Recently, Professor Chen Xi'en, a young faculty member of the Crop Pest Disaster Mechanism and Control Team from our college, published a research paper titled "Chronologically inappropriate morphogenesis (Chinmo) is required for maintenance of larval stages of fall armyworm" in the Proceedings of the National Academy of Sciences of the United States of America (PNAS). The study found that the transcription factor Chinmo participates in regulating chromatin remodeling in the regions of metamorphosis genes (Br-C, a key gene promoting pupal development, and E93, a key gene promoting adult development) during the early larval stages of the fall armyworm, Spodoptera frugiperda. By inhibiting the expression of these genes during early larval stages, Chinmo suppresses premature metamorphosis independently of hormonal signals.
Holometabolous insects undergo four life stages: egg, larva, pupa, and adult. Larvae differ significantly from pupae and adults in external morphology and physiological characteristics. The transition between larvae, pupae, and adults is strictly controlled by juvenile hormone (JH, which maintains larval morphology) and ecdysone (which promotes molting/metamorphosis). Studies have found that functional loss of JH signaling pathway genes Met and Kr-h1 causes premature metamorphosis in older larvae but has no effect on younger larvae. For a long time, the key factors and mechanisms by which younger larvae maintain larval morphology independently of JH signals have remained unclear. Recent studies in the model organism Drosophila melanogaster have identified BTB-zinc finger protein Chinmo as one of the key factors maintaining larval morphology during early larval stages. However, questions remain about how Chinmo regulates metamorphosis-related genes such as Br-C and E93 and whether there is an interaction between Chinmo and JH signals.
In this study, Sf9 cell lines with knockout, overexpression of Chinmo and Kr-h1 were first established, confirming at the cellular level that Chinmo and Kr-h1 can directly regulate the expression of Br-C and E93. Subsequently, mutant strains with individual knockouts of Chinmo, Kr-h1, and double knockouts of both genes were obtained using a transgenic CRISPR system. It was found that Chinmo mutants exhibited molting failure during the 3rd to 4th instars; the cuticles of 5th and 6th instar larvae of Kr-h1 mutants showed chimeric pupal cuticle phenotypes, ultimately leading to pupation failure; some larvae of Chinmo/Kr-h1 double knockout mutants exhibited chimeric melanization and molting failure at the end of the 2nd instar, while others could grow to the 4th instar but failed to complete molting, showing chimeric pupal cuticle phenotypes on their bodies. Significant increases in Br-C and E93 expression were detected in these mutants. These results indicate that both Chinmo and Kr-h1 are indispensable for larval development, with Chinmo inhibiting premature metamorphosis during early larval stages and the JH signaling pathway playing a role in inhibiting premature metamorphosis during late larval stages. RNA-seq analysis of the mutants revealed that Chinmo may have a function in regulating chromatin accessibility. To test this hypothesis, single-cell ATAC-seq and overall differential accessibility analysis were performed on the previously established knockout cell lines, finding that the Chinmo knockout samples had more open chromatin accessibility, making the promoter regions of metamorphosis genes such as Br-C and E93 more accessible to regulatory factors and thus increasing their expression levels.
The results which were mentioned above ultimately demonstrate that Chinmo has the function of regulating chromatin accessibility, closing the accessibility of promoters of metamorphosis genes such as Br-C and E93 independently of hormonal signals, inhibiting the expression of these genes, and thus preventing premature metamorphosis in younger larvae. This study not only elucidates the molecular mechanism by which Chinmo inhibits premature metamorphosis in insects but also provides a promising molecular target for pest control.
Original link: https://doi.org/10.1073/pnas.2411286121
