Ecdysone

Biosynthesis and metabolism of the ecdysone

Bodies of biosynthesis

The ecdysone is primarily produced by the glands of moult, known under the name of prothoracic glands in many insects or of Organe Y in many shellfish. But it is also produced by other bodies, in particular the Gonade S, in the adult arthropods.

Sometimes, the ecdysone is replaced or accompanied by other precursors by the hormone by moult, like the 3-déhydro-ecdysone or the 2-désoxy-ecdysone, but in a rather general way, the active hormone, the 20-hydroxy-ecdysone, is not secreted by glands of moult or the gonades. It is generally formed in peripheral fabrics, where it exerts its action.

Ways of biosynthesis

The Insect S, like undoubtedly the majority of the Arthropod S, are not able to carry out the biosynthesis of the core sterol and they must thus draw from their food a sterol which will be the precursor of their hormone of moult. This one is generally the Cholestérol, for the species nourishing itself at the expense of animal organizations, but it is about another sterol, like the stigmastérol or the sitostérol, for the phytophagous species, that the animal must generally transform into cholesterol (which is practically absent at the plants).

The stages of biosynthesis are not all known. The first is a transfomation of cholesterol in 7-déhydrocholestérol, but the following stages remain still nondefined (one currently speaks about " box noire"). Then, the steroid is finalized by a certain number of Hydroxylation S, each one carried out by a specific hydroxylase, but using the Cytochrome P450 like cofactor. These enzymes were discovered at the embryo of Drosophile, thanks to changes which stopped the development of it by creating a hormonal deficiency.

Control neuroendocrine biosynthesis

The biosynthesis of the ecdysone is controlled by several mécanimes of control, activators or inhibiters. In the insects, the biosynthesis of the ecdysone is primarily placed under control activator of a cerebral neurohormone, called Hormone prothoracotrope, released on the level of a body neuro-hémal, the allato-cardiac bodies. In shellfish, on the contrary, the activity of gland of moult is primarily controlled by an inhibiting hormone of the moult, released by a gland located in the ocular stalk. In fact, mechanisms activators and inhibiters probably coexist in all the arthropods, to differing degree according to the various groups.

Degradation of the ecdystéroïdes

The ecdystéroïdes hydroxylated and are combined various manners to be inactivated, then excreted, by the animal.

Biological functions

Role in the control of the moult

The 20-hydroxy-ecdysone is the hormone which starts the pre-exuviale phase of the Mue of the arthropods. The first demonstration is the Apolyse, i.e. the separation of the skin and the cuticule, followed cellular rehandlings, and synthesis of a news cuticule, under the old one which will be rejected with the Exuviation. In the insects, if the ecdystéroïdes act as synergy with the youthful Hormone (emitted by the bodies allates), one assists then has a larval driven , leading to an increase in size of the individual (driven growth). In the absence of youthful hormone, the ecdystéroïdes lead to a driven of metamorphosis (leading to the Imago).

Role in the control of the reproduction

In addition to its role of hormone of moult in the larvae, the ecdysone is also implied in the control of the functions of reproduction in the adult insect. It is in particular secreted by the ovary during the process of Vitellogenèse. In many insects, the ecdysone and its derivatives are also stored in eggs where they take part in the control of the embryonic development.

Modes of action

The hormone of moult generally acts via a Récepteur nuclear power, present in the target cells. One knows however also particular cases where it interacts with a membrane Récepteur.

The Récepteur nuclear power of the hormone of moult is called, by simplification, Récepteur of the ecdysone, but it is before all the receiver of the most active hormone, which is the 20-hydroxy-ecdysone, like that of all the active ecdystéroïdes or the their mimetic ones (see low). The functional receiver, which activates the transcription Gène S targets in the presence of its ligand, is a complex (hétérodimère) of two nuclear proteins belonging to the superfamille of the nuclear receiving , EcR and Usp. The protein EcR (for ecdysone receptor in English) has a structure made up of several fields, in particular a Domaine of connection of the ligand (ecdystéroïde), and a Domaine of connection to the DNA. In that, EcR has many structural homologies with the receiving of the hormones steroids the vertebrate ones. However, on the functional level, the EcR protein does not seem able, with it only, of a capacity of sufficient connection with its ligand or the DNA to activate the transcription. For that, EcR imperatively owes dimériser with another nuclear receiver, Usp (for ultraspiracle , name of a change which made it possible to identify this protein). Usp is the orthologist of RXR, another protein of the superfamille of the nuclear receiving , known at the vertebrate ones to also function in hétérodimère (for example with RAR).

Phytoecdystéroïdes

Many plants contain variable quantities of ecdystéroïdes, being able to represent up to 2% of their dry weight. Some of these ecdystéroïdes are identical to those found in the animals, others are very different, but sometimes much more biologically active than the 20-hydroxy-ecdysone. These ecdystéroïdes probably does not have a hormonal role in the development of the plants, although those have also hormones steroids, the Brassinostéroïde S, which control their growth. The phytoecdystéroïdes rather seem to take part in the defense of the plants against the insects or other arthropods which parasitize them. However, certain insects are very well protected from ingestion from such phytoecdystéroïdes, because they have, on the level of the intestine, the enzymes capable of the inactiver. However, the ecdystéroïdes of plants also naturally have effective anti-appétants effects on the insects.

Agonists of the ecdystéroïdes used as insecticides

The ecdystéroïdes are too complex molecules to synthesize, or to extract from plants, to plan to use them like insecticides. However, the search for molecules not steroids, but able to start the premature moult of the insects, led to the description of the diacylhydrazines (derivatives of hydrazides of the Benzoic acid ), like the tébufénozide. They are molecules relatively inexpensive to synthesize, having the possibility of binding to the receiver of the ecdysone, but that the insect cannot eliminate as quickly as the ecdystéroïdes. Their pulverization on insects and/or their ingestion cause anticipated moults, or failures in the mechanisms of reproduction, having a lethal effect on the insects only. Moreover, progress of chemistry, associated with a search for optimization of the connection of such molecules with the receivers of the various groups of insects, lets hope for the development very specific insecticide against certain vermin, but not having practically an effect against the other insects.

See too

Moults of the arthropods
Ecdysozoaires

External bonds

EcdyBase The Ecdysone Handbook - has free online ecdysteroids database

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