Mechanism of programmed cell death in the posterior silk gland of the silkworm, Bombyx mori, during pupation based on Ca2+ homeostasis

• Published in: Insect molecular biology 2024-12, Vol.33 (6), p.551-559
• Main Authors: Gu, Haoyi, Zou, Hongbin, Cheng, Jialu, Liu, Xiaohan, Jiang, Zhe, Peng, Peilin, Li, Fanchi, Li, Bing
• Format: Article
• Language: English
• Subjects: Apoptosis; Autophagy; Bombyx mori; Calcium (intracellular); Calcium homeostasis; Calcium ions; Calpain; Cell death; Degeneration; Homeostasis; Insects; Metamorphosis; PCD; Protein biosynthesis; Protein synthesis; Pupation; Regulatory mechanisms (biology); Silk; Silk gland; Silkworms; Western blotting
• ISSN: 0962-1075; 1365-2583; 1365-2583
• DOI: 10.1111/imb.12911

The silkworm, Bombyx mori, is a complete metamorphosed economic insect, and the silk gland is a significant organ for silk protein synthesis and secretion. The silk gland completely degenerates during pupation, but the regulatory mechanism of programmed cell death (PCD) has not yet been understood. In the present study, we investigated the non‐genetic pathway of 20E‐induced PCD in the posterior silk gland (PSG) based on intracellular Ca2+ levels. Silk gland morphology and silk gland index indicated rapid degeneration of silk gland during metamorphosis from mature silkworm (MS) to pupal day 1 (P1), and Ca2+ levels within the PSG were found to peak during the pre‐pupal day 1 (PP1) stage. Moreover, the results of autophagy and apoptosis levels within the PSG showed that autophagy was significantly increased in MS‐PP1 periods, and significantly decreased in PP2 and P1 periods. Apoptosis was almost absent in MS‐PP1 periods and significantly increased in PP2 and P1 periods. Additionally, western blotting results showed that autophagy preceded apoptosis, and the autophagy‐promoting ATG5 was cleaved by calpain to the autophagy‐inhibiting and apoptosis‐promoting NtATG5 since PP1 period, while decreased autophagy was accompanied by increased apoptosis. Collectively, these findings suggest that Ca2+ is a key factor in the shift from autophagy to apoptosis. The silk gland of the silkworm rapidly degenerates and shrinks during metamorphosis from mature silkworm (MS) to pupal day 1 (P1). Autophagy preceded apoptosis in the silk gland, and autophagy occurred significantly at the MS‐PP1 (pupal day 1) stage, while apoptosis hardly occurred. The Ca2+ peaked in PP1 stage, and the autophagy‐promoting ATG5 was cleaved by calpain to the apoptosis‐promoting NtATG5 since PP1 period, while decreased autophagy was accompanied by increased apoptosis.