YTHDC1/CRM1 Facilitates m6A-Modified circRNA388 Nuclear Export to Induce Coelomocyte Autophagy via the miR-2008/ULK Axis in Apostichopus japonicus.

N 6-methyladenosine (m6A), the most prevalent internal modification in eukaryotic RNA, was able to mediate circular RNA (circRNA) function in many immune processes. Nevertheless, the functional role of m6A-modified circRNAs in innate immunity of invertebrates remained unclear. In this study, we identified m6A-modified circRNA388 from cultured sea cucumber (Apostichopus japonicus) coelomocytes, which was mainly detected in cytoplasm after Vibrio splendidus infection. A knockdown assay indicated that cytoplasm circRNA388 promoted coelomocyte autophagy and decreased the number of intracellular V. splendidus. Mechanistically, the circRNA388 in the cytoplasm directly sponged miR-2008 to block its interaction with Unc-51-like kinase 1 from A. japonicus (AjULK) and further promoted autophagy to resist V. splendidus infection. More importantly, we found that m6A modification was vital to circRNA388 nuclear export with YTH domain-containing protein 1 from A. japonicus (AjYTHDC1) as the reader. AjYTHDC1 facilitated the nuclear export of m6A-modified circRNA388 via interaction with exportin-1 (chromosomal maintenance 1) from A. japonicus (AjCRM1). Knockdown of AjCRM1 could significantly decrease the content of cytoplasm circRNA388. Overall, our results provide the first evidence that nuclear export of m6A-modified circRNA388 is dependent on the novel AjCRM1 to our knowledge, which was further promoted coelomocyte autophagy by miR-2008/AjULK axis to clear intracellular V. splendidus.