Preliminary Characterization of Two Small Insulinase-Like Proteases in Cryptosporidium parvum

Involvement of INS15 in the development and pathogenicity of the zoonotic pathogen Cryptosporidium parvum

BACKGROUND

Cryptosporidium parvum is a common protozoan pathogen responsible for moderate to severe diarrhea in humans and animals. The C. parvum genome contains 22 genes encoding insulinase-like M16 proteases (INS) with diverse structures and sequences, suggesting that members of the protein family may have distinct biological functions in the life cycle of parasites. Here, we investigated the role of INS15 and INS16, two proteases encoded by neighboring genes with high sequence identity, in the growth and development of C. parvum in vivo and in vitro.

METHODOLOGY/PRINCIPAL FINDINGS

INS15 and INS16 genes were tagged and knocked out using CRISPR/Cas9 technology in C. parvum IIdA20G1-HLJ isolate. The expression of INS15 and INS16 was determined by immunofluorescence analysis and immunoelectron microscopy. The effect of depletion of INS15 and INS16 on parasite growth and pathogenicity were assessed on HCT-8 cells and in interferon-γ knockout mice. Endogenous tagging showed that INS15 and INS16 expressed in the oocyst, trophozoite, meront and female gametes. INS15 also expressed in male gamonts, while INS16 was not detected in the male gamonts. Although depletion of the INS15 or INS16 gene affected late development of C. parvum in vitro, only depletion of INS15 significantly reduced parasite burden in infected mice. Mice infected with the INS15-depleted strain had reduced clinical signs, body weight, intestinal villus length to crypt height ratio, and survival time compared to infected with the tagging mutant.

CONCLUSIONS/SIGNIFICANCE

The results of this study indicate that INS15 is mainly involved in the late development of C. parvum. Depletion of this gene attenuates the pathogenicity of this important zoonotic parasite.

Characterization of CpCaM, a protein potentially involved in the growth of Cryptosporidium parvum

Cryptosporidium parvum is an important apicomplexan parasite causing severe diarrhea in both humans and animals. Calmodulin (CaM), a multifunctional and universal calcium-binding protein, contributes to the growth and development of apicomplexan parasites, but the role of CaM in C. parvum remains unknown. In this study, the CaM of C. parvum encoded by the cgd2_810 gene was expressed in Escherichia coli, and the biological functions of CpCaM were preliminarily investigated. The transcriptional level of the cgd2_810 gene peaked at 36 h post infection (pi), and the CpCaM protein was mainly located around the nucleus of the whole oocysts, in the middle of sporozoites and around the nucleus of merozoites. Anti-CpCaM antibody reduced the invasion of C. parvum sporozoites by 30.69%. The present study indicates that CpCaM is potentially involved in the growth of C. parvum. Results of the study expand our knowledge on the interaction between host and Cryptosporidium.

Characterization of Dense Granule Metalloproteinase INS-16 in Cryptosporidium parvum

The protozoan pathogen Cryptosporidium parvum infects intestinal epithelial cells and causes diarrhea in humans and young animals. Among the more than 20 genes encoding insulinase-like metalloproteinases (INS), two are paralogs with high sequence identity. In this study, one of them, INS-16 encoded by the cgd3_4270 gene, was expressed and characterized in a comparative study of its sibling, INS-15 encoded by the cgd3_4260 gene. A full-length INS-16 protein and its active domain I were expressed in Escherichia coli, and antibodies against the domain I and an INS-16-specific peptide were produced in rabbits. In the analysis of the crude extract of oocysts, a ~60 kDa fragment of INS-16 rather than the full protein was recognized by polyclonal antibodies against the specific peptide, indicating that INS-16 undergoes proteolytic cleavage before maturation. The expression of the ins-16 gene peaked at the invasion phase of in vitro C. parvum culture, with the documented expression of the protein in both sporozoites and merozoites. Localization studies with antibodies showed significant differences in the distribution of the native INS-15 and INS-16 proteins in sporozoites and merozoites. INS-16 was identified as a dense granule protein in sporozoites and macrogamonts but was mostly expressed at the apical end of merozoites. We screened 48 candidate INS-16 inhibitors from the molecular docking of INS-16. Among them, two inhibited the growth of C. parvum in vitro (EC₅₀ = 1.058 µM and 2.089 µM). The results of this study suggest that INS-16 may have important roles in the development of C. parvum and could be a valid target for the development of effective treatments.