Differential location of peptide hormones in the secretory pathway of insect adipokinetic cells

Abstract

Immunoreactivity of granules containing secretory material in the adipokinetic cells of the insect Locusta migratoria was studied using antisera specific for the adipokinetic hormone-associated peptides (AAP) I, II and III. Immunocytochemical detection of these associated peptides represents a new strategy for studying the intracellular location of the adipokinetic hormones and their prohormones. Fixation with 2% glutaraldehyde and 2% formaldehyde with low-temperature embedding in Lowicryl HM20 allowed highly selective immunogold labelling of both secretory and intracisternal granules. All three associated peptides were co-localized in secretory granules. This indicates that also all three adipokinetic hormones can be co-localized in these granules, which was confirmed by experiments in which, after secretory stimulation, adipokinetic hormone III was released from the adipokinetic cells together with adipokinetic hormones I and II. The immunopositivity of the intracisternal granules was similar to that of the secretory granules, although with the exception that the intracisternal granules did not show any specific reaction with anti-AAP III. The presence of AAP I and AAP II in intracisternal granules indicates that these granules only function as stores of adipokinetic prohormones I and II and not of adipokinetic prohormone III. The observed differences in storage in intracisternal granules among the three adipokinetic prohormones suggest differences in physiological significance of the three adipokinetic hormones in L. migratoria.

Induced hyperlipaemia and immune challenge in locusts

“…Passive immunisation experiments have been undertaken using IgG prepared against Lom-AKH-II (data not shown), and give similar results to those shown here for AKH-I. The possibility remains, of course, that the release of AKH-III is responsible for the changes in lipid metabolism induced by the immunogens, but this is considered very unlikely: the three AKHs in Locusta are thought to be released together (Harthoorn et al, 1999;Flanigan and Gade, 1999), and AKH-III is present in the corpora cardiaca in the least quantities of the three neurohormones (Oudejans et al, 1991;Oudejans et al, 1993). It is interesting to note that the particular preparation of LPS used here had unusual biological activity, in that it was able to activate prophenoloxidase in the haemolymph when injected alone: other commercial preparations of LPS from a variety of bacteria, including Pseudomonas, were reported to have no effect on the phenoloxidase activity in the haemolymph unless co-injected with AKH (Goldsworthy et al, 2002a).…”

Two peptide transmitters co-packaged in a single neurosecretory vesicle

“…On the other hand, peptide co-packaging has also been observed. For example, adipokinetic hormone (AKH) I, II and III are found in the same vesicles of secretory cells in the corpus cardiacum of Schistocerca gregaria [7], although they are cleaved from three different preprohormones [11]. Sossin et al [23] suggest that co-packaging may arise from fusion of two segregated vesicles, with peptides displaying segregation within the vesicle.…”

Neuronal co-localization of different isoforms of tachykinin-related peptides (LemTRPs) in the cockroach brain

“…An interesting example of cellular co-localization of related peptides is constituted by the locust metabolic hormones designated the adipokinetic hormones (AKHs). In Locusta migratoria, these are derived from three different precursors (Bogerd et al 1995), and yet, they are co-localized in the same glandular cells of the corpora cardiaca (Diederen et al 1987;Harthoorn et al 1999).…”