Abstract
Insect fat body is the organ for intermediary metabolism, comparable to vertebrate liver and adipose tissue. Larval fat body is disintegrated to individual fat body cells and then adult fat body is remodeled at the pupal stage. However, little is known about the dissociation mechanism. We find that the moth Helicoverpa armigera cathepsin L (Har-CL) is expressed heavily in the fat body and is released from fat body cells into the extracellular matrix. The inhibitor and RNAi experiments demonstrate that Har-CL functions in the fat body dissociation in H. armigera. Further, a nuclear protein is identified to be transcription factor Har-Relish, which was found in insect immune response and specifically binds to the promoter of Har-CL gene to regulate its activity. Har-Relish also responds to the steroid hormone ecdysone. Thus, the dissociation of the larval fat body is involved in the hormone (ecdysone)-transcription factor (Relish)-target gene (cathepsin L) regulatory pathway.
Highlights
In holomatabolous insects, larva undergoes a complete transformation during metamorphosis to form adult
Insect fat body is the intermediary metabolism organ and the main source of hemolymph components, and it is crucial for insect development and metamorphosis
A pupal diapause species the cotton bollworm, Helicoverpa armigera (Har), is a useful model to study individual or tissue remodeling, because larval fat body will remain integral in diapause-type pupae for months, whereas the dissociation of larval fat body will start on day 0 after pupation in nondiapause-type ones
Summary
Larva undergoes a complete transformation during metamorphosis to form adult. This transformation is accomplished by the destruction of larval tissues and organogenesis of the adult tissues, and is called as tissue remodeling. The insect fat body is an important organ, comparable to vertebrate liver and adipose tissue, which performs a myriad of metabolic activities including intermediary metabolism and the homeostatic maintenance of hemolymph proteins, lipids, and carbohydrates [5,6]. Molecular regulatory mechanism showed that fat body can regulate growth and development through mediating release of the brain hormone [7,8]. Understanding the fat body remodeling is crucial for insect development and metamorphosis, and the fat body dissociation is the first step to understand the remodeling of the fat body
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