The actin cytoskeleton of host cells has been implicated in Cryptosporidium invasion. However, the underlying mechanism of how actin filaments and associated
proteins modulate this process remains unclear. In this study, we use in vitro cultured cell lines, human ileocecal
adenocarcinoma HCT-8 and Chinese hamster ovary (CHO), and an in vivo mouse model to investigate the roles of
tropomyosin isoforms in Cryptosporidium invasion. Using
isoform-specific
monoclonal antibodies, we found that the major human
tropomyosin (hTM)
isoforms expressed in HCT-8 cells are hTM4 and hTM5. HCT-8 cells also express hTM1 at low levels but not hTM2 and hTM3. During Cryptosporidium parvum
infection, hTM5 colocalized to the
infection sites with a novel parasite
membrane protein, CP2. Neither hTM1 nor hTM4 accumulated at
infection sites. Similarly, a high level of TM5 and varying amounts of TM4 accumulated at the C. parvum
infection sites in CHO cells. CHO cells overexpressing hTM5 exhibit a significantly higher percent of mature meronts early in the
infection process relative to CHO cells or CHO cells overexpressing a
tropomyosin mutant, chimeric
isoform hTM5/3. These results suggest that functional TM5 enhances Cryptosporidium invasion of host cells. In C. parvum-infected mice, accumulation and rearrangement of TM5 and TM4 were detected throughout the infected ileum. Similarly, in the Cryptosporidium muris-infected mice, TM5 accumulated in discrete regions of the epithelial cells of gastric glands and in the oocyst-laden stomach gland lumen.
Cryptosporidium infection appears to rearrange and recruit host TM
isoforms in both culture cells and in the mouse. Localized accumulation of
tropomyosin at the
infection sites may facilitate parasite invasion.