Defining Stage-Specific Activity of Potent New Inhibitors of Cryptosporidium parvum Growth In Vitro

Abstract

Cryptosporidium parvum and Cryptosporidium hominis have emerged as major enteric pathogens of infants in the developing world, in addition to their known importance in immunocompromised adults. Although there has been recent progress in identifying new small molecules that inhibit Cryptosporidium sp. growth in vitro or in animal models, we lack information about their mechanism of action, potency across the life cycle, and cidal versus static activities. Here, we explored four potent classes of compounds that include inhibitors that likely target phosphatidylinositol 4 kinase (PI4K), phenylalanine-tRNA synthetase (PheRS), and several potent inhibitors with unknown mechanisms of action. We utilized monoclonal antibodies and gene expression probes for staging life cycle development to define the timing of when inhibitors were active during the life cycle of Cryptosporidium parvum grown in vitro. These different classes of inhibitors targeted different stages of the life cycle, including compounds that blocked replication (PheRS inhibitors), prevented the segmentation of daughter cells and thus blocked egress (PI4K inhibitors), or affected sexual-stage development (a piperazine compound of unknown mechanism). Long-term cultivation of C. parvum in epithelial cell monolayers derived from intestinal stem cells was used to distinguish between cidal and static activities based on the ability of parasites to recover from treatment. Collectively, these approaches should aid in identifying mechanisms of action and for designing in vivo efficacy studies based on time-dependent concentrations needed to achieve cidal activity. IMPORTANCE Currently, nitazoxanide is the only FDA-approved treatment for cryptosporidiosis; unfortunately, it is ineffective in immunocompromised patients, has varied efficacy in immunocompetent individuals, and is not approved in infants under 1 year of age. Identifying new inhibitors for the treatment of cryptosporidiosis requires standardized and quantifiable in vitro assays for assessing potency, selectivity, timing of activity, and reversibility. Here, we provide new protocols for defining which stages of the life cycle are susceptible to four highly active compound classes that likely inhibit different targets in the parasite. We also utilize a newly developed long-term culture system to define assays for monitoring reversibility as a means of defining cidal activity as a function of concentration and time of treatment. These assays should provide valuable in vitro parameters to establish conditions for efficacious in vivo treatment.

A Conditional Protein Degradation System To Study Essential Gene Function in Cryptosporidium parvum

“…2F ). Asexual stages (trophozoites, meronts, merozoites) have been reported to occur in cell culture at 12 to 36 hpi, followed by predominantly sexual stages and few meronts at 48 hpi ( 44 , 45 ). To visualize the localization of CDPK1, we performed immunofluorescence assays and super-resolution microscopy of CDPK1-HA transgenic parasites.…”

Faculty Opinions recommendation of A Conditional Protein Degradation System To Study Essential Gene Function in Cryptosporidium parvum.

“…2F). Asexual stages (trophozoites, meronts, merozoites) have been reported to occur in cell culture at 12 to 36 hpi, followed by predominantly sexual stages and few meronts at 48 hpi (44,45). To visualize the localization of CDPK1, we performed immunofluorescence assays and super-resolution microscopy of CDPK1-HA Table S1 in the supplemental material.…”

Intestinal organoid/enteroid-based models for Cryptosporidium

“…Additionally, the authors created transgenic parasite lines using CRiSPR/Cas9 technology and performed a genetic cross in vitro , to produce viable, recombinant parasites, which is currently not possible with other apicomplexan parasites. In another recent report, the long-term murine ALI organoid model capable of supporting continuous parasite growth has proved promising in studying cidal activity and mechanism of action of small molecule inhibitors of C. parvum infection [ 49 ].…”