Coherence between biosynthesis, storage, and release of adipokinetic hormones

Abstract

In insects, specific parts of the retrocerebral complex are essential for homeostatic control in response to changes in both internal and external environments. Neuroendocrine cells in the glandular part of the coprus cardiacum of Locusta migratoria represent the site of synthesis and release of three structurally related adipokinetic hormones (AKHs), which control lipid and carbohydrate mobilization from the fat body to meet the dramatic energy demand during long-distance flight. Flight activity is the in vivo stimulus for the adipokinetic cells to release their hormones. This regulation is intrinsically achieved by secretomotor neurons located in the lateral part of the protocerebrum. A multitude of regulatory stimulating, inhibiting, and modulating substances has been established to influence the secretion of AKHs. In the adipokinetic cells the amounts of AKHs as well as the number of secretory granules where these AKHs are stored increase with age, reflecting that the biosynthesis of AKHs is a continuous process. Gene expression studies have revealed that flight activity as well as in vitro influences on the secreory process resulting from the peptidergic regulatory ligands crustacean cardioactive peptide (CCAP) and locustamyoinhibiting peptide do not affect the steady-state levels of AKH-specific mRNAs. Equally, stimulation of signal transduction routes involved in activating the secretory process such as protein kinase A and C activation does not alter these mRNA levels. Results of incorporation of radiolabel into AKHs and pro-AKHs, showed that in vitro secretory stimulation by CCAP also has no effects on the biosynthetic activity for AKHs. The reason for a clear absence of coupling of release and biosynthesis of AKHs is most likely that the continuous biosynthesis of these peptide hormones does not require any external regulatory signal for upregulation. The bulky pool of secretory material in ageing adipokinetic cells resulting from the continuous biosynthesis of AKHs is not predetermined to be stored only in ordinary secretory granules, but can also be accumulated into dilated cisternae of the rough endoplasmic reticulum, called intracisternal granules. These intracisternal granules appear to be not degraded and were shown to represent supplementary stores of intact secretory material, which after proper processing can be mobilized and released if called upon. Although the large amounts of stored hormones by far surpass the relatively small amounts needed for the control of the physiological response, adipokinetic cells preferentially release newly synthesized hormones. Data involving blocking of secretion and production of newly synthesized AKHs by the specific intracellular secretory transport inhibitor brefeldin A indicate that older secretory granules can no longer play a role in the secretory process. Consequently, regulated release of AKHs is dependent on the readily releasable pool of newly synthesized hormones. In conclusion, the strategy of the neuroendocrine adipokinetic cells to cope with variations in physiological stimulation appears to be the reliance on the generation of a sufficiently large storage pool of secretory material by means of a continuous biosynthesis.