Isolation and characterization of a novel serine protease inhibitor, SjSPI, from Schistosoma japonicum

Surviving in the fish gut: Comparative inhibitory capacities against the host proteinases in cestodes of the genus Proteocephalus

Currently, little is known about inhibitory substances enabling tapeworms to settle in fish intestines thereby avoiding proteolysis. Contrary to previous studies with certain host-parasite pairs, this research compares the inhibitory capacities in three tapeworm species of the same genus Proteocephalus from four different fishes (P. torulosus from dace and zope, P. sagittus from stone loach and P. cernuae from ruffe). The tapeworm extracts studied significantly reduced the activity of commercial trypsin (although to a lesser degree than the synthetic inhibitor of serine proteinases PMSF), displaying clear inter-specific variation in worms' inhibitory ability. We also measured the proteolytic activity of the host intestinal mucosa exposed to tapeworm extracts which served as inhibitors. Based on per cent inhibition values, all tapeworm extracts significantly suppressed the mucosal proteolytic activity, with marked differences between certain host-parasite pairs. SDS-PAGE electrophoresis of the incubation media and extracts detected in each tapeworm species 20-36 protein bands with apparent molecular weights from 10-12 to 312.5 kDa, mostly below 50 kDa. The incubation medium and extract of each parasite shared one to six bands ranging from 12 to 35 kDa, depending on its species, with only four bands common for two or more species. The band profiles suggest that in various Proteocephalus species inhibitory capacities against host proteinases can be ensured by different proteins.

Localization of the proteinase inhibitor activity in the fish cestode Eubothrium rugosum

The mechanisms enabling fish tapeworms to avoid proteolytic attacks by digestive enzymes of their fish host have been studied in less detail compared with mammalian cestodes. This study aimed to assess the inhibitory ability towards trypsin and chymotrypsin in Eubothrium rugosum, an intestinal parasite of burbot Lota lota, and establish its localization in the tapeworm. To this end, the worms were treated with Triton X-100 followed by differential centrifugation to isolate the tegumental brush border membrane. The protease inhibitory abilities of the worms were mostly determined by their excretory/secretory products released into the incubation medium. These inhibitory abilities proved to be linked mainly with the brush border fractions. Notably, the per cent inhibition of both studied digestive enzymes (trypsin and chymotrypsin) hardly depended on the duration of the parasite exposure in the incubation medium, probably due to intermittent glycocalyx renewal. Improved knowledge on functions of the excretory/secretory proteins produced by fish tapeworms may contribute to a better understanding of host-parasite relations and development of new approaches to the treatment and prevention of diseases caused by pathogenic helminths.

The trypsin inhibitor-like domain is required for a serine protease inhibitor of Haemonchus contortus to inhibit host coagulation

Haemonchus contortus, a blood-feeding nematode, inhibits blood coagulation at the site of infection to facilitate blood-sucking and digesting for successful parasitism. However, the mechanism underlying anti-coagulation at the host-parasite interface is largely unknown. In the current study, Hc-spi-i8, which has two greatly different transcripts named Hc-spi-i8a and Hc-spi-i8b, respectively, was described. Hc-SPI-I8A was a serine protease inhibitor containing a trypsin inhibitor-like cysteine rich (TIL) domain, while Hc-SPI-I8B was not. Hc-SPI-I8A/B were primarily expressed in the hypodermis, intestines and gonads in the parasitic stages of H. contortus. Hc-SPI-I8A interacted with Ovis aries TSP1-containing protein (OaTSP1CP), which was determined by yeast two-hybrid, co-immunoprecipitation (Co-IP), pull down and co-localization experiments. The blood clotting time contributed by the TIL domain was prolonged by Hc-SPI-I8A. Hc-SPI-I8A is most likely interfering in the extrinsic coagulation cascade by interacting with OaTSP1CP through its TIL domain and intrinsic coagulation cascade by an unknown mechanism. These findings depict a crucial point in the host-parasite interaction during H. contortus colonization, which should contribute to drug discovery and vaccine development in fighting against this important parasite worldwide.

Serpin-type serine protease inhibitor mediates coelomocyte apoptosis in Apostichopus japonicus

Serine protease inhibitors (SPIs, serpins) are a protein superfamily involved in almost all physiological processes in all organisms. In this study, a novel serpin was identified from Apostichopus japonicus (Ajserpin) by using high-throughput sequencing and RACE approaches. The full-length cDNA of Ajserpin was 1893 bp with a 5'-untranslated region (UTR) of 130 bp, a 3'-UTR of 587 bp, and an open reading frame of 1176 bp encoding a polypeptide of 391 amino acids with a deduced molecular weight of 43.8 kDa. Ajserpin shares the standard structure of SPI, including three β-sheets and eight α-helices. The deduced amino acid sequences of Ajserpin had no nuclear location signal and signal peptide structure. The phylogenetic tree and immunofluorescence showed that Ajserpin belonged to the clade B subfamily and was mainly located in the cytoplasm and nucleus. Sequence comparison and protein inhibition experiments showed that the active site (P1-P1' site) of Ajserpin was Arginine and Serine, which displayed inhibitory activity toward trypsin in a dose-dependent manner. Tissue distribution analysis showed that Ajserpin transcripts were constitutively expressed in all examined tissues with the peak in the body wall. Ajserpin mRNA transcripts could be induced in Vibrio splendidus-challenged sea cucumber or lipopolysaccharide-exposed coelomocytes. Furthermore, Ajserpin knockdown by small interfering RNAs could inhibit coelomocytes apoptosis. All our results revealed that Ajserpin might serve as an immune regulator in sea cucumber.

Fasciola hepatica serine protease inhibitor family (serpins): Purposely crafted for regulating host proteases

Serine protease inhibitors (serpins) regulate proteolytic events within diverse biological processes, including digestion, coagulation, inflammation and immune responses. The presence of serpins in Fasciola hepatica excretory-secretory products indicates that the parasite exploits these to regulate proteases encountered during its development within vertebrate hosts. Interrogation of the F. hepatica genome identified a multi-gene serpin family of seven members that has expanded by gene duplication and divergence to create an array of inhibitors with distinct specificities. We investigated the molecular properties and functions of two representatives, FhSrp1 and FhSrp2, highly expressed in the invasive newly excysted juvenile (NEJ). Consistent with marked differences in the reactive centre loop (RCL) that executes inhibitor-protease complexing, the two recombinant F. hepatica serpins displayed distinct inhibitory profiles against an array of mammalian serine proteases. In particular, rFhSrp1 efficiently inhibited kallikrein (K_i = 40 nM) whilst rFhSrp2 was a highly potent inhibitor of chymotrypsin (K_i = 0.07 nM). FhSrp1 and FhSrp2 are both expressed on the NEJ surface, predominantly around the oral and ventral suckers, suggesting that these inhibitors protect the parasites from the harmful proteolytic effects of host proteases, such as chymotrypsin, during invasion. Furthermore, the unusual inhibition of kallikrein suggests that rFhSrp1 modulates host responses such as inflammation and vascular permeability by interfering with the kallikrein-kinin system. A vaccine combination of rFhSrp1 and rFhSrp2 formulated in the adjuvant Montanide ISA 206VG elicited modest but non-significant protection against a challenge infection in a rat model, but did induce some protection against liver pathogenesis when compared to a control group and a group vaccinated with two well-studied vaccine candidates, F. hepatica cathepsin L2 and L3. This work highlights the importance of F. hepatica serpins to regulate host responses that enables parasite survival during infection and, coupled with the vaccine data, encourages future vaccine trials in ruminants.

Serpins are protease inhibitors that regulate various biological processes, including digestion, blood coagulation, inflammation and immune responses. The liver fluke, Fasciola hepatica, produces an array of inhibitors to regulate proteolytic enzymes they encounter during development within the mammalian host. In this study, we identified seven different serpins that have evolved to inhibit a range of host proteases. In particular, we characterized two representatives, FhSrp1 and FhSrp2, that we found highly expressed on the surface of the invasive newly excysted juvenile (NEJ), suggesting that they protect the parasites from harmful proteolytic effects during invasion. Contrasting inhibitory profiles were observed; while recombinant FhSrp1 inhibited kallikrein, recombinant FhSrp2 was a highly potent inhibitor of chymotrypsin. The unusual inhibition of kallikrein suggests that rFhSrp1 influences host responses such as inflammation and vascular permeability by interfering with the kallikrein-kinin system. Conversely, chymotrypsin is typically inhibited by trematode-specific serpins, implying a conserved mechanism to regulate digestive enzymes. The ability of the liver fluke serpin family to inhibit such an array of proteases highlights the importance of these inhibitors in parasite-host interactions and encourages future investigations of serpins as candidate anti-parasite vaccine targets for the control of fasciolosis in ruminants.

Serpins in Fasciola hepatica: insights into host-parasite interactions

Protease inhibitors play crucial roles in parasite development and survival, modulating the immune responses of their vertebrate hosts. Members of the serpin family are irreversible inhibitors of serine proteases and regulate systems related to defence against parasites. Limited information is currently available on protease inhibitors from the liver fluke Fasciola hepatica. In this study, we characterised four serpins from F. hepatica (FhS-1-FhS-4). Biochemical characterisation revealed that recombinant FhS-2 (rFhS) inhibits the activity of human neutrophil cathepsin G, while rFhS-4 inhibits the activity of bovine pancreatic chymotrypsin and cathepsin G. Consistent with inhibitor function profiling data, rFhS-4 inhibited cathepsin G-activated platelet aggregation in a dose-responsive manner.Similar to other serpins, rFhS2 and rFhS-4 bind to heparin with high affinity. Tissue localisation demonstrated that these serpins have different spatial distributions. FhS-2 is localised in the ovary, while FhS-4 was found in gut cells. Both of them co-localised in the spines within the tegument. These findings provide the basis for study of functional roles of these proteins as part of an immune evasion mechanism in the adult fluke, and in protection of eggs to ensure parasite life cycle continuity. Further understanding of serpins from the liver fluke may lead to the discovery of novel anti-parasitic interventions.

Molecular cloning and characterization of serine protease inhibitor from food-borne nematode, Gnathostoma spinigerum

Gnathostoma spinigerum is a causative agent of human gnathostomiasis and infects people residing in endemic areas as well as travelers. Cutaneous and visceral larval migrants cause clinical manifestations, resulting in severe morbidity and mortality. To survive in hosts, these parasites have evolved various immune evasion mechanisms, including the release of regulatory molecules. Serine protease inhibitors (serpins) that are present in many parasitic helminths are proteins suspected of suppressing host serine protease-related digestion and immune responses. In this study, the serpin secreted by G. spinigerum (GsSerp) was characterized using bioinformatics and molecular biology techniques. The bioinformatics revealed that GsSerp contains 9 helices, 3 β-sheets, and a reactive central loop, which are conserved structures of the serpin superfamily. Recombinant GsSerp (rGsSerp) was expressed in E. coli (molecular weight, 39 kDa) and could inhibit chymotrypsin. Mouse polyclonal antibody against GsSerp could detect the native GsSerp in crude worm antigen but not the excretory-secretory product (ES) of infective-stage larva (aL3Gs). Moreover, the expression of GsSerp in the aL3Gs tissue was located in the hemolymph and intestinal tissue, indicating its role in parasite homeostasis. Our findings may help develop effective strategies for preventing and controlling gnathostomiasis.

A serpin from the tick Rhipicephalus haemaphysaloides: Involvement in vitellogenesis

Tick serpins are involved in enzyme activity, food digestion, blood-feeding, immune response and anticoagulation. Little is known about the potential roles of serpins in tick reproduction. RHS8, a serpin from the tick Rhipicephalus haemaphysaloides, has an open reading frame 1212 bp long and encodes a protein that has 404 amino acids and a predicted molecular weight of 45 kDa. RHS8 exhibits 89.58 % amino acid identity with RmS15 in Rhipicephalus microplus. RHS8 was expressed primarily in larvae and nymphs. RHS8 mRNA expression in the ovaries, fat bodies and salivary glands were up-regulated from feeding to ovipositing ticks. RNAi results showed that RHS8 dsRNA-injected ticks had a lower body weight, longer feeding time, fewer eggs laid and lower egg hatchability. Tick reproduction, such as egg laying and hatching, was disrupted by RNAi. Compared with the control group, ovaries of the RHS8 interference group were light brown color, indicating a reduction in yolk granule accumulation. Western blot results showed that the expression of RHVg3 and RHVg4 proteins in ovaries was reduced in the RHS8 dsRNA-injected group. These results indicate that RHS8 is related to tick reproduction and its interference affects vitellogenesis.

A membrane-associated metalloprotease of Schistosoma japonicum structurally related to the FACE-1/Ste24p protease family

The CaaX proteases are closely related in the post-translational modification of many membrane-bound or secreted proteins and play a key role in the activation or stabilization of these molecules belonging to the CAAX family. In this study, a full-length cDNA putatively encoding a FACE-1/Ste24p CaaX protease (type I) of the Schistosoma japonicum was isolated. The cDNA, named SjSte24p, composed of 1646 bp and encoded 473 amino acids with predicted Mr/pI as 54.77 kDa/8.04. SjSte24p is a monoexonic gene constantly expressed in the parasite from cercariae to adult stages. It contained the characteristic of CaaX protease topology, including seven trans-membrane domains and a metallo-protease segment with a zinc-binding motif (HEXXH). SjSte24p shared a considerable degree of sequence identity with the type I CaaX proteases. A phylogenetic analysis showed that this protein family is tightly conserved from fungi to vertebrates. The expressed recombinant SjSte24p protein showed a proteolytic activity, which was inhibited by EDTA. Its activity was increased at low doses of the Zn²⁺ (0.001-0.01 mM); but was reversibly down-regulated at high doses (>0.1 mM). The native SjSte24p appeared to function in insoluble from. The protein was mainly localized in the tegument on the surface of adult worms. These results indicated that the SjSte24p is a practical zinc-dependent metalloprotease, which belongs to the FACE-1/Ste24p protease family.

Transcriptome and excretory-secretory proteome of infective-stage larvae of the nematode Gnathostoma spinigerum reveal potential immunodiagnostic targets for development

Background: Gnathostoma spinigerum is a harmful parasitic nematode that causes severe morbidity and mortality in humans and animals. Effective drugs and vaccines and reliable diagnostic methods are needed to prevent and control the associated diseases; however, the lack of genome, transcriptome, and proteome databases remains a major limitation. In this study, transcriptomic and secretomic analyses of advanced third-stage larvae of G. spinigerum (aL3Gs) were performed using next-generation sequencing, bioinformatics, and proteomics. Results: An analysis that incorporated transcriptome and bioinformatics data to predict excretory–secretory proteins (ESPs) classified 171 and 292 proteins into classical and non-classical secretory groups, respectively. Proteins with proteolytic (metalloprotease), cell signaling regulatory (i.e., kinases and phosphatase), and metabolic regulatory function (i.e., glucose and lipid metabolism) were significantly upregulated in the transcriptome and secretome. A two-dimensional (2D) immunomic analysis of aL3Gs-ESPs with G. spinigerum-infected human sera and related helminthiases suggested that the serine protease inhibitor (serpin) was a promising antigenic target for the further development of gnathostomiasis immunodiagnostic methods. Conclusions: The transcriptome and excretory–secretory proteome of aL3Gs can facilitate an understanding of the basic molecular biology of the parasite and identifying multiple associated factors, possibly promoting the discovery of novel drugs and vaccines. The 2D-immunomic analysis identified serpin, a protein secreted from aL3Gs, as an interesting candidate for immunodiagnosis that warrants immediate evaluation and validation.