# Bombyx E75 isoforms display stage- and tissue-specific responses to 20-hydroxyecdysone

## TL;DR

This study shows how different versions of the E75 gene in silkworms respond to a hormone called 20-hydroxyecdysone in a tissue- and stage-specific way.

## Contribution

The study reveals stage- and tissue-specific regulation of E75 isoforms by 20-hydroxyecdysone and identifies unknown factors influencing their protein levels.

## Key findings

- BmE75A mRNA and protein levels in prothoracic glands correlate with ecdysteroid titer.
- RNAi of EcR-USP reduces all BmE75 isoform mRNA and protein levels in prothoracic glands and fat body.
- BmE75 isoforms show distinct 20E responses at mRNA and protein levels in a stage- and tissue-specific manner.

## Abstract

Resulted from alternative splicing of the 5′ exons, the nuclear receptor gene E75 in the silkworm, Bombyx mori, processes three mRNA isoforms, BmE75A, BmE75B and BmE75C. From the early 5th larval instar to the prepupal stages, BmE75A mRNA and protein levels in the prothoracic glands display developmental profiles similar to ecdysteroid titer. In the fat body, mRNA levels but not protein levels of all three BmE75 isoforms correlate with ecdysteroid titer; moreover, proteins of all three BmE75 isoforms disappear at the prepupal stages, and a modified BmE75 protein with smaller molecular weight and cytoplasm localization occurs. At the early 5th larval instar stage, treatment of the prothoracic glands and fat body with 20-hydroxyecdysone (20E) and/or cycloheximide (CHX) revealed that BmE75A is 20E primary-responsive at both mRNA and protein levels, while BmE75B and BmE75C exhibit various responses to 20E. At the early wandering stage, RNAi-mediated reduction of gene expression of the 20E nuclear receptor complex, EcR-USP, significantly decreased mRNA and protein levels of all three BmE75 isoforms in both tissues. In conclusion, BmE75 isoforms display stage- and tissue-specific responses to 20E at both mRNA and protein levels; moreover, they are regulated by other unknown factors at the protein level.