to highly harmful zoonotic parasitic diseases. Most previous transcriptomic studies have been performed on this parasite, but mainly focus on stages inside the mammalian host. Moreover, few larval transcriptomic data are available in public databases. Here we mapped the detailed transcriptome profiles of four S. japonicum larval stages including eggs, miracidia, sporocysts and cercariae, providing a comprehensive development picture outside of the mammalian host. By analyzing the stage-specific/enriched genes, we identified functional genes associated with the biological characteristic at each stage: e.g. we observed enrichment of genes necessary for DNA replication only in sporocysts, while those involved in proteolysis were upregulated in sporocysts and/or cercariae. This data indicated that miracidia might use leishmanolysin and neprilysin to penetrate the snail, while elastase (SjCE2b) and leishmanolysin might contribute to the cercariae invasion. The expression profile of stem cell markers revealed potential germinal cell conversion during larval development. Additionally, our analysis indicated that tandem duplications had driven the expansion of the papain family in S. japonicum. Notably, all the duplicated cathepsin B-like proteases were highly expressed in cercariae. Utilizing our 3rd version of S. japonicum genome, we further characterized the alternative splicing profiles throughout these four stages. Taken together, the present study provides compressive gene expression profiles of S. japonicum larval stages and identifies a set of genes that might be involved in intermediate and definitive host invasion.

Plasmepsin-like Aspartyl Proteases in Babesia

Apicomplexan genomes encode multiple pepsin-family aspartyl proteases (APs) that phylogenetically cluster to six independent clades (A to F). Such diversification has been powered by the function-driven evolution of the ancestral apicomplexan AP gene and is associated with the adaptation of various apicomplexan species to different strategies of host infection and transmission through various invertebrate vectors. To estimate the potential roles of Babesia APs, we performed qRT-PCR-based expressional profiling of Babesia microti APs (BmASP2, 3, 5, 6), which revealed the dynamically changing mRNA levels and indicated the specific roles of individual BmASP isoenzymes throughout the life cycle of this parasite. To expand on the current knowledge on piroplasmid APs, we searched the EuPathDB and NCBI GenBank databases to identify and phylogenetically analyse the complete sets of APs encoded by the genomes of selected Babesia and Theileria species. Our results clearly determine the potential roles of identified APs by their phylogenetic relation to their homologues of known function—Plasmodium falciparum plasmepsins (PfPM I–X) and Toxoplasma gondii aspartyl proteases (TgASP1–7). Due to the analogies with plasmodial plasmepsins, piroplasmid APs represent valuable enzymatic targets that are druggable by small molecule inhibitors—candidate molecules for the yet-missing specific therapy for babesiosis.

pH-Dependent Structural Dynamics of Cathepsin D-Family Aspartic Peptidase of Clonorchis sinensis

Cathepsin D (CatD; EC 3.4.23.5) family peptidases of parasitic organisms are regarded as potential drug targets as they play critical roles in the physiology and pathobiology of parasites. Previously, we characterized the biochemical features of cathepsin D isozyme 2 (CatD2) in the carcinogenic liver fluke Clonorchis sinensis (CsCatD2). In this study, we performed all-atomic molecular dynamics simulations by applying different systems for the ligand-free/bound forms under neutral and acidic conditions to investigate the pH-dependent structural alterations and associated functional changes in CsCatD2. CsCatD2 showed several distinctive characteristics as follows: (1) acidic pH caused major conformational transitions from open to closed state in this enzyme; (2) during 30–36-ns simulations, acidic pH contributed significantly to the formation of rigid β-sheets around the catalytic residue Asp₂₁₉, higher occupancy (0% to 99%) of hydrogen bond than that of Asp₃₃, and enhanced stabilization of the CsCatD2-inhibtor complex; (3) neutral pH-induced displacement of the N-terminal part to hinder the accessibility of the active site and open allosteric site of this enzyme; and (4) the flap dynamics metrics, including distance (d₁), TriCα angles (θ₁ and θ₂), and dihedral angle (ϕ), account for the asymmetrical twisting motion of the active site of this enzyme. These findings provide an in-depth understanding of the pH-dependent structural dynamics of free and bound forms of CsCatD2 and basic information for the rational design of an inhibitor as a drug targeting parasitic CatD.

Protease Inhibition-An Established Strategy to Combat Infectious Diseases

Therapeutic agents with novel mechanisms of action are urgently needed to counter the emergence of drug-resistant infections. Several decades of research into proteases of disease agents have revealed enzymes well suited for target-based drug development. Among them are the three recently validated proteolytic targets: proteasomes of the malarial parasite Plasmodium falciparum, aspartyl proteases of P. falciparum (plasmepsins) and the Sars-CoV-2 viral proteases. Despite some unfulfilled expectations over previous decades, the three reviewed targets clearly demonstrate that selective protease inhibitors provide effective therapeutic solutions for the two most impacting infectious diseases nowadays-malaria and COVID-19.

In silico identification of excretory/secretory proteins and drug targets in monogenean parasites

The Excretory/Secretory (ES) proteins of parasites are involved in invasion and colonization of their hosts. In addition, since ES proteins circulate in the extracellular space, they can be more accessible to drugs than other proteins, which makes ES proteins optimal targets for the development of new and better pharmacological strategies. Monogeneans are a group of parasitic Platyhelminthes that includes some pathogenic species problematic for finfish aquaculture. In the present study, 8297 putative ES proteins from four monogenean species which genomic resources are publicly available were identified and functionally annotated by bioinformatic tools. Additionally, for comparative purposes, ES proteins in other parasitic and free-living platyhelminths were identified. Based on data from the monogenean Gyrodactylus salaris, 15 ES proteins are considered potential drug targets. One of them showed homology to 10 cathepsins with known 3D structure. A docking molecular analysis uncovered that the anthelmintic emodepside shows good affinity to these cathepsins suggesting that emodepside can be experimentally tested as a monogenean's cathepsin inhibitor.

De novo transcriptome sequencing and comparative profiling of the ovary in partially engorged and fully engorged Haemaphysalis flava ticks

Haemaphysalis flava is the vector of several pathogens and has medical and veterinary importance. Transcriptome information of the ovary of H. flava is unavailable and limits understanding of its molecular basis of reproduction. We studied the ovary transcriptome of partially engorged and fully engorged H. flava using high-throughput RNA sequencing technology. A total of 53,025,360 and 57,942,890 clean reads were obtained with 7.95 GB and 8.69 GB clean bases in partially engorged ticks (PETs) and fully engorged ticks (FETs), respectively. The clean reads were assembled into 138,711 unigenes. A total of 72,043 unigenes (51.93%) were annotated and 66,668 unigenes (48.07%) were unknown. A total of 38,487 differentially expressed genes (DEGs) were found between PET and FET with 19,031 upregulated genes and 19,456 downregulated genes. The RNA-seq results were validated by qRT-PCR, including six upregulated genes and three downregulated genes. Some unigenes coding for nutrient transporters, proteases, and protease inhibitors were found and analyzed. This study was the first time to perform the transcriptome sequences of the ovary of partially engorged and fully engorged H. flava. The results can benefit the understanding of the molecular basis of ovary maturation and oogenesis of the H. flava and boost the development of the strategies for control of H. flava.

Proteolytic profile of larval developmental stages of Penaeus vannamei: An activity and mRNA expression approach

In arthropods, the cleavage of specific proteins by peptidases has pivotal roles in multiple physiological processes including oogenesis, immunity, nutrition, and parasitic infection. These enzymes are also key players in the larval development, and well-described triggers of molting and metamorphosis. In this work the peptidase complement throughout the larvae development of Penaeus vannamei was quantified at the transcript and activity level using qPCR and fluorogenic substrates designed to be hydrolyzed by class-specific peptidases respectively, providing a detailed identification of the proteolytic repertoire in P. vannamei larvae. Significant changes in the peptidase activity profile were observed. During the lecithotrophic naupliar instars, the dominant peptidase activity and expression derive from cysteine peptidases, suggesting that enzymes of this class hydrolyze the protein components of yolk as the primary amino acid source. At the first feeding instar, zoea, dominant serine peptidase activity was found where trypsin activity is particularly high, supporting previous observations that during zoea the breakdown of food protein is primarily enzymatic. At decapodid stages the peptidase expression and activity is more diverse indicating that a multienzyme network achieves food digestion. Our results suggest that proteolytic enzymes fulfill specific functions during P. vannamei larval development.

Proteases and pseudoproteases in parasitic arthropods of clinical importance

Parasitic arthropods feed on blood or skin tissue and share comparable repertoires of proteases involved in haematophagy, digestion, egg development and immunity. While proteolytically active proteases of multiple classes dominate, an increasing number of pseudoproteases have been discovered that have no proteolytic function but are pharmacologically active biomolecules, evolved to carry out alternative functions as regulatory, antihaemostatic, anti-inflammatory or immunomodulatory compounds. In this review, we provide an overview of proteases and pseudoproteases from clinically important arthropod parasites. Many of these act in central biological pathways of parasite survival and host-parasite interaction and may be potential targets for therapeutic interventions.

Novel pseudo-aspartic peptidase from the midgut of the tick Rhipicephalus microplus

The characterization of Rhipicephalus microplus tick physiology can support efforts to develop and improve the efficiency of control methods. A sequence containing a domain with similarity to one derived from the aspartic peptidase family was isolated from the midgut of engorged female R. microplus. The lack of the second catalytic aspartic acid residue suggest that it may be a pseudo-aspartic peptidase, and it was named RmPAP. In this work we confirm the lack of proteolytic activity of RmPAP and investigate it’s non-proteolytic interaction with bovine hemoglobin by Surface Plasmon Resonance and phage display. Moreover we carried out RNAi interference and artificial feeding of ticks with anti-RmPAP antibodies to assess it’s possible biological role, although no changes were observed in the biological parameters evaluated. Overall, we hypothesize that RmPAP may act as a carrier of hemoglobin/heme between the tick midgut and the ovaries.

Activity profiling of peptidases in Angiostrongylus costaricensis first-stage larvae and adult worms

BACKGROUND

Angiostrongylus costaricensis is a relatively uncharacterized nematode that causes abdominal angiostrongyliasis in Latin America, a human parasitic disease. Currently, no effective pharmacological treatment for angiostrongyliasis exists. Peptidases are known to be druggable targets for a variety of diseases and are essential for several biological processes in parasites. Therefore, this study aimed to systematically characterize the peptidase activity of A. costaricensis in different developmental stages of this parasitic nematode.

METHODOLOGY/PRINCIPAL FINDINGS

A library of diverse tetradecapeptides was incubated with cellular lysates from adult worms and from first-stage larvae (L1) and cleaved peptide products were identified by mass spectrometry. Lysates were also treated with class specific peptidase inhibitors to determine which enzyme class was responsible for the proteolytic activity. Peptidase activity from the four major mechanistic classes (aspartic, metallo, serine and cysteine) were detected in adult worm lysate, whereas aspartic, metallo and serine-peptidases were found in the larval lysates. In addition, the substrate specificity profile was found to vary at different pH values.

CONCLUSIONS/SIGNIFICANCE

The proteolytic activities in adult worm and L1 lysates were characterized using a highly diversified library of peptide substrates and the activity was validated using a selection of fluorescent substrates. Taken together, peptidase signatures for different developmental stages of this parasite has improved our understanding of the disease pathogenesis and may be useful as potential drug targets or vaccine candidates.

Asymmetric peptidomimetics containing L-tartaric acid core inhibit the aspartyl peptidase activity and growth of Leishmania amazonensis promastigotes

Aspartyl-type peptidases are promising chemotherapeutic targets in protozoan parasites. In the present work, we identified an aspartyl peptidase activity from the soluble extract of Leishmania amazonensis promastigotes, which cleaved the fluorogenic peptide 7-methoxycoumarin-4-acetyl-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys(DNP)-D-Arg-amide (cathepsin D substrate) under acidic pH conditions at 37°C, showing a KM of 0.58 μM and Vmax of 129.87 fluorescence arbitrary units/s mg protein. The leishmanial aspartyl peptidase activity was blocked by pepstatin A (IC50 = 6.8 μM) and diazo-acetyl-norleucinemetilester (IC50 = 10.2 μM), two classical aspartyl peptidase inhibitors. Subsequently, the effects of 6 asymmetric peptidomimetics, containing L-tartaric acid core, were tested on both aspartyl peptidase and growth of L. amazonensis promastigotes. The peptidomimetics named 88, 154 and 158 promoted a reduction of 50% on the leishmanial aspartyl peptidase activity at concentrations ranging from 40 to 85 μM, whereas the peptidomimetic 157 was by far the most effective, presenting IC50 of 0.04 μM. Furthermore, the peptidomimetics 157 and 154 reduced the parasite proliferation in a dose-dependent manner, displaying IC50 values of 33.7 and 44.5 μM, respectively. Collectively, the peptidomimetic 157 was the most efficient compound able to arrest both aspartyl peptidase activity and leishmanial proliferation, which raises excellent perspectives regarding its use against this human pathogenic protozoan.

Serine proteases in schistosomes and other trematodes

Trematodes, also known as flukes, are phylogenetically ancient parasitic organisms. Due to their importance as human and veterinary parasites, their proteins have been investigated extensively as drug and vaccine targets. Among those, proteases, as crucial enzymes for parasite survival, are considered candidate molecules for anti-parasitic interventions. Surprisingly however, trematode serine proteases, in comparison with other groups of proteases, are largely neglected. Genes encoding serine proteases have been identified in trematode genomes in significant abundance, but the biological roles and biochemical functions of these proteases are poorly understood. However, increasing volumes of genomic and proteomic studies, and accumulated experimental evidence, indicate that this class of proteases plays a substantial role in host-parasite interactions and parasite survival. Here, we discuss in detail serine proteases at genomic and protein levels, and their known or hypothetical functions.

Multiple legumain isoenzymes in ticks

By searching nucleotide databases for the North American Lyme disease vector, Ixodes scapularis, we have complemented the previously characterized European Ixodes ricinus legumain IrAE1 with a full set of nine analogous genes (isae1-9). Six of these were PCR confirmed as genes present in all tick genomes tested. The absolute mRNA copy number examined by quantitative (q)PCR enabled expression profiling and an absolute comparison of mRNA levels for individual I. scapularis (Is)AEs in tick tissues. Four IsAEs (1, 2, 4, 9) were expressed solely in the gut and thus are proposed to be involved in host blood digestion. Expression qPCR profiling over developmental stages confirmed IsAE1, the direct analogue of previously characterized I. ricinus IrAE1, as the principle legumain transcript in partially engorged females, and demonstrated its strong regulation by on-host feeding in larvae, nymphs and females. In contrast, IsAE2 was the predominant gut legumain in unfed nymphs, unfed females and males. In-silico, IsAE1 and IsAE2 protein three-dimensional structural models displayed minimal differences in overall proenzyme structures, even in comparison with recently resolved crystal structures of mammalian prolegumain. Three functional studies were performed in I. ricinus with IsAE1/IsAE2 analogues: double IrAE1/IrAE2 RNA interference silencing, feeding of ticks on IrAE1+IrAE2 immunized hosts and in vitro membrane tick feeding on blood containing a legumain-specific inhibitor. The latter experiment led to reduced weights of fully engorged ticks and limited oviposition, and indicated the potential of legumain inhibitors for novel anti-tick interventions.

Is digestive cathepsin D the rule in decapod crustaceans?

Cathepsin D is an aspartic endopetidase with typical characteristics of lysosomal enzymes. Cathepsin D activity has been reported in the gastric fluid of clawed lobsters where it acts as an extracellular digestive enzyme. Here we investigate whether cathepsin D is unique in clawed lobsters or, instead, common in decapod crustaceans. Eleven species of decapods belonging to six infraorders were tested for cathepsin D activity in the midgut gland, the muscle tissue, the gills, and when technically possible, in the gastric fluid. Cathepsin D activity was present in the midgut gland of all 11 species and in the gastric fluid from the seven species from which samples could be taken. All sampled species showed higher activities in the midgut glands than in non-digestive organs and the activity was highest in the clawed lobster. Cathepsin D mRNA was obtained from tissue samples of midgut gland, muscle, and gills. Analyses of deduced amino acid sequence confirmed molecular features of lysosomal cathepsin D and revealed high similarity between the enzymes from Astacidea and Caridea on one side, and the enzymes from Penaeoidea, Anomura, and Brachyura on the other side. Our results support the presence of cathepsin D activity in the midgut glands and in the gastric fluids of several decapod species suggesting an extracellular function of this lysosomal enzyme. We discuss whether cathepsin D may derive from the lysosomal-like vacuoles of the midgut gland B-cells and is released into the gastric lumen upon secretion by these cells.

Proteases of haematophagous arthropod vectors are involved in blood-feeding, yolk formation and immunity - a review

Ticks, triatomines, mosquitoes and sand flies comprise a large number of haematophagous arthropods considered vectors of human infectious diseases. While consuming blood to obtain the nutrients necessary to carry on life functions, these insects can transmit pathogenic microorganisms to the vertebrate host. Among the molecules related to the blood-feeding habit, proteases play an essential role. In this review, we provide a panorama of proteases from arthropod vectors involved in haematophagy, in digestion, in egg development and in immunity. As these molecules act in central biological processes, proteases from haematophagous vectors of infectious diseases may influence vector competence to transmit pathogens to their prey, and thus could be valuable targets for vectorial control.

Identification of candidate infection genes from the model entomopathogenic nematode Heterorhabditis bacteriophora

Background

Despite important progress in the field of innate immunity, our understanding of host immune responses to parasitic nematode infections lags behind that of responses to microbes. A limiting factor has been the obligate requirement for a vertebrate host which has hindered investigation of the parasitic nematode infective process. The nematode parasite Heterorhabditis bacteriophora offers great potential as a model to genetically dissect the process of infection. With its mutualistic Photorhabdus luminescens bacteria, H. bacteriophora invades multiple species of insects, which it kills and exploits as a food source for the development of several nematode generations. The ability to culture the life cycle of H. bacteriophora on plates growing the bacterial symbiont makes it a very exciting model of parasitic infection that can be used to unlock the molecular events occurring during infection of a host that are inaccessible using vertebrate hosts.

Results

To profile the transcriptional response of an infective nematode during the early stage of infection, we performed next generation RNA sequencing on H. bacteriophora IJs incubated in Manduca sexta hemolymph plasma for 9 h. A subset of up-regulated and down-regulated genes were validated using qRT-PCR. Comparative analysis of the transcriptome with untreated controls found a number of differentially expressed genes (DEGs) which cover a number of different functional categories. A subset of DEGs is conserved across Clade V parasitic nematodes revealing an array of candidate parasitic genes.

Conclusions

Our analysis reveals transcriptional changes in the regulation of a large number of genes, most of which have not been shown previously to play a role in the process of infection. A significant proportion of these genes are unique to parasitic nematodes, suggesting the identification of a group of parasitism factors within nematodes. Future studies using these candidates may provide functional insight into the process of nematode parasitism and also the molecular evolution of parasitism within nematodes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3468-6) contains supplementary material, which is available to authorized users.