Mechanism of differential efficiency in expression of sericin 3 and silk fibroin heavy chain-like protein in the posterior silk gland of the domestic silkworm (Bombyx mori)

Abstract

The silk gland of the domestic silkworm (Bombyx mori) holds tremendous application prospects as a bioreactor. However, expressing exogenous proteins in the silk gland faces bottlenecks, especially aberrant silk gland development and low efficiency in silk protein synthesis and secretion. This study aimed to investigate the mechanisms underlying the differential outcomes of genic engineering to promote protein synthesis of posterior silk gland (PSG)-specific transgenic expression of Sericin 3 (Ser3) and the silk fibroin heavy chain-like protein HPL gene (Hpl), which influence silk gland development and silk synthesis and secretion. The results demonstrated that PSG-specific expression of the Hpl gene reduced silk protein synthesis and secretion by inhibiting endoreplication. Concurrently, activation of the Hippo signaling pathway and elevated autophagy/apoptosis in the PSG resulted in imbalanced cell development across the gland, ultimately leading to structural collapse. In contrast, PSG-specific expression of the Ser3 gene did not activate the Hippo pathway, nor increase autophagy or apoptosis. Furthermore, enhanced endoreplication not only maintained normal tissue morphology but also significantly improved silk protein synthesis and secretion as compared to the wild-type group. These findings clarified the role of the Hippo signaling pathway in silkworm silk gland development and provided insights to improve silk gland development in transgenic silkworms and optimized PSG-specific transgene targeting strategies.