Schistosome Invasion of Human Skin and Degradation of Dermal Elastin Are Mediated by a Single Serine Protease

Abstract

Aquatic larvae (cercariae) of the trematode parasite Schistosoma mansoni rapidly penetrate human skin by degrading host proteins including elastin. Two serine proteases, one chymotrypsin-like and the second trypsin-like, have been proposed to be involved. To evaluate the relative roles of these two proteases in larval invasion, both were purified, identified by sequence, and then biochemically characterized. The trypsin-like activity was resolved into two distinct serine proteases 76% similar in predicted amino acid sequence. Southern blot analysis, genomic polymerase chain reaction, and immunolocalization demonstrated that the trypsin-like proteases are in fact not from the schistosome, but are released with larvae from the snail host Biomphalaria glabrata. Invasion inhibition assays using selective inhibitors confirmed that the chymotrypsin-like protease is the enzyme involved in skin penetration. Its ability to degrade skin elastin was confirmed, and the three sites of cleavage within elastin help define a new family of elastases.Infection of a human host by the trematode parasite Schistosoma mansoni begins with invasion of intact skin by an aquatic larva, the cercaria (1). Exiting the freshwater snail Biomphalaria glabrata, cercariae locate a human host by thermal (2) and chemical (3) signals and rapidly penetrate the skin, entering the vascular system in the dermis (4). S. mansoni cercariae are ϳ150 m long and 70 m wide and require lysis of skin tissues to migrate into blood vessels. Host macromolecules representing barriers to cercarial invasion are known to be cleaved by proteolytic activities present in cercarial secretions. These include elastin (5); chondromucoprotein (6); keratin (7); fibronectin, laminin, and collagens IV and VIII (8). Two distinct serine proteases have been reported in extracts of cercariae or in secretions from cercariae induced upon contact with skin lipids. One is a "chymotrypsin-like" protease with a preference for large hydrophobic side chains at P1 (9). The second is a "trypsin-like" protease with a preference for positively charged side chains at P1 (10). The class of proteases responsible for host protein degradation has been demonstrated by several independent studies to be serine proteases (6), but the relative contributions to invasion of the trypsin-like or chymotrypsin-like proteases are not known.To analyze the relative contributions of each of these proteases to the degradation of host proteins, cercarial secretions were fractionated, and the two proteases purified. The trypsinlike activity, which had not been previously purified or sequenced, was purified, and a cDNA was cloned by reverse transcription-PCR 1 based on amino-terminal amino acid sequence. Specific inhibitors were identified to evaluate the role of each protease in skin invasion. Proteases were localized by immunohistochemistry, and specific sites of cleavage in elastin, the most protease-resistant target in host skin, were analyzed.

Identification of a serine protease gene expressed by Myxobolus cerebralis during development in rainbow trout Oncorhynchus mykiss

“…The conserved catalytic triad of MyxSP-1 is linearly arranged, suggesting functions similar to that of chymotrypsin-like serine proteases. The chymotrypsin-like serine proteases secreted by the human pathogen Schistosoma mansoni are responsible for parasite penetration of skin and passage through the extracellular matrix of host tissues , 1985, Newport et al 1988, Salter et al 2000. We suspect that MyxSP-1 serves a similar role for M. cerebralis in the fish host.…”

Peptidases of Trichobilharzia regenti (Schistosomatidae) and its molluscan host Radix peregra s. lat. (Lymnaeidae): construction and screening of cDNA library from intramolluscan stages of the parasite

“…In the case of Schistosoma mansoni, the skin penetration is mediated by a serine peptidase known as cercarial elastase (Salter et al 2000). Further molecular characterisation of the enzyme showed that the gene family for this enzyme is highly conserved among several species of schistosomes, including S. mansoni, S. haematobium and Schistosomatium douthitti (Salter et al 2002).…”

Cercarial Elastase Is Encoded by a Functionally Conserved Gene Family across Multiple Species of Schistosomes

“…Considering the diverse set of macromolecular barriers the cercariae must breach during invasion, we previously investigated the possibility that multiple enzyme activities were required. However, only a single protease activity, cercarial elastase, was found to be present in acetabular gland secretions and required for invasion (3).…”