Single-cell transcriptome profiles and E-64 inhibitor data reveal the essential role of cysteine proteases in the ontogeny of Ichthyophthirius multifiliis.

Ichthyophthiriasis (Ich), also known as white spot disease, causes significant economic losses to fish farmers once an outbreak occurs. For fish survival, it is therefore crucial to understand the pathogenic mechanism and find effective prevention methods. In this study, we obtained data for four stages (theront, trophont, protomont and tomont) of Ichthyophthirius multifiliis by single-cell RNA sequencing (scRNA-seq). We found that the invasion-related proteins encoded by highly expressed genes in the theront stage mainly belong to the leishmanolysin family proteins, heat shock proteins, transmembrane proteins and cysteine proteases (CPs). Additionally, the exosome pathway appears to play a significant role in the invasion process of the theront. Since cysteine proteases are expressed at all stages of the I. multifiliis, and five CP-related genes were significantly upregulated at the theront stage of its life cycle - two of which are enriched in the exosome pathway - we incubated I. multifiliis theronts and protomonts with cysteine protease inhibitor (E-64). Our findings revealed that E-64 could kill both stages of the parasite in vitro and affected tomont division and subsequent release. Furthermore, infection experiment showed that E-64 could significantly inhibit the invasion of theronts. Based on our preliminary analysis from the transcriptomic and E-64 experiments, we have confirmed that CPs play a crucial role in I. multifiliis. This research establishes a foundation for future strategies in the prevention and control of Ich.