Systemic positive feedback regulation of ecdysone biosynthesis via sugar metabolism in insects.

Insects are recognized as the most diverse species in the world. They are classified into three separate groups based on their developmental characteristics: ametabola, hemimetabola, and holometabola. Holometabolous insects have attracted the attention of many developmental biologists because of their dramatic metamorphosis as a key model for studying regulatory mechanisms of development. Central to this process is ecdysone, a steroid hormone that governs insect growth and maturation. Extensive research has clarified ecdysone's structure, functions, and signaling pathways. In particular, the regulatory mechanisms of ecdysone synthesis have been a core research topic. Classical experiments, including ligation and tissue transplantation, established that the prothoracic glands are the primary site of ecdysone production and are activated by brain-derived prothoracicotropic hormone (PTTH). PTTH was first purified from silkworm brains, where it was shown to stimulate ecdysone secretion from the prothoracic glands directly. Although PTTH was identified decades ago, the molecular mechanisms linking PTTH signaling to enhanced ecdysone biosynthesis remain poorly understood. Recent findings suggest that PTTH stimulates ecdysone synthesis in the prothoracic glands via sugar metabolism. Given that ecdysone levels fail to rise when trehalose metabolism-an essential blood sugar in insects-is disrupted, these findings are particularly intriguing. This opinion paper discusses how PTTH activates prothoracic glands through sugar metabolism and proposes a model for systemic, midgut-mediated positive feedback regulation of ecdysone synthesis, integrating interorgan communication to coordinate developmental transitions.