Bombyx mori Jun gene is involved in endoreplication of silk gland cells by regulating S-phase cell cycle genes

Endoreplication as a variant of the cell cycle is an important avenue of development and reproduction for organisms. Endoreplication-induced polyploid cells are pivotal in ensuring the normal size and function of tissues and organs. However, it is becoming increasingly evident that a multitude of signaling pathways exert regulatory effects on endoreplication, and the regulatory network still needs further clarification. Herein, we uncovered the contributions of the JNK signaling pathway and the downstream BmJun gene to the silkworm cell cycle and silk gland endoreplication. Our data indicated that the BmJun gene induced G0/G1 phase arrest in BmN-SWU1 cells, decreased the rate of DNA synthesis, inhibited the process of cell division in the M phase, and decreased the proliferative activity of the cells. We revealed that BmJun knockout in the middle silk gland (MSG) resulted in a 30 % enlargement of the cells, triggering a 20 % expansion of the MSG diameter and a thickening of cocoon silk filaments by 17 %, accompanied by a 16 % increase in silk yield. Furthermore, we showed that MSG-specific BmJun knockout promoted endoreplication by inducing the expression of S-phase genes. We confirmed that the BmJun gene could target the promoters of the BmCDK2, BmCyclinA and BmCyclinE genes, thereby exerting negative regulatory functions. These findings offer crucial perspectives regarding the functionality of the Jun protein subfamily in cell cycle regulation and further clarify the molecular mechanisms underlying endoreplication.