The Caenorhabditis elegans muscle specific serpin, SRP-3, neutralizes chymotrypsin-like serine peptidases

Evaluation of proteases and protease inhibitors in Heterodera glycines cysts obtained from laboratory and field populations

Proteases and protease inhibitors were evaluated in preparations ofHeterodera glycinescysts (nHglCE) obtained from glasshouse cultures (GH) and a field (LR) population. Using a FRET-peptide library comprising 512 peptide substrate pools that detect four endoprotease types (aspartic, cysteine, metallo- and serine), we found that the relative distributions of six endoproteases within the four catalytic types were similar among GH and LR preparations. However, levels of mean protease activity ( s−1(μg nHglCE)−1) across all 512 pools varied nearly eight-fold among the preparations. This variation was not related to cyst source. These qualitative (type distribution) and quantitative relationships persisted when analysis was restricted to the top 40% (activity) pools. Analysis of the top 4% of activity pools revealed some substrate cleavage site preferences between the GH and LR proteases. GH and LR preparations also differed significantly in digestion rates of trypsin and matrix metalloprotease (MMP) substrates, with LR rates two-fold greater than GH rates for each protease. By contrast, inhibition of trypsin activity inMeloidogyne incognitaextracts by heat-denatured preparations (hHglCE) was 1.6-fold greater with GH hHglCE than with LR hHglCE. Inhibition of MMP activity was the same (>60%) for each hHglCE. Fractionation of GH and LR hHglCE preparations by RP-HPLC (CH3CN/0.1% TFA system) yielded protease inhibition profiles that were similar for GH and LR. MMP inhibitors eluted at 35-40% CH3CN, and trypsin inhibitors eluted at both 5% and 35-40% CH3CN, suggesting the possibility of peptide as well as other small molecular weight inhibitors. These discoveries illustrate the importance of examiningH. glycinescysts as a source of materials for novel nematode controls.

Heterodera glycines cysts contain an extensive array of endoproteases as well as inhibitors of proteases in H. glycines and Meloidogyne incognita infective juvenile stages

Heterodera glycinescysts contain proteases, and inhibitors of protease activities in various nematode species. In this investigation, proteases inH. glycinescysts were identified using a commercially available FRET-peptide library comprising 512 peptide pools qualified to detect up to four endoprotease types (aspartic, cysteine, metallo- and serine). Native cyst content (nHglCE) digested peptides in over 96% of the pools with all four protease types identified. Serine and metalloproteases represented nearly 70% of all proteases detected and were examined further. Trypsin (serine) and matrix metalloprotease (MMP) activities were compared among nHglCE, andH. glycinessecond-stage juvenile (J2) andMeloidogyne incognitaJ2 extracts. The relative levels of activity were different for all three enzyme sources. Trypsin activity was up to 60-fold greater inM. incognitathan in eitherH. glycinessource, while MMP activity was highest in nHglCE and lowest inM. incognitaJ2. Heat-denatured cyst content (hHglCE) inhibited proteases in all three nematode preparations and was generally greater inM. incognitathan inH. glycines. Largest differences (5.2- to 6.4-fold) were observed betweenM. incognitaand nHglCE trypsin and MMP inhibition. In infective juveniles, hHglCE inhibitedM. incognitaJ2 trypsin (IC50 = 0.64 hHglCEeq reaction−1) and MMP (IC50 = 0.54) more potently than eitherH. glycinestrypsin (IC50 = 1.34) or MMP (IC50 = 1.84). Use of three MMP substrates (73, 74 and 80) revealed clear species differences as well as complex associations between activity and inhibition. MMP73 digestion rates were the same inH. glycinesandM. incognitabut responses to hHglCE inhibition were different. MMP80 digestion rates were different but inhibition was the same. MMP74 digestion rates and inhibition levels were each different between species. These experiments provide further evidence that theH. glycinescyst should be examined as a source of compounds useful for developing nematode control methods.

The aggregation-prone intracellular serpin SRP-2 fails to transit the ER in Caenorhabditis elegans

Familial encephalopathy with neuroserpin inclusions bodies (FENIB) is a serpinopathy that induces a rare form of presenile dementia. Neuroserpin contains a classical signal peptide and like all extracellular serine proteinase inhibitors (serpins) is secreted via the endoplasmic reticulum (ER)-Golgi pathway. The disease phenotype is due to gain-of-function missense mutations that cause neuroserpin to misfold and aggregate within the ER. In a previous study, nematodes expressing a homologous mutation in the endogenous Caenorhabditis elegans serpin, srp-2, were reported to model the ER proteotoxicity induced by an allele of mutant neuroserpin. Our results suggest that SRP-2 lacks a classical N-terminal signal peptide and is a member of the intracellular serpin family. Using confocal imaging and an ER colocalization marker, we confirmed that GFP-tagged wild-type SRP-2 localized to the cytosol and not the ER. Similarly, the aggregation-prone SRP-2 mutant formed intracellular inclusions that localized to the cytosol. Interestingly, wild-type SRP-2, targeted to the ER by fusion to a cleavable N-terminal signal peptide, failed to be secreted and accumulated within the ER lumen. This ER retention phenotype is typical of other obligate intracellular serpins forced to translocate across the ER membrane. Neuroserpin is a secreted protein that inhibits trypsin-like proteinase. SRP-2 is a cytosolic serpin that inhibits lysosomal cysteine peptidases. We concluded that SRP-2 is neither an ortholog nor a functional homolog of neuroserpin. Furthermore, animals expressing an aggregation-prone mutation in SRP-2 do not model the ER proteotoxicity associated with FENIB.

Functional characterization of SjB10, an intracellular serpin from Schistosoma japonicum

SUMMARY Serine protease inhibitors (serpin) play essential roles in many organisms. Mammalian serpins regulate the blood coagulation, fibrinolysis, inflammation and complement activation pathways. In parasitic helminths, serpins are less well characterized, but may also be involved in evasion of the host immune response. In this study, a Schistosoma japonicum serpin (SjB10), containing a 1212 bp open reading frame (ORF), was cloned, expressed and functionally characterized. Sequence analysis, comparative modelling and structural-based alignment revealed that SjB10 contains the essential structural motifs and consensus secondary structures of inhibitory serpins. Transcriptional profiling demonstrated that SjB10 is expressed in adult males, schistosomula and eggs but particularly in the cercariae, suggesting a possible role in cercarial penetration of mammalian host skin. Recombinant SjB10 (rSjB10) inhibited pancreatic elastase (PE) in a dose-dependent manner. rSjB10 was recognized strongly by experimentally infected rat sera indicating that native SjB10 is released into host tissue and induces an immune response. By immunochemistry, SjB10 localized in the S. japonicum adult foregut and extra-embryonic layer of the egg. This study provides a comprehensive demonstration of sequence and structural-based analysis of a functional S. japonicum serpin. Furthermore, our findings suggest that SjB10 may be associated with important functional roles in S. japonicum particularly in host-parasite interactions.

Characterisation of a secretory serine protease inhibitor (SjB6) from Schistosoma japonicum

BACKGROUND

Proteins belonging to the serine protease inhibitor (serpin) superfamily play essential physiological roles in many organisms. In pathogens, serpins are thought to have evolved specifically to limit host immune responses by interfering with the host immune-stimulatory signals. Serpins are less well characterised in parasitic helminths, although some are thought to be involved in mechanisms associated with host immune modulation. In this study, we cloned and partially characterised a secretory serpin from Schistosoma japonicum termed SjB6, these findings provide the basis for possible functional roles.

METHODS

SjB6 gene was identified through database mining of our previously published microarray data, cloned and detailed sequence and structural analysis and comparative modelling carried out using various bioinformatics and proteomics tools. Gene transcriptional profiling was determined by real-time PCR and the expression of native protein determined by immunoblotting. An immunological profile of the recombinant protein produced in insect cells was determined by ELISA.

RESULTS

SjB6 contains an open reading frame of 1160 base pairs that encodes a protein of 387 amino acid residues. Detailed sequence analysis, comparative modelling and structural-based alignment revealed that SjB6 contains the essential structural motifs and consensus secondary structures typical of inhibitory serpins. The presence of an N-terminal signal sequence indicated that SjB6 is a secretory protein. Real-time data indicated that SjB6 is expressed exclusively in the intra-mammalian stage of the parasite life cycle with its highest expression levels in the egg stage (p < 0.0001). The native protein is approximately 60 kDa in size and recombinant SjB6 (rSjB6) was recognised strongly by sera from rats experimentally infected with S. japonicum.

CONCLUSIONS

The significantly high expression of SjB6 in schistosome eggs, when compared to other life cycle stages, suggests a possible association with disease pathology, while the strong reactivity of sera from experimentally infected rats against rSjB6 suggests that native SjB6 is released into host tissue and induces an immune response. This study presents a comprehensive demonstration of sequence and structural-based analysis of a secretory serpin from a trematode and suggests SjB6 may be associated with important functional roles in S. japonicum, particularly in parasite modulation of the host microenvironment.

The biochemical and immunological characterization of two serpins from Clonorchis sinensis

Serpins (serine proteinase inhibitors) are evidenced to regulate numerous biological processes such as immunoregulation in parasitic helminths. The functions of serpins from Clonorchis sinensis remain unclear to date. In this study, two serpin genes, respectively denominated as CsproSERPIN and CsSERPIN2, had been selected from metacercaria cDNA library of C. sinensis. The biochemical activities of both recombinant proteins (rCsproSERPIN and rCsSERPIN2) were analyzed by assays of inhibition on some serine or cysteine proteases, the results showed that rCsproSERPIN significantly inhibited trypsin, chymotrypsin and thrombin, while rCsSERPIN2 inhibited only chymotrypsin. Moreover, cytokine and antibody measurements indicated that rats subcutaneously immunized with rCsproSERPIN and rCsSERPIN2 respectively developed a strong IFN-γ production and IgG2a levers of sera were higher than IgG1. Besides, immunoblot assays revealed that the rCsproSERPIN and rCsSERPIN2 could be recognized by the sera of rats infected with C. sinensis and the sera of rabbits immunized by excretory/secretory products. Furthermore, immunofluorescence assays illuminated the two were similarly localized in the reproductive organs such as vitelline glands, testis and eggs in adult stage. In short, all the results collectively indicated that CsproSERPIN and CsSERPIN2 might play important role in the parasite development by preventing the parasite from digestion by exogenous serine proteases, as well as CsproSERPIN and CsSERPIN2 probably involved in immunoregulation of host by inducing Th1-biased type cytokines in rats.

Using C. elegans to identify the protease targets of serpins in vivo

Most serpins inhibit serine and/or cysteine proteases, and their inhibitory activities are usually defined in vitro. However, the physiological protease targets of most serpins are unknown despite many years of research. This may be due to the rapid degradation of the inactive serpin:protease complexes and/or the conditions under which the serpin inhibits the protease. The model organism Caenorhabditis elegans is an ideal system for identifying protease targets due to powerful forward and reverse genetics, as well as the ease of creating transgenic animals. Using combinatorial approaches of genetics and biochemistry in C. elegans, the true in vivo protease targets of the endogenous serpins can be elucidated.

Serpins flex their muscle: I. Putting the clamps on proteolysis in diverse biological systems

Serpins compose the largest superfamily of peptidase inhibitors and are well known as regulators of hemostasis and thrombolysis. Studies using model organisms, from plants to vertebrates, now show that serpins and their unique inhibitory mechanism and conformational flexibility are exploited to control proteolysis in molecular pathways associated with cell survival, development, and host defense. In addition, an increasing number of non-inhibitory serpins are emerging as important elements within a diversity of biological systems by serving as chaperones, hormone transporters, or anti-angiogenic factors.

Peptidase inhibitors in the MEROPS database

The MEROPS website (http://merops.sanger.ac.uk) includes information on peptidase inhibitors as well as on peptidases and their substrates. Displays have been put in place to link peptidases and inhibitors together. The classification of protein peptidase inhibitors is continually being revised, and currently inhibitors are grouped into 67 families based on comparisons of protein sequences. These families can be further grouped into 38 clans based on comparisons of tertiary structure. Small molecule inhibitors are important reagents for peptidase characterization and, with the increasing importance of peptidases as drug targets, they are also important to the pharmaceutical industry. Small molecule inhibitors are now included in MEROPS and over 160 summaries have been written.

Serpins in plants and green algae

Control of proteolysis is important for plant growth, development, responses to stress, and defence against insects and pathogens. Members of the serpin protein family are likely to play a critical role in this control through irreversible inhibition of endogenous and exogenous target proteinases. Serpins have been found in diverse species of the plant kingdom and represent a distinct clade among serpins in multicellular organisms. Serpins are also found in green algae, but the evolutionary relationship between these serpins and those of plants remains unknown. Plant serpins are potent inhibitors of mammalian serine proteinases of the chymotrypsin family in vitro but, intriguingly, plants and green algae lack endogenous members of this proteinase family, the most common targets for animal serpins. An Arabidopsis serpin with a conserved reactive centre is now known to be capable of inhibiting an endogenous cysteine proteinase. Here, knowledge of plant serpins in terms of sequence diversity, inhibitory specificity, gene expression and function is reviewed. This was advanced through a phylogenetic analysis of amino acid sequences of expressed plant serpins, delineation of plant serpin gene structures and prediction of inhibitory specificities based on identification of reactive centres. The review is intended to encourage elucidation of plant serpin functions.