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Juvenile hormones ( JHs) are a group of acyclic that regulate many aspects of insect physiology. The first discovery of a JH was by Vincent Wigglesworth. JHs regulate development, reproduction, diapause, and . (1996). 9780120242269 ISBN 9780120242269
In , JH (formerly neotenin) refers to a group of , which ensure growth of the larva, while preventing metamorphosis. Because of their rigid exoskeleton, insects grow in their development by successively shedding their exoskeleton (a process known as molting).
Juvenile hormones are secretion by a pair of behind the brain called the corpus allatum. JHs are also important for the production of eggs in female insects.
JH was isolated in 1965 by Karel Sláma and Carroll Williams and the first molecular structure of a JH was solved in 1967.
Most insect species contain only juvenile growth hormone (JH) III. To date JH 0, JH I, and JH II have been identified only in the Lepidoptera (butterflies and moths). The form JHB3 (JH III bisepoxide) appears to be the most important JH in the Diptera, or flies. Certain species of crustaceans have been shown to produce and secrete methyl farnesoate, which is juvenile hormone III lacking the epoxide group. Methyl farnesoate is believed to play a role similar to that of JH in crustaceans.
Being a sesquiterpenoid, JH chemical structure differs significantly from the structure of other animal hormones. Some JH analogs have been found in .
This has been demonstrated in various studies, most prominently that by V. B. Wigglesworth in 1960s. In this study, two adult Rhodnius had their blood systems linked, ensuring that the JH titre in both would be equal. One was a third instar Rhodnius, the other was a fourth instar. When the corpora allata of the third instar insect were removed, the level of JH was equal in both insects to that in the fourth instar animal, and hence both proceeded to the fifth instar at the next moult. When the fourth instar Rhodnius had its corpora allata removed, both contained a third instar level of JH and hence one proceeded to instar four, and the other remained at this instar.
Generally, the removal of the corpora allata from juveniles will result in a diminutive adult at the next moult. Implantation of corpora allata into last larval instars will boost JH levels and hence produce a supernumerary (extra) juvenile instar etc.
JH titers in Worker bee progressively increase through the first 15 or so days of the worker's life before the onset of foraging.Elekonich, M. M., Schulz, D. J., Bloch, G. and Robinson, G. E. (2001). Juvenile hormone levels in honey bee (Apis mellifera L.) foragers: foraging experience and diurnal variation. Journal for Insect Physiology. 47,1119 -1125 During the first 15 days, workers perform tasks inside the hive, such as nursing larvae, constructing comb, and cleaning cells. JH titers peak around day 15; workers this age guard, remove dead bees from the colony, and fan at the colony entrance to cool the nest. Aggressiveness of guard bees is correlated with their blood JH levels. Even though guards have high JH levels, their ovaries are relatively undeveloped.Pearce A.N., Huang Z.Y., Breed M.D. (2001) Juvenile hormone and aggression in honey bees, Journal for Insect Physiology. 47, 1243– 1247.Breed, Michael D. (2002) Juvenile Hormone Although, JH does not activate foraging. Rather it is involved in controlling the pace at which bees develop into foragers.Sullivan, J. P., Jassim, O., Fahrbach, S. E. and Robinson, G. E. (2000). Juvenile hormone paces behavioral development in the adult worker honey bee. Hormones and Behavior. 37, 1-14
Vitellogenin titers are high at the beginning of adult life and slowly decrease.
JH has been known to be involved in the queen-worker caste differentiation during the larval stage.Rachinsky A., Hartfelder K. (1990) Corpora allata activity, a primer regulating element for caste juvenile hormone titer in honey bee larvae ( Apis mellifera carnica), Journal of Insect Physiology. 36, 189–194 The unique negative relationship between JH and Vitellogenin may be important to the understanding of queen longevity.Miguel Corona, Rodrigo A. Velarde, Silvia Remolina, Adrienne Moran-Lauter, Ying Wang, Kimberly A. Hughes, and Gene E. Robinson Vitellogenin, juvenile hormone, insulin signaling, and queen honey bee longevity Proceedings of the National Academy of Sciences of the United States of America, April 2007; 104: 7128 - 7133
JH stimulates the accessory glands of adult males, promoting gland growth and the production of accessory gland secretion. Yolk production (vitellogenesis) in female ovaries is also stimulated by JH action. JH may also regulate reproductive behaviour in both sexes.
Cholesterol biosynthesis has been exhaustively studied in animals. All steps occur in the cytosol. The starting material is citrate, which is exported by the mitochondrion when metabolic fuels are high. It is converted into acetyl-CoA, ADP, CO2, and oxaloacetate by ATP-citrate lyase, together with ATP and CoASH as substrates. Three acetyl-CoAs are converted into HMG-CoA by the cytosolic isoforms of thiolase and 3-hydroxy-3-methylglutaryl-CoA synthase. The HMG-CoA is then reduced by NADPH to mevalonate by HMG-CoA reductase, the rate controlling enzyme of cholesterol biosynthesis. This enzyme has 8 helical domains anchoring it in the Golgi membrane of the ER;Burg, J.S., Espenshade, P.J., 2011. Regulation of HMG-CoA reductase in mammals and yeast. Prog. Lipid Res. 50, 403-410 the catalytic domain is in the cytosol. It is strongly inhibited by the statins, a class of drugs based on a mold metabolite which, at least at one time, were the largest selling class of drugs in the world. Mevalonate is acted of by a series of 3 kinases to give the highly labile 1,2-diphosphomevalonate-3-phosphate, which is acted on by a lyase to give phosphate, CO2, and isopentenyl diphosphate. Isopentenyl diphosphate isomerase converts the latter to the less stable dimethylallyl diphosphate. Farnesyl diphosphate synthase takes one DMAPP and two IPP to give the C15 metabolite farnesyl diphosphate. There are a large number of additional steps to generate cholesterol from IPP, the ubiquitous precursor of all isoprenoids.
It appears that the biosynthesis of JH III is identical to that of cholesterol, from production of IPP to FPP, although there appear to be no studies on export of citrate or other metabolites from the mitochondrion into the cytosol, or formation of acetyl-CoA. The enzymes of this pathway were first studied in Manduca sexta, which produces both homoisoprenoid and isoprenoid (JHIII) JHs.
Very early on propionate was shown to incorporate very highly efficiently into JH II and JH I in cell free extracts of M. sexta] corpora allata. Mevalonate and acetate also incorporate into JH I, II, JH III from M. sexta, albeit far less efficiently that propionate. Jennings et al. showed that homomevalonate incorporates into JH II in M. sexta. Baker identified 3-hydroxy-3-ethylglutarate and 3-hydroxy-3-methylglutarate from the same enzyme source incubated with acetyl and propionyl-CoA. Lee et al. showed that the same source of enzymes efficiently make both mevalonate and its 3-ethyl homolog, homomevalonate. Bergot showed that the mevalonate and homomevalonate produced by these enzymes has the same 3S optical isomer configuration as the vertebrate enzymes. Baker showed that isopentenyl diphosphate, and its homolog, 3-ethyl-butenyl diphosphate (homoisopentenyl diphosphate) are metabolized to their corresponding allyic diphosphates, DMAPP and homoDMAPP (3-ethyl-3-methylallyl diphosphate). The latter is required for biosynthesis of JH I, JH II, and 4-methylJH I. 2 units of homoDMAPP are required for JH I and 4methyl JH I biosynthesis, and one for JH II biosynthesis.Baker, F.C., Lee, E., Bergot, B.J., Schooley, D.A., 1981. Isomerization of isopentenyl pyrophosphate and homoisopentenyl pyrophosphate by Manduca sexta corpora cardiaca - corpora allata homogenates., in: Pratt, G.E., Brooks, G.T. (Eds.), Juvenile Hormone Biochemistry. Elsevier, Amsterdam, pp. 67-80
All parts of the carbon skeleton comes from IPP. Then an enzyme prenyl transferase/farnesyl diphosphate synthase binds IPP, strips the diphosphate off it to give an allylic carbocation, and adds this to an IPP to give geranyl diphosphate (C10). Then it does the same thing to geranyl diphosphate, giving farnesyl diphosphate (C15). This reaction appears to be the only known enzymatic reaction involving the coupling of two molecules with a carbocation. The free electron pair adds to the double bond of IPP, also isomerizing IPP so that the product is an allylic diphosphate. Thus, this part of the isoprenoid pathway appears nearly identical with that of cholesterol with the exception of the insect specific homoisoprenoid units. NAD+-dependent farnesol dehydrogenase, a corpora allata enzyme involved in juvenile hormone synthesis showed that the same source of enzymes efficiently make both mevalonate and its 3-ethyl homolog, homomevalonate.
Absolute configuration of homomevalonate and 3-hydroxy-3-ethylglutaryl and 3-hydroxy-3-methylglutaryl coenzyme a, produced by cell-free extracts of insect corpora allata. A cautionary note on prediction of absolute stereochemistry based on liquid chromatographic elution order of diastereomeric derivatives. Journal of the American Chemical Society 101, 7432-7434 showed that the mevalonate and homomevalonate produced by these enzymes has the same 3S optical isomer configuration as the vertebrate enzymes showed that isopentenyl diphosphate, and its homolog, 3-ethyl-butenyl diphosphate (homoisopentenyl diphosphate) are metabolized to their corresponding allyic diphosphates, DMAPP and homoDMAPP (3-ethyl-3-methylallyl diphosphate). The latter is required for biosynthesis of JH I, JH II, and 4-methylJH I. 2 units of homoDMAPP are required for JH I and 4methyl JH I biosynthesis, and one for JH II biosynthesis.
However, at this point these pathways diverge. While the vast majority of farnesyl diphosphate is converted into ultimately cholesterol in animals, in insects it is apparently acting on by a diphosphatase to give farnesol, which in turn is acted on by an NAD+ dependent enzyme, farnesol/farnesal dehydrogenase in M. sexta to give farnesoic acid. Subsequent work has shown that the enzyme is highly specific for trans allylic alcohols with at least three isoprene units, and to also be present in mosquitoes.
The next steps of JH biosynthesis differ between orders. In Lepidoptera and mosquitoes farnesoic acid or its homologs is epoxidized by a P450 dependent farnesoic acid methyl epoxidase, then it is methylated by a JH acid methyl transferase In most orders, farnesoic acid is methylated by farensoic acid methyl transferase, and then is epoxidized by a P450 dependent methyl transferas.
A recent publication by Nouzova et al. (2015) shows that allatostatin C, the peptide which inhibits JH production by the corpora allata, blocks the transport of citrate out of the mitochondrion in Aedes aegypti. This is a very logical control mechanism for JH biosynthesis.
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