UCSF

Serine protease inhibitors (serpins) are a superfamily of structurally conserved proteins that are key regulators of proteolysis in plants, animals, and certain viruses. The S. mansoni genome contains eight full-length serpins. Semi-quantitative PCRs performed on S. mansoni cDNA samples verified the transcription of these eight putative serpins. All identified serpin sequences had homology to other members of the serpin family and contained all the structural components needed for functionality. Some schistosome serpins are predicted to be non-inhibitory while others showed clear tryptic or chymotryptic protease specificity. Two of these serpins, SmSrpQ and SmSerp_C, were of particular interest owing to their secretion by the infective larva of S. mansoni. Little published data exists examining the function of SmSrpQ, SmSerp_C, or any other of the schistosome serpin.

SmSrpQ and SmSerp_C are only expressed during larval development and, in the case of SmSrpQ, in the early stages of mammalian infection. 35S-SmSrpQ was able to form an SDS-stable complex with a component of the larval lysate but no complex was detected 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. Similar experiments were carried out with 35S-SmSerp_C, which formed a complex with trypsin, human plasma kallikrein, and a molluscan protease, most likely the tryptases found in the intermediate host of S. mansoni, Biomphalaria glabrata.

Invasion by S. mansoni larvae is facilitated by cercarial elastase (SmCE), a serine protease. The band detected between 35S-SmSrpQ and a protease in cercarial lysates was also detected in western blot analysis of parasite lysates. These blots showed that when active SmCE was present, a complex of comparable size to SmCE bound to SmSrpQ was observed. This data, together with the results from immunolocalization of these proteins in the parasite and in simulated infections, suggests that SmSrpQ carefully regulates SmCE degradation of skin tissue during larval invasion.

SmSrpQ inhibition of SmCE and SmSerp_C inhibition of kallikrein and a snail protease confirm the inhibitory nature of these serpins and suggests a role in the pathogenesis/ biology of the parasite.