A blood fluke serine protease inhibitor regulates an endogenous larval elastase.

The larvae of Schistosoma mansoni invade their mammalian host by utilizing a serine protease, cercarial elastase (SmCE), to degrade macromolecular proteins in host skin. The catalytic activity of serine and cysteine proteases can be regulated after activation by serpins. SmSrpQ, one of two S. mansoni serpins found in larval secretions, is only expressed during larval development and in the early stages of mammalian infection. In vitro, (35)S-SmSrpQ was able to form an SDS-stable complex with a component of the larval lysate, but no complex was detected when (35)S-SmSrpQ was incubated with several mammalian host proteases. Formation of a complex was sensitive to the protease active site inhibitors PMSF, Z-AAPF-CMK, and Z-AAPL-CMK. Western blot analysis of parasite lysates from different life stages detected a complex of comparable size to SmCE bound to SmSrpQ using anti-SmSrpQ or anti-SmCE antibodies. SmSrpQ and SmCE are located in adjacent but discrete compartments in the secretion glands of the parasite. Fluorescence immunohistochemical analysis of simulated infection showed co-localization of SmCE and SmSrpQ in host tissue suggesting a post release regulation of parasite protease activity during skin transversal. The results of this study suggest that cercarial elastase degradation of skin tissue is carefully regulated by SmSrpQ.

A multienzyme network functions in intestinal protein digestion by a platyhelminth parasite.

Proteases frequently function not only as individual enzymes but also in cascades or networks. A notable evolutionary switch occurred in one such protease network that is involved in protein digestion in the intestine. In vertebrates, this is largely the work of trypsin family serine proteases, whereas in invertebrates, cysteine proteases of the papain family and aspartic proteases assume the role. Utilizing a combination of protease class-specific inhibitors and RNA interference, we deconvoluted such a network of major endopeptidases functioning in invertebrate intestinal protein digestion, using the parasitic helminth, Schistosoma mansoni as an experimental model. We show that initial degradation of host blood proteins is ordered, occasionally redundant, and substrate-specific. Although inhibition of parasite cathepsin D had a greater effect on primary cleavage of hemoglobin, inhibition of cathepsin B predominated in albumin degradation. Nevertheless, in both cases, inhibitor combinations were synergistic. An asparaginyl endopeptidase (legumain) also synergized with cathepsin B and L in protein digestion, either by zymogen activation or facilitating substrate cleavage. This protease network operates optimally in acidic pH compartments either in the gut lumen or in vacuoles of the intestinal lining cells. Defining the role of each of these major enzymes now provides a clearer understanding of the function of a complex protease network that is conserved throughout invertebrate evolution. It also provides insights into which of these proteases are logical targets for development of chemotherapy for schistosomiasis, a major global health problem.

Functional expression and characterization of Schistosoma mansoni cathepsin B and its trans-activation by an endogenous asparaginyl endopeptidase.

Peptidases are essential for the establishment and survival of the medically important parasite, Schistosoma mansoni. This helminth expresses a number of gut-associated peptidases that degrade host blood proteins, including hemoglobin, as a means of nutrition. Using irreversible affinity probes, we demonstrate that S. mansoni cathepsin B-like endopeptidase 1 (SmCB1) is the most abundant papain family cysteine peptidase in both the parasite gut and somatic extracts. SmCB1 zymogen (SmCB1pm) was functionally expressed in Pichia pastoris (4-11mgl(-1)). Monospecific and immunoselected antibodies raised against SmCB1pm localized the enzyme exclusively to the gut lumen and surrounding gastrodermis of adult worms. Recombinant SmCB1pm was unable to catalyze its activation, even at low pH. However, recombinant S. mansoni asparaginyl endopeptidase (SmAE), another gut-associated cysteine peptidase, processed and activated SmCB1pm in trans. Consistent with the known specificity of AEs, processing occurred on the carboxyl side of an asparagine residue, two residues upstream of the start of the mature SmCB1 sequence. The remaining pro-region dipeptide was removed by rat cathepsin C (dipeptidyl-peptidase I)-an action conceivably performed by an endogenous cathepsin C in vivo. The activated recombinant SmCB1 is biochemically identical to the native enzyme with respect to dipeptidyl substrate kinetics and pH profiles. Also, the serum proteins, hemoglobin, serum albumin, IgG, and alpha-2 macroglobulin were efficiently degraded. Further, a novel application of an assay to measure the peptidyl carboxypeptidase activity of SmCB1 and other cathepsins B was developed using the synthetic substrate benzoyl-glycinyl-histidinyl-leucine (Bz-Gly-His-Leu). This study characterizes the major digestive cysteine peptidase in schistosomes and defines novel trans-processing events required to activate the SmCB1 zymogen in vitro which may facilitate the digestive process in vivo.