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Osmoregulation is essential for organisms to adapt to the exterior environment and plays an important role in embryonic organogenesis. Tubular organ formation usually involves a hyperosmotic lumen environment. The mechanisms of how the cells respond and regulate lumen formation remain largely unknown. Here, we reported that the nuclear factor of activated T cells-5 (NFAT5), the only transcription factor in the NFAT family involved in the cellular responses to hypertonic stress, regulated notochord lumen formation in chordate Ciona. Ciona NFAT5 (Ci-NFAT5) was expressed in notochord, and its expression level increased during notochord lumen formation and expansion. Knockout and expression of the dominant negative of NFAT5 in Ciona embryos resulted in the failure of notochord lumen expansion. We further demonstrated that the Ci-NFAT5 transferred from the cytoplasm into nuclei in HeLa cells under the hyperosmotic medium, indicating Ci-NFAT5 can respond the hypertonicity. To reveal the underly mechanisms, we predicted potential downstream genes of Ci-NFAT5 and further validated Ci-NFAT5-interacted genes by the luciferase assay. The results showed that Ci-NFAT5 promoted SLC26A6 expression. Furthermore, expression of a transport inactivity mutant of SLC26A6 (L421P) in notochord led to the failure of lumen expansion, phenocopying that of Ci-NFAT5 knockout. These results suggest that Ci-NFAT5 regulates notochord lumen expansion via the SLC26A6 axis. Taken together, our results reveal that the chordate NFAT5 responds to hypertonic stress and regulates lumen osmotic pressure via an ion channel pathway on luminal organ formation.

The golden cuttlefish (Sepia esculenta Hoyle) which has one-time reproduction and die after reproduction, what changes have taken place in the ovarian development in the whole life cycle is a very noteworthy issue. In this study, we performed RNA-Seq technology of ovary tissue from female golden cuttlefish collected at three key developmental periods: growth period (IG), spawning period (IS) and post-reproductive death period (IA). Then, we obtained 66.65 Gb of clean data, with Q20 > 96%. Comparison of the transcriptome data for the IG vs. IS, IG vs. IA and IS vs. IA revealed 10,290, 17,107 and 380 differentially expressed genes (DEGs) respectively. Gene ontology (GO) enrichment analysis of DEGs revealed that oxidative phosphorylation, NADH dehydrogenase (ubiquinone) activity, respiratory chain and other GO entries were significantly enriched. Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathway database alignment and annotation revealed that 317 metabolic pathways differed between IG vs. IS, 317 differed between IG vs. IA; and 161 differed between IS vs. IA. Significantly enriched pathways included oxidative phosphorylation, cell cycle, mTOR signalling pathway, AMPK signalling pathway and ubiquitin-mediated proteolytic pathway were associated and interacted, and the post-reproductive death of golden cuttlefish was regulated by a complex signalling network. This study clarified the regulation of transcripts expression in the ovary of golden cuttlefish at different developmental periods, which not only filled the blank of molecular mechanism of cephalopod ovarian development, but also provided research ideas and experimental models for solving the complex biological proposition of post-reproductive death.

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