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 ^[1]. 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 ^[2].

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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 ^[3].

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 ^[4].

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 ^[5][6]. 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 ^[7]. 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 ^[8]. Infection with parasites in mosquitoes has been shown to have a negative effect on egg production, possibly because of the stimulation of Akh ^[9]. 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 ^[10].