Ecdysteroid secretion

The occurrence and secretion of ecdysteroids by helminths has led to the proposal to use ecdysteroids secreted by the parasite into the host serum as a means of diagnosis. However, the correlation between occurrence of infection and ecdysteroid level seems too variable and unreliable to be of diagnostic value (68). 

The ecdysteroids are responsible for cellular programming in cooperation with the juvenile hormones secreted from the corpora allata. The juvenile hormones (JHs) regulate the result of a molt — whether a new larva or a pupa will be the next stage following the molt. When the amount of JH secreted is high, the epidermis is programmed for a larval molt. When the level declines, the cells are programmed for metamorphosis and pupa formations (in holometabolous insects). 

Secretion of ecdysone starts the many biochemical processes that are necessary for the molting. The cells in the epidermis are stimulated to produce a new cuticle, and when ready, the insect will creep out of its old skin. The molecular mechanism of ecdysone has been studied in some detail. The molecular target of ecdysone and other ecdysteroids consists of at least two proteins, the ecdysteroid receptor (EcR) and ultraspiracle (USP). Both EcR and USP are members of the steroid hormone receptor superfamily with characteristic ligand-binding domains. An EcR-USP-ecdysteroid complex is formed, which activates several genes that code for transcription factors, i.e., proteins that activate or repress the activity of other genes, and the appropriate amounts of proteases and other enzymes necessary to degrade old structures and rebuild new ones are formed in a time-controlled sequence. 

Two groups of hormones regulate development and reproduction in insects, namely the ecdysteroids and the JHs. In crustaceans too, ecdysteroids are involved in the hormonal control of growth, and a chemical compound, which is similar to the JHs of insects, is present and thought to play a role in crustacean reproduction and development. The insect JH is a species-specific acyclic sesquiterpenoid epoxide, which is synthesized in a pair of retrocerebral epithelial organs called the corpora allata (CA see Fig. 1). In decapod crustaceans, MF is the unepoxidated form of the insect JH III and it is synthesized and secreted from the MOs (see Section 3.2.4). 

Graf, R. Neuenschwander, S. Brown, M.R. Ackermann, U. Insulin-mediated secretion of ecdysteroids from mosquito ovaries and molecular... 

Tomaschko K-H (1994a) Defensive secretion of ecdysteroids in Pycnogonum litorale (Arthro-poda, Pantopoda). Z Naturforsch 49c 367-371... 

Similarly, parasitism of P. separata, by A. kariyai or injection of the calyx fluid or virus material from the parasitoid, caused a decline in the host ecdysteroid titer and arrested metamorphosis of the host (54). Again, injection of exogenous 20-hydroxyecdysone reversed the developmental arrest. In this case, administration of prothoracicotropic hormone caused a reactivation of the prothoracic glands in the treated host. These results showed that the virus material inhibited the synthesis or secretion of prothoracicotropic hormone and also lowered the ecdysteroid level in the host. Also in this case, a mixture of both venom and calyx fluid were needed to obtain the full prolongation of the larval stage in the host (110). 

Studies on the pre-adult ecdysteroidogenins are extensive but include few species. Much of the work was recently reviewed, including studies on the biosynthesis and secretion of ecdysteroids by the PGs (796). More specific reviews on B. mori have dealt primarily with peptide and gene isolation, cellular localization and titer 197-201), specific reviews on M. sexta have dealt primarily with cellular localization, mode of action and titer 202-205), and those on the gypsy moth, Lymantria dispar primarily with isolation and titer 206,207),... 

---