Adipokinetic hormone

Adipokinetic hormone ("Akh") is a short peptide hormone that has been studied in insects. It is a lipid mobilising hormone and is responsible for regulating fuel transport in the haemolymph, for redirecting energy to other processes as required by the insect. "Akh" was initially discovered in the desert locust "Schistocerca gregaria" and is generally associated with aiding flight [Kaufmann, C. and M. Brown, Adipokinetic hormones in the African malaria mosquito, Anopheles gambiae: Identification and expression of genes for two peptides and a putative receptor. Insect Biochemistry and Molecular Biology, 2006. 36(6): p. 466-481] . Early observations of locusts showed that despite the fact that lipids are metabolised by flight muscle in order to maintain flight, which would be transported from the haemolymph, there was often still a high concentration of lipids in the haemolymph, implying that an agent may be responsible for activating lipid transport into the haemolymph and this was thought most likely to be hormonal regulation. [ [http://entomology.unl.edu/ent801/xport.html Chapter Eleven ] ]

"Akh" was first purified by an English group in 1976 and the chemical structure was determined to be a peptide hormone formed from 10 amino acids. This was the first instance where an insect peptide hormone had been identified. [ [http://entomology.unl.edu/ent801/xport.html Chapter Eleven ] ]

Later, after "Akh" hormones were identified in cockroaches, locust "Akh" was inserted into a cockroach and a similar increase in lipid mobilisation was observed. Conversely, cockroach "Akh" led to similar activity within a locust. [ [http://entomology.unl.edu/ent801/xport.html Chapter Eleven ] ]

"Akh" has become an important area of study, particularly in insect crop pests and insects that act as intermediate or vector hosts for parasites that can affect humans or agricultural animals. It is believed that "Akh" does not only aid flight in insects, but it has also been shown that an increase in "Akh" can lead to stronger immune responses in locusts [Goldsworthy, G.J., K. Opoku-Ware, and L.M. Mullen, Adipokinetic hormone and the immune responses of locusts to infection. Annals of the New York Academy of Science, 2005. 1040: p. 106-113] [Goldsworthy, G., K. Opoku-Ware, and L. Mullen, Adipokinetic hormone enhances laminarin and bacterial lipopolysaccharide-induced activation of the prophenoloxidase cascade in the African migratory locust Locusta migratoria. Journal of Insect Physiology, 2002. 48: p. 601-608] . One group have performed experiments where locusts received an injection including "Akh" and lipopolysaccharide (LPS) – an immune elicitor found in the cell walls of bacteria – led to a stronger immune response than locusts which only received an LPS injection [Goldsworthy, G., K. Opoku-Ware, and L. Mullen, Adipokinetic hormone enhances laminarin and bacterial lipopolysaccharide-induced activation of the prophenoloxidase cascade in the African migratory locust Locusta migratoria. Journal of Insect Physiology, 2002. 48: p. 601-608] . This implies that lipid mobilisation by "Akh" can have a variety of uses for energy dynamics within insects and may be important in a range of functions that are related to insect survival. It is now believed that parasites which infect insects may stimulate the production of "Akh" in order to mobilise lipids which can then be used for parasite gains.

Effectively, mobilisation of these lipids will occur from the fat body of insects where synthesis of proteins and peptides take place for a range of processes. This includes the production of proteins that will form yolk in the ovaries which can be an important factor. As one process within an insect is started, such as flight, the stimulation of "Akh" could result in the redirection of energy from the production of yolk proteins causing a lower egg yield from insects that are forced into flight while producing eggs, which goes some way to explaining the necessity of a rest in anautogenous insects (insects which feed on proteins because they are unable to synthesise their own) after a protein meal in order to maximise the amount of energy focussed on producing eggs [Collins, L.A., Are the energy costs of insecticide resistance in the malaria mosquito Anopheles gambiae (Diptera:culicidae) expressed in the reduced gene regulation of immune peptides?, in School of Tropical Medicine. 2007, University of Liverpool: Liverpool] . Infection with parasites in mosquitoes has been shown to have a negative effect on egg production, possibly because of the stimulation of "Akh" [Hopwood, J.A., et al., Malaria-induced apoptosis in mosquito ovaries: a mechanism to control vector egg production. Journal of Experimental Biology, 2001. 204(16): p. 2773-80.] . Stimulation of the immune response in mosquitoes also has an effect on the production of yolk proteins in mosquitoes. These are all possible areas that could be affected by "Akh". [Collins, L.A., Are the energy costs of insecticide resistance in the malaria mosquito Anopheles gambiae (Diptera:culicidae) expressed in the reduced gene regulation of immune peptides?, in School of Tropical Medicine. 2007, University of Liverpool: Liverpool]