Cell cycle and differentiation in Giardia Lamblia

Abstract

Giardia lamblia is the major cause of waterborne diarrhea worldwide. Giardiasis is initiated by ingestion of cysts, which after passing through the stomach are triggered to excyst. Excystation, or awakening from dormancy, is central to successful colonization of the host. An investigation of the role of calcium signaling throughout excystation was initiated to study the cellular regulation of this special differentiation. Calcium signaling was most crucial during late excystation where the excyzoite emerges. A calcium pump inhibitor, thapsigargin, inhibited excystation, calcium signaling and localized to a calcium storage compartment in cysts. Inhibitors of the calcium signaling protein calmodulin blocked excystation and calmodulin localized to Giardia s basal bodies suggesting that the basal bodies are Giardia s cellular control center. Basal bodies were isolated and 310 proteins identified using proteomics. Functional orthologs of these proteins were identified bioinformatically and used to build a network model. Differentiation-specific nodes were identified in the network using transcriptional data from the Giardia lifecycle. The model correctly predicts that calmodulin is involved in cytoskeletal remodeling and this was verified by affinity purifying 10 calmodulin-specific binding proteins.

For cysts to survive in nature and the pass through the stomach successfully they need a protective wall. A study was undertaken to look for new cyst wall proteins. One cyst wall protein identified was identified by SAGE and localized to the cyst body. This new cyst wall protein was found to be an invariant cysteine-rich Type 1 membrane protein and a member of a larger cysteine-rich family. This new family of novel cysteine-rich Giardia proteins was shown bioinformatically to have homologs in two other cyst-forming protozoans.

The initiation of differentiation is associated with cell cycle arrest in many cells. Giardia differentiates and forms cysts by arresting from the cell cycle and encysting. We looked at the role of the cell cycle in Giardia during encystation. We developed for the first time a method of synchronizing Giardia for use to determine where the encystation restriction point is in the cell cycle. We found using encystation-specific organelle biogenesis as a marker, that it was late in G2. In addition we used quantitative real-time PCR to determine the periodic cell cycle regulation of histones. Cyclin B is normally up-regulated in the late G2 stage of the cell cycle and promotes G2/M transition. We phylogenomically identified a Giardia cyclin B and found that expression gradually increased reaching a maximum at 3 h corresponding to G2, and decreased again with entry into mitosis after 4 h. We also identified bioinformatically 217 cell cycle orthologs and studies are in progress to verify these using synchronized populations and Giardia microarrays.