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

Ascorbic acid as serine protease inhibitor in lung cancer cell line and human serum albumin

Serine proteases (SPs) are distributed among all living cells accounting for almost one-third of all proteases. Dysregulation of SPs during inflammation and/or infection can result in devastating consequences, such as skin and lung inflammation, neuroinflammation, arthritis, as well as metastasis of cancerous cells. Such activities are tightly regulated by various inhibitors known as serine protease inhibitors (SERPIN). The thermodynamic investigations previously revealed that L-ascorbic acid binds to trypsin more firmly than pepsin and the binding force of L-ascorbic acid is driven by hydrogen bonds and van der Waals forces. However, the physiochemical effects of such interaction on trypsin and/or pepsin have not yet been reported. Ascorbic acid, also known as vitamin C, is one of the essential nutrients and most common food supplements, fortificants, and preservatives. The aim of this study was to explore the inhibitory effects of ascorbic acid on serine proteases at various concentrations on the in-vitro digestion and/or hydrolysis of intercellular matrix of cell monolayer and human serum albumin (HSA). The inhibitory effects of ascorbic on trypsin are investigated by qualitative and quantitative analysis using SDS-PAGE imaging and NIH densitometric software. Upon the addition of ascorbic acid in both indicator systems, the detachment and/or dissociation of cell monolayer and the digestion of HSA were inhibited in the presence of EDTA-Trypsin. The inhibitory effect of ascorbic acid on the digestion of intercellular matrix and/or hydrolysis of HSA showed a dose-dependent trend until it reached the maximum extent of inhibition. At an equal concentration (2.5mg/mL) ascorbic acid and EDTA-Trypsin exhibited the most potent inhibitory effect on the in vitro digestion of protein either in the form of intercellular matrix in cell monolayer and/or HSA respectively. Overall, our results based on two indicator systems strongly indicate that ascorbic acid may function as a serine protease inhibitor (SERPIN) beyond other important functions.

Plasmodium falciparum proteases as new drug targets with special focus on metalloproteases

Malaria poses a significant global health threat particularly due to the prevalence of Plasmodium falciparum infection. With the emergence of parasite resistance to existing drugs including the recently discovered artemisinin, ongoing research seeks novel therapeutic avenues within the malaria parasite. Proteases are promising drug targets due to their essential roles in parasite biology, including hemoglobin digestion, merozoite invasion, and egress. While exploring the genomic landscape of Plasmodium falciparum, it has been revealed that there are 92 predicted proteases, with only approximately 14 of them having been characterized. These proteases are further distributed among 26 families grouped into five clans: aspartic proteases, cysteine proteases, metalloproteases, serine proteases, and threonine proteases. Focus on metalloprotease class shows further role in organelle processing for mitochondria and apicoplasts suggesting the potential of metalloproteases as viable drug targets. Holistic understanding of the parasite intricate life cycle and identification of potential drug targets are essential for developing effective therapeutic strategies against malaria and mitigating its devastating global impact.

Maturation of subtilisin-like protease NbSLP1 from microsporidia Nosema bombycis

Microsporidia are obligate intracellular parasites and possess a unique way of invading hosts, namely germination. Microsporidia are able to infect almost all animal cells by germination. During the process, the polar tube extrudes from the spores within, thus injecting infectious sporoplasm into the host cells. Previous studies indicated that subtilisin-like protease 1 (NbSLP1) of microsporidia Nosema bombycis were located at the polar cap of germinated spores where the polar tube extrusion. We hypothesized that NbSLP1 is an essential player in the germination process. Normally, SLP need to be activated by autoproteolysis under conditions. In this study, we found that the signal peptide of NbSLP1 affected the activation of protease, two self-cleavage sites were involved in NbSLP1 maturation between Ala¹⁰⁴Asp¹⁰⁵ and Ala¹²⁴Asp¹²⁵ respectively. Mutants at catalytic triad of NbSLP1 confirmed the decreasing of autoproteolysis. This study demonstrates that intramolecular proteolysis is required for NbSLP1 maturation. The protease undergoes a series of sequential N-terminal cleavage events to generate the mature enzyme. Like other subtilisin-like enzymes, catalytic triad of NbSLP1 are significant for the self-activation of NbSLP1. In conclusion, clarifying the maturation of NbSLP1 will be valuable for understanding the polar tube ejection mechanism of germination.

Serine protease inhibitors of the whirling disease parasite Myxobolus cerebralis (Cnidaria, Myxozoa): Expression profiling and functional predictions

Here, we studied the expression pattern and putative function of four, previously identified serine protease inhibitors (serpins) of Myxobolus cerebralis, a pathogenic myxozoan species (Cnidaria: Myxozoa) causing whirling disease of salmonid fishes. The relative expression profiles of serpins were determined at different developmental stages both in fish and in annelid hosts using serpin-specific qPCR assays. The expression of serpin Mc-S1 was similar throughout the life cycle, whereas a significant decrease was detected in the relative expression of Mc-S3 and Mc-S5 during the development in fish, and then in the sporogonic stage in the worm host. A decreasing tendency could also be observed in the expression of Mc-S4 in fish, which was, however, upregulated in the worm host. For the first time, we predicted the function of M. cerebralis serpins by the use of several bioinformatics-based applications. Mc-S1 is putatively a chymotrypsin-like inhibitor that locates extracellularly and is capable of heparin binding. The other three serpins are caspase-like inhibitors, and they are probably involved in protease and cell degradation processes during the early stage of fish invasion.

Genetic Diversity of Serine Protease Inhibitors in Myxozoan (Cnidaria, Myxozoa) Fish Parasites

We studied the genetic variability of serine protease inhibitors (serpins) of Myxozoa, microscopic endoparasites of fish. Myxozoans affect the health of both farmed and wild fish populations, causing diseases and mortalities. Despite their global impact, no effective protection exists against these parasites. Serpins were reported as important factors for host invasion and immune evasion, and as promising targets for the development of antiparasitic therapies. For the first time, we identified and aligned serpin sequences from high throughput sequencing datasets of ten myxozoan species, and analyzed 146 serpins from this parasite group together with those of other taxa phylogenetically, to explore their relationship and origins. High intra- and interspecific variability was detected among the examined serpins. The average sequence identity was 25–30% only. The conserved domains (i.e., motif and signature) showed taxon-level differences. Serpins clustered according to taxonomy rather than to serpin types, and myxozoan serpins seemed to be highly divergent from that of other taxa. None of them clustered with their closest relative free-living cnidarians. The genetic distinction of myxozoan serpins further strengthens the idea of an independent origin of Myxozoa, and may indicate novel protein functions potentially related to parasitism in this animal group.

A malaria parasite subtilisin propeptide-like protein is a potent inhibitor of the egress protease SUB1

Subtilisin-like serine peptidases (subtilases) play important roles in the life cycle of many organisms, including the protozoan parasites that are the causative agent of malaria, Plasmodium spp. As with other peptidases, subtilase proteolytic activity has to be tightly regulated in order to prevent potentially deleterious uncontrolled protein degradation. Maturation of most subtilases requires the presence of an N-terminal propeptide that facilitates folding of the catalytic domain. Following its proteolytic cleavage, the propeptide acts as a transient, tightly bound inhibitor until its eventual complete removal to generate active protease. Here we report the identification of a stand-alone malaria parasite propeptide-like protein, called SUB1-ProM, encoded by a conserved gene that lies in a highly syntenic locus adjacent to three of the four subtilisin-like genes in the Plasmodium genome. Template-based modelling and ab initio structure prediction showed that the SUB1-ProM core structure is most similar to the X-ray crystal structure of the propeptide of SUB1, an essential parasite subtilase that is discharged into the parasitophorous vacuole (PV) to trigger parasite release (egress) from infected host cells. Recombinant Plasmodium falciparum SUB1-ProM was found to be a fast-binding, potent inhibitor of P. falciparum SUB1, but not of the only other essential blood-stage parasite subtilase, SUB2, or of other proteases examined. Mass-spectrometry and immunofluorescence showed that SUB1-ProM is expressed in the PV of blood stage P. falciparum, where it may act as an endogenous inhibitor to regulate SUB1 activity in the parasite.

Synthetic DNA Vaccines Adjuvanted with pIL-33 Drive Liver-Localized T Cells and Provide Protection from Plasmodium Challenge in a Mouse Model

The need for a malaria vaccine is indisputable. A single vaccine for Plasmodium pre-erythrocytic stages targeting the major sporozoite antigen circumsporozoite protein (CSP) has had partial success. Additionally, CD8+ T cells targeting liver-stage (LS) antigens induced by live attenuated sporozoite vaccines were associated with protection in human challenge experiments. To further evaluate protection mediated by LS antigens, we focused on exported pre-erythrocytic proteins (exported protein 1 (EXP1), profilin (PFN), exported protein 2 (EXP2), inhibitor of cysteine proteases (ICP), transmembrane protein 21 (TMP21), and upregulated in infective sporozoites-3 (UIS3)) expressed in all Plasmodium species and designed optimized, synthetic DNA (synDNA) immunogens. SynDNA antigen cocktails were tested with and without the molecular adjuvant plasmid IL-33. Immunized animals developed robust T cell responses including induction of antigen-specific liver-localized CD8+ T cells, which were enhanced by the co-delivery of plasmid IL-33. In total, 100% of mice in adjuvanted groups and 71%–88% in non-adjuvanted groups were protected from blood-stage disease following Plasmodium yoelii sporozoite challenge. This study supports the potential of synDNA LS antigens as vaccine components for malaria parasite infection.

Potential role of salt-bridges in the hinge-like movement of apicomplexa specific β-hairpin of Plasmodium and Toxoplasma profilins: A molecular dynamics simulation study

Profilin is one of the actin-binding proteins that regulate dynamics of actin polymerization. It plays a key role in cell motility and invasion. It also interacts with several other proteins notably through its poly-L-proline (PLP) binding site. Profilin in apicomplexa is characterized by a unique mini-domain consisting of a large β-hairpin extension and an acidic loop which is relatively longer in Plasmodium species. Profilin is essential for the invasive blood stages of Plasmodium falciparum. In the current study, unbound profilins from Plasmodium falciparum (Pf), Toxoplasma gondii (Tg), and Homo sapiens (Hs) were subjected to molecular dynamics (MD) simulations for a timeframe of 100 ns each to understand the conformational dynamics of these proteins. It was found that the β-hairpin of profilins from Pf and Tg shows a hinge-like movement. This movement in Pf profilin may possibly be driven by the loss of a salt-bridge within profilin. The impact of this conformational change on actin binding was assessed by docking three dimensional (3D) structures of profilin from Pf and Tg with their corresponding actins using ClusPro2.0. The stability of docked Pf profilin-actin complex was assessed through a 50 ns MD simulation. As Hs profilin I does not have the apicomplexa specific mini-domain, MD simulation was performed for this protein and its dynamics was compared to that of profilins from Pf and Tg. Using an immunoinformatics approach, potential epitope regions were predicted for Pf profilin. This has a potential application in the design of vaccines as they mapped to its unique mini-domain.

A critical review on serine protease: Key immune manipulator and pathology mediator

Proteolytic activity is fundamental to survival, so it is not surprising that all living organisms have proteases, especially seine protease. This enzyme in its numerous isoforms and homologues, constitutes the quintessential offence and defence factors, in the form of surface proteins, secreted molecules, gut digestive enzymes, venom in specialised glands or plant latex, among other manifestations. Occurring as trypsin, chymotrypsin, elastase, collagenase, thrombin, subtilisin etc., it mediates a diverse array of functions, including pathological roles as inflammatory, coagulatory to haemorrhagic. This review emphasizes that despite the superficial differences in mechanisms, most health issues, be they infectious, allergic, metabolic, or neural have a common conduit. This enzyme, in its various glycosylated forms leads to signal misinterpretations, wreaking havoc. However, organisms are endowed with serine protease inhibitors which might restrain this ubiquitous yet deleterious enzyme. Hence, serine proteases-driven pathogenesis and antagonising role of inhibitors is the focal point of this critical review.

Plasmodium berghei PIMMS2 Promotes Ookinete Invasion of the Anopheles gambiae Mosquito Midgut

Mosquito midgut stages of the malaria parasite present an attractive biological system to study host-parasite interactions and develop interventions to block disease transmission. Mosquito infection ensues upon oocyst development that follows ookinete invasion and traversal of the mosquito midgut epithelium. Here, we report the characterization of PIMMS2 (Plasmodium invasion of mosquito midgut screen candidate 2), a Plasmodium berghei protein with structural similarities to subtilisin-like proteins. PIMMS2 orthologs are present in the genomes of all plasmodia and are mapped between the subtilisin-encoding genes SUB1 and SUB3. P. berghei PIMMS2 is specifically expressed in zygotes and ookinetes and is localized on the ookinete surface. Loss of PIMMS2 function through gene disruption by homologous recombination leads to normal development of motile ookinetes that exhibit a severely impaired capacity to traverse the mosquito midgut and transform to oocysts. Genetic complementation of the disrupted locus with a mutated PIMMS2 allele reveals that amino acid residues corresponding to the putative subtilisin-like catalytic triad are important but not essential for protein function. Our data demonstrate that PIMMS2 is a novel ookinete-specific protein that promotes parasite traversal of the mosquito midgut epithelium and establishment of mosquito infection.

Every member of the kingdom Animalia is a potential vector of human pathogens

Zoonotic diseases are a subset of infectious diseases, which account for enormous morbidity and mortality. Pathologies like malaria, rabies, Lyme disease, leptospirosis, avian flu etc. are microbe- and parasite-caused ailments, where the etiological agents are introduced into or on the human body via ticks, mosquitoes, birds, rodents, bats, and deer, among other members of kingdom Animalia. While some of the zoonotic diseases are well-investigated and caution taken against, a lot many are yet to be recognized. This ignorance costs health, and lives, especially in developing countries. To promote awareness regarding the risks of immunogenicity and pathogen dissemination by hitherto unknown non-plant organisms, the members of kingdom Animalia, this letter has been compiled. The vector exploitation mechanisms of the pathogens, and in silico evidences of conserved protein domains across the potential pathogen reservoirs have been mentioned to underline the importance of this topic.

Trichinella spiralis newborn larvae: characterization of a stage specific serine proteinase expression, NBL1, using monoclonal antibodies

Trichinella spiralis is an intracellular parasitic nematode of mammalian skeletal muscle, causing a serious zoonotic disease in humans and showing a high economic impact mainly in pig breeding. Serine proteinases of T. spiralis play important roles in the host-parasite interactions mediating host invasion. In this study, we have focused on newborn larvae (NBL-1), the first identified serine proteinase from the NBL stage of T. spiralis. Five monoclonal antibodies (mAbs) directed against the C-terminal part of NBL1, were produced. These mAbs were IgG1κ isotype and specifically recognized as a common motif of 10 amino acids (PSSGSRPTYP). Selected mAbs were further characterized using antigens from various developmental stages of T. spiralis. Western blot revealed that selected mAbs reacted with the native NBL1 at Mr 50 kDa in the adult and NBL mixed antigens and NBL stage alone. Indirect immunofluorescence analysis revealed that selected mAbs intensely stained only the embryos within the gravid females and the NBL. Thus, the produced mAbs are useful tools for the characterization of NBL1 as a major antigen of Trichinella involved in the invasion of the host but also for the development of new serological tests with an early detection of T. spiralis infection.

The genome of the myxosporean Thelohanellus kitauei shows adaptations to nutrient acquisition within its fish host

Members of Myxozoa, a parasitic metazoan taxon, have considerable detrimental effects on fish hosts and also have been associated with human food-borne illness. Little is known about their biology and metabolism. Analysis of the genome of Thelohanellus kitauei and comparative analysis with genomes of its two free-living cnidarian relatives revealed that T. kitauei has adapted to parasitism, as indicated by the streamlined metabolic repertoire and the tendency toward anabolism rather than catabolism. Thelohanellus kitauei mainly secretes proteases and protease inhibitors for nutrient digestion (parasite invasion), and depends on endocytosis (mainly low-density lipoprotein receptors-mediated type) and secondary carriers for nutrient absorption. Absence of both classic and complementary anaerobic pathways and gluconeogenesis, the lack of de novo synthesis and reduced activity in hydrolysis of fatty acids, amino acids, and nucleotides indicated that T. kitauei in this vertebrate host-parasite system has adapted to inhabit a physiological environment extremely rich in both oxygen and nutrients (especially glucose), which is consistent with its preferred parasitic site, that is, the host gut submucosa. Taking advantage of the genomic and transcriptomic information, 23 potential nutrition-related T. kitauei-specific chemotherapeutic targets were identified. This first genome sequence of a myxozoan will facilitate development of potential therapeutics for efficient control of myxozoan parasites and ultimately prevent myxozoan-induced fish-borne illnesses in humans.

A novel Plasmodium-specific prodomain fold regulates the malaria drug target SUB1 subtilase

The Plasmodium subtilase SUB1 plays a pivotal role during the egress of malaria parasites from host hepatocytes and erythrocytes. Here we report the crystal structure of full-length SUB1 from the human-infecting parasite Plasmodium vivax, revealing a bacterial-like catalytic domain in complex with a Plasmodium-specific prodomain. The latter displays a novel architecture with an amino-terminal insertion that functions as a 'belt', embracing the catalytic domain to further stabilize the quaternary structure of the pre-protease, and undergoes calcium-dependent autoprocessing during subsequent activation. Although dispensable for recombinant enzymatic activity, the SUB1 'belt' could not be deleted in Plasmodium berghei, suggesting an essential role of this domain for parasite development in vivo. The SUB1 structure not only provides a valuable platform to develop new anti-malarial candidates against this promising drug target, but also defines the Plasmodium-specific 'belt' domain as a key calcium-dependent regulator of SUB1 during parasite egress from host cells.

Serine Proteases of Malaria Parasite Plasmodium falciparum: Potential as Antimalarial Drug Targets

Malaria is a major global parasitic disease and a cause of enormous mortality and morbidity. Widespread drug resistance against currently available antimalarials warrants the identification of novel drug targets and development of new drugs. Malarial proteases are a group of molecules that serve as potential drug targets because of their essentiality for parasite life cycle stages and feasibility of designing specific inhibitors against them. Proteases belonging to various mechanistic classes are found in P. falciparum, of which serine proteases are of particular interest due to their involvement in parasite-specific processes of egress and invasion. In P. falciparum, a number of serine proteases belonging to chymotrypsin, subtilisin, and rhomboid clans are found. This review focuses on the potential of P. falciparum serine proteases as antimalarial drug targets.