DjSoxP-1 and DjSox5 are Essential for Tissue Homeostasis and Regeneration in Dugesia Japonica

Sox genes encode a family of transcription factors that regulate multiple biological processes during metazoan development, including embryogenesis, tissue homeostasis, sex determination, nervous system specification, and stem cell maintenance. However, the underlying molecular mechanism of Sox proteins during cellular turnover and regeneration in the context of the complexity of the whole organism is unclear. The planarian Dugesia japonica contains a reservoir of stem cells that grow and divide continuously to support cellular turnover. In this study, three Sox gene homologs, DjSoxP-1, DjSoxP-5, and DjSox5, were discovered in the planarian transcriptome, and their roles were investigated. The results showed that the amino acid deduced from the three Sox genes all contained high-mobility group (HMG) domain sequences, which are highly conserved in Sox family members. Whole-mount in situ hybridization and real-time quantitative PCR results indicated that the three Sox genes were mainly expressed in parenchymal tissues and regenerative blastema. The intact worms with DjSoxP-1 and DjSox5 RNA interference (RNAi) exhibited head regression and the regenerating planarians had smaller blastemas in both head or tail fragments compared to the control animals, suggesting that DjSoxP-1 and DjSox5 have essential roles during cellular turnover and regeneration in planarians, conversely, there was no obvious phenotypic abnormalities or regeneration defect in DjSoxP-5 RNAi animals. Knockdown of DjSoxP-1 or DjSox5 decreased neoblast proliferation and promoted cell apoptosis. In conclusion, our findings demonstrate that DjSoxP-1 and DjSox5 are involved in cellular turnover and regeneration in planarians by modulating coordination between cell proliferation and apoptosis.