SOX12 and DELTA mediate ecdysone-driven adult neural cell proliferation during metamorphosis

During metamorphic development from larva to adult, insect brains exhibit cell proliferation; however, the molecular mechanisms governing this process remain poorly understood. Employing the lepidopteran insect Helicoverpa armigera (cotton bollworm), an agricultural pest, as a model system, this study demonstrates that the steroid hormone 20-hydroxyecdysone (20E) regulates neural cell proliferation during imaginal brain development by upregulating the expression of SOX12 and DELTA via nuclear receptor EcRA. SOX12 exhibits significantly elevated expression and is localized in the brain during metamorphosis. Overexpression of SOX12-GFP in the H. armigera epidermal cell line (HaEpi) promotes cell proliferation. RNA interference (RNAi)-mediated knockdown of Sox12 in larvae leads to lethality, delayed pupation, impaired imaginal brain development, downregulation of cell proliferation-related genes, and reduced neural cell proliferation in the brain. CRISPR/Cas9 knockout of Sox12 can also impair imaginal brain development and reduce neural cell proliferation in the brain. Sox12 is upregulated by 20E through its nuclear receptor EcRA, which subsequently induces Delta expression to facilitate imaginal brain development during metamorphosis. Notably, Delta knockdown recapitulates the phenotypic effects observed on Sox12 silencing and knockout. Collectively, these findings establish that the steroid hormone 20E, acting through the EcRA-SOX12-DELTA regulatory axis, drives neural cell proliferation in the developing imaginal brain during insect metamorphosis.