Inositol trisphosphate mediates the action of hypertrehalosemic hormone on fat body of the American cockroach, Periplaneta americana

Rhythmic change of adipokinetic hormones diurnally regulates locust vitellogenesis and egg development

Adipokinetic hormones (AKHs), the neurohormones synthesized in the insect corpora cardiaca are known to mobilize lipids and carbohydrates for energy-consuming activities including reproduction. However, both inhibitory and stimulatory effects of AKHs on insect reproduction have been reported, and the underlying mechanisms remain elusive. Using the migratory locust, Locusta migratoria, as a model system, we report here that AKHs are expressed in response to rhythmic diel change, and AKH III expression increases markedly at photophase. Diurnal injection of AKH III but not AKH I or AKH II in adult females stimulates vitellogenesis and egg development. In contrast, AKH treatment at scotophase represses female reproduction. RNA interference-mediated knockdown of AKH receptor (AKHR) results in significantly reduced vitellogenin (Vg) expression in the fat body at photophase along with reduced Vg deposition in the ovary. AKHR knockdown also leads to decreased expression of Brummer, triacylglycerol lipase and trehalose transporter, accompanied by suppressed mobilization of triacylglycerol and trehalose. We propose that in addition to stimulating Vg expression at photophase, AKH/AKHR signalling is likely to regulate ovarian uptake of Vg via triacylglycerol mobilization and trehalose homeostasis. This study provides new insights into the understanding of AKH/AKHR signalling in the regulation of insect reproduction.

Carbohydrate and lipid metabolism in cockroach (Periplaneta americana) fat body are both activated by low and similar concentrations of Peram-AKH II

Injection of 0.1 pmol of the octapeptide Peram-AKH II (pGlu-Leu-Thr-Phe-Thr-Pro-Asn-TrpNH(2)) elicits a significant hypertrehalosemic response in the American cockroach, Periplaneta americana; a maximal effect is obtained with 1pmol. The latter amount also lowers the level of neutral lipid (NL) and phospholipid (PL) in the hemolymph. The evidence supports the idea that Peram-AKH II promotes the liberation of fatty acids from hemolymph phospholipid, and indirectly diacylglycerol in the same compartment. The fatty acids are then transported into the fat body where they are converted into triacylglycerol for storge. Because lipolysis and trehalose synthesis are initiated by a common concentration of Peram-AKH II it is reasonable to suggest that the physiological function of Peram-AKH II involves the participation of both metabolic pathways.

Mode of action of neuropeptides from the adipokinetic hormone family

Neuropeptides of the adipokinetic hormone (AKH) family regulate inter alia mobilisation of various substrates from stores in the fat body of insects during episodes of flight. How is this achieved? In insects which exclusively oxidise carbohydrates for flight (cockroaches), or which oxidise carbohydrates in conjunction with lipids (locusts) or proline (a number of beetles), the endogenous AKHs bind to a G(q)-protein-coupled receptor, activate a phospholipase C and the resulting inositol trisphosphate releases Ca(2+) from internal stores. In addition, influx of extracellular Ca(2+) is increased and, via a kinase cascade, glycogen phosphorylase is activated, glucose-1-phosphate produced, and transformed to trehalose, which is released into the haemolymph. In locusts, additionally, adenylate cyclase is activated and cyclic AMP is synthesised. In insects which use lipids for sustained flight (locust, tobacco hornworm moth) or proline for flight (certain beetles), adenylate cyclase is activated after the AKHs bind to their respective G(s)-protein-coupled receptor. The resulting cyclic AMP, together with the messengers intra- and extracellular Ca(2+), activate a triacylglycerol lipase, which results in the production of 1,2 diacylglycerols (in locusts, moths) or (hypothetically) free fatty acids (fruit beetle).

The role of Ins(1,4,5)P(3) in signal transduction of the metabolic neuropeptide Mem-CC in the cetoniid beetle, Pachnoda sinuata

We have investigated the role of inositol triphosphate, Ins(1,4,5)P(3), in the transduction of the hypertrehalosaemic and hyperprolinaemic signal of the endogenous neuropeptide Mem-CC in the cetoniid beetle Pachnoda sinuata. Flight and injection of Mem-CC into the haemocoel of the beetle induce an increase of Ins(1,4,5)P(3) levels in the fat body of the beetle. When Mem-CC is co-injected with U 73122, which is an inhibitor of phospholipase C, this effect is abolished. Mem-CC also elevates Ins(1,4,5)P(3) concentration in fat body pieces in vitro. The increase in Ins(1,4,5)P(3) levels is tissue-specific and does not occur in brain and flight muscles. Elevation of the Ins(1,4,5)P(3) levels upon injection of Mem-CC is time- and dose-dependent: the maximum response is reached after 3 min and a dose of 10 pmol is needed. Compounds that mimic the action of cAMP (cpt-cAMP, forskolin) do not influence the concentration of Ins(1,4,5)P(3), while those that stimulate G-proteins (aluminium fluoride and cholera toxin) cause an increase of Ins(1,4,5)P(3) levels. The application (in vivo and in vitro) of F-Ins(1,4,5)P(3), an Ins(1,4,5)P(3) analogue that penetrates the cell membrane, causes a mobilisation of carbohydrate reserves via the activation of glycogen phosphorylase but does not stimulate proline synthesis. In addition, U 73122 abolishes the hypertrehalosaemic but not the hyperprolinaemic effect of Mem-CC. The results suggest that the hypertrehalosaemic signal of Mem-CC is mediated via an increase of Ins(1,4,5)P(3) levels in the fat body of P. sinuata.

Regulation of phospholipase A(2) activity in cockroach (Periplaneta americana) fat body by hypertrehalosemic hormone: evidence for the participation of protein kinase C

Phospholipase A(2) (PLA(2)) associated with the membrane fraction of trophocytes from Periplaneta americana fat body increases by as much as 100% when the cells are incubated with hypertrehalosemic hormone (HTH-II). Activation with HTH-II is approximately halved by inclusion of the PKC inhibitor sphingosine in the incubation medium. Because activation of PLA(2) by HTH-II is blocked by the GDP analogue GDP-beta-S, and the unactivated enzyme is activated by the GTP analogue GTP-gamma-S it is likely that a G protein is involved in activation of the enzyme. Activation of PLA(2) was also achieved by treating the trophocytes with the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol in the presence of thapsigargin. This supports the view that protein kinase C is also involved in the activation process.

Stimulation of trehalose efflux from cockroach (Periplaneta americana) fat body by hypertrehalosemic hormone is dependent on protein kinase C and calmodulin

Protein kinase C and calmodulin play key roles in cockroach fat body during activation of phosphorylase and trehalose efflux by HTH-II. The data support the view that an increase in cytosolic Ca2+ is prerequisite for enhanced activity of protein kinase C and calmodulin. Chelation of Ca2+ (i) with BAPTA blocks HTH-II-induced trehalose efflux from the fat body whereas thapsigargin, which raises [Ca2+]i to the same level as HTH-II, produces only a small, yet significant increase in trehalose efflux. Sphingosine, an inhibitor of protein kinase C, inhibits HTH-II-induced trehalose efflux in a concentration-dependent manner. Trehalose efflux is not activated by the protein kinase C activators OAG or PMA alone but in the presence of thapsigargin both agents increase trehalose efflux to a level comparable to that obtained with HTH-II. Thapsigargin has only a moderate activating effect on phosphorylase but in combination with OAG produces an activation indistinguishable from that provoked by HTH-II. Each of the structurally different calmodulin inhibitors, trifluoperazine, W-7, and calmidazolium, blocks completely the action of HTH-II on trehalose efflux, thus confirming the importance of calmodulin in HTH-II initiated trehalose efflux.