Differential proteomic analysis of laser-microdissected penetration glands of avian schistosome cercariae with a focus on proteins involved in host invasion

Schistosome invasive stages, cercariae, leave intermediate snail hosts, penetrate the skin of definitive hosts, and transform to schistosomula which migrate to the final location. During invasion, cercariae employ histolytic and other bioactive products of specialized holocrine secretory cells – postacetabular (PA) and circumacetabular (CA) penetration glands. Although several studies attempted to characterize protein composition of the in vitro-induced gland secretions in Schistosoma mansoni and Schistosoma japonicum, the results were somewhat inconsistent and dependent on the method of sample collection and processing. Products of both gland types mixed during their secretion did not allow localization of identified proteins to a particular gland. Here we compared proteomes of separately isolated cercarial gland cells of the avian schistosome Trichobilharzia szidati, employing laser-assisted microdissection and shotgun LC-MS/MS, thus obtaining the largest dataset so far of the representation and localization of cercarial penetration gland proteins. We optimized the methods of sample processing with cercarial bodies (heads) first. Alizarin-pre-stained, chemically non-fixed samples provided optimal results of MS analyses, and enabled us to distinguish PA and CA glands for microdissection. Using 7.5 × 10⁶ μm³ sample volume per gland replicate, we identified 3347 peptides assigned to 792 proteins, from which 461 occurred in at least two of three replicates in either gland type (PA = 455, 40 exclusive; CA = 421, six exclusive; 60 proteins differed significantly in their abundance between the glands). Peptidases of five catalytic types accounted for ca. 8% and 6% of reliably identified proteins in PA and CA glands, respectively. Invadolysin, nardilysin, cathepsins B2 and L3, and elastase 2b orthologs were the major gland endopeptidases. Two cystatins and a serpin were highly abundant peptidase inhibitors in the glands. While PA glands generally had rich enzymatic equipment, CA glands were conspicuously abundant in venom allergen-like proteins.