Depletion of splicing factor Cdc5 in Toxoplasma disrupts transcriptome integrity, induces stress-driven abortive bradyzoite formation, and triggers host protective immunity

Toxoplasma gondii , a member of the Apicomplexa phylum, has over 75% of genes with predicted introns; however, RNA splicing, a major source of post-transcriptional regulation of gene expression during stage transitions, is not fully understood. Here, we demonstrate the role of pre-mRNA splicing factor Cdc5 in maintaining transcriptome integrity by harmonizing interaction with spliceosomal proteins and snRNAs in Toxoplasma . TgCdc5 is an essential splicing factor, and its depletion generates significant alternative splicing with widespread changes in gene expression demonstrated by RNA-seq and proteomic studies. Loss of TgCdc5 leads to catastrophic effects on the parasites, concomitantly triggering a switch from rapidly replicating tachyzoite to dormant bradyzoite cysts in many parasites, likely due to the formation of misfolded protein aggregates caused by the translation of erroneous transcripts. However, these dormant state parasites could not survive due to lacking functional proteins for bradyzoite development. Remarkably, the knockdown of TgCdc5 in vivo protects mice from lethal infection, and the immune response generated during initial parasite exposure completely protects these mice from future infection and offers partial protection in vertical transmission. Overall, this study unveils a novel role of TgCdc5-mediated pre-mRNA splicing in governing Toxoplasma stage conversion, providing new insights into developmental stage gene regulation.