Background: the apicomplexan parasite Neospora caninum causes important reproductive problems in farm animals, most notably in cattle. After infection via oocysts or tissue cysts, rapidly dividing tachyzoites infect various tissues and organs, and in immunocompetent hosts, they differentiate into slowly dividing bradyzoites, which form tissue cysts and constitute a resting stage persisting within infected tissues. Bumped kinase inhibitors (BKIs) of calcium dependent protein kinase 1 are promising drug candidates for the treatment of Neospora infections. BKI-1294 exposure of cell cultures infected with N. caninum tachyzoites results in the formation of massive multinucleated complexes (MNCs) containing numerous newly formed zoites, which remain viable for extended periods of time under drug pressure in vitro. MNC and tachyzoites exhibit considerable antigenic and structural differences.
Methods: Using shotgun mass spectrometry, we compared the proteomes of tachyzoites to BKI-1294 induced MNCs, and analyzed the mRNA expression levels of selected genes in both stages.
Results: More than half of the identified proteins are downregulated in MNCs as compared to tachyzoites. Only 12 proteins are upregulated, the majority of them containing SAG1 related sequence (SRS) domains, and some also known to be expressed in bradyzoites Conclusions: MNCs exhibit a proteome different from tachyzoites, share some bradyzoite-like features, but may constitute a third stage, which remains viable and ensures survival under adverse conditions such as drug pressure. We propose the term "baryzoites" for this stage (from Greek βαρυσ = massive, bulky, heavy, inert).
Keywords: antigenic variation; chemotherapy; drug adaptation; drug resistance.