BmHR3 Is Essential for Silk Gland Development and Silk Protein Synthesis in Silkworms (Bombyx mori)

The steroid hormone 20-hydroxyecdysone (20E), which is known to regulate insect molting and metamorphosis, is crucial for the normal development of silk glands (SGs) in the silkworm <i>Bombyx mori</i>. However, how the 20E signaling pathway and its core members function in the SG remains largely unclear. Here, we report that the orphan nuclear receptor BmHR3, a 20E-response factor, plays an essential role in regulating SG development and silk protein synthesis. First, we showed that tissue-specific <i>BmHR3</i> overexpression and knockout result in severe developmental defects in posterior silk glands (PSGs). Second, we revealed that <i>BmHR3</i> dysfunction in PSGs dramatically represses the transcription of silk fibroin protein-coding genes, thereby inhibiting fibroin protein synthesis. Finally, we confirmed that BmHR3 can regulate fibroin protein-coding gene expression via direct and indirect mechanisms. This study elucidates the vital function of BmHR3 in <i>B. mori</i> SG and provides valuable information for thoroughly understanding the regulatory roles of 20E signaling in specialized insect organs.