Secondly and more importantly, reactivation of bradyzoites to tachyzoites presents profound clinical complications in the immune-compromised host and may lead to potentially fatal neurological diseases as a result of unrestrained tissue destruction (52,53). Understanding the molecular basis of this process, therefore, holds promise for the identification of novel drug targets to effectively eliminate Toxoplasma cysts and/or prevent their reactivation. Stage differentiation is marked by significant morphological and physiological remodelling, which is prompted by extensive alterations in gene expression (35,54). The first unbiased

genome-wide PI3K inhibitor query for developmentally regulated genes compared ESTs isolated from tissue cysts with a tachyzoite EST library (55,56). Many genes with unique ESTs in bradyzoites were identified including some previously known bradyzoite-specific genes. The most comprehensive published analysis of developmentally regulated gene expression to date has been performed using serial analysis of gene expression (SAGE) (41). With a 4× coverage of the total mRNA pool of Toxoplasma, transcript abundance was examined progressively through the tachyzoite-to-bradyzoite differentiation process. Almost 700 unique SAGE tags were found to be up-regulated in bradyzoites

relative to tachyzoites. Conversely, genes whose products are involved in high-activity DNA Damage inhibitor functions

such as DNA replication and cell division, endocytosis and metabolism were observed to be down-regulated in day 15 in vitro-induced bradyzoites. These findings are consistent with the characteristic decreased growth and activity of bradyzoites and provide an important lead for addressing the regulatory mechanism of this critical stage of the asexual cycle. Analysis of gene expression in stage differentiation mutants has also been explored 5-Fluoracil in vivo quite extensively in an attempt to identify gene interactions and pathways that might be required for this process (36,37,57). Using microarrays, global gene expression changes have been compared between differentiation mutants and wild-type strains under different bradyzoite induction conditions. Results from these studies suggest a common pathway for bradyzoite induction, downstream of individual stress response genes, which is able to integrate different induction stimuli to produce bradyzoite phenotypes. Similar expression profiles were observed for a core set of genes under different induction conditions, suggesting that these genes may play a critical role in differentiation (37). All these and other major advances counted, our understanding of stage differentiation still remains incomplete. For instance, a sensory mechanism that detects environmental stress and triggers the differentiation cascade has yet to be identified.