Biological function and Structural of Allatotropin
Discovery
In 1989, allatotropin was originally characterized in the tobacco hornworm Manduca sexta as a neuropeptide that stimulates the biosynthesis of juvenile hormone (JH) by the corpora allata (CA) in vitro.1 Subsequently, functional studies have revealed multiple bioactivities of this peptide. Homologous peptides have been identified in other families of insects. Some of the myotropic factors identified in other phyla, showing high similarity to insect allatotropins, constitute a large family of peptides.
Structural
features M. sexta allatotropin (Manse-AT) is a tridecapeptide with its C-terminal amidation. The family of this peptides, either isolated or predicted from cDNA sequences and transcriptomic data, have a similar sequence length (13–16 aa residues) and an amidated C-terminus.
Gene, mRNA, and precursor
Three alternatively spliced mRNAs for Manse-AT have been characterized.Each mRNA encodes a precursor protein, from which one copy of Manse-AT is derived. The two longer mRNAs contain the insertion of one or two alternative exons, each of which codes for one or two additional peptides structurally related to ManseAT (Manse-ATL (AT-like) -I, -II, -III). A cDNA coding for a precursor protein containing Manse-AT and a peptide similar to Manse-ATL-III have been reported in other Lepidoptera, suggesting that the presence of the ATL peptide in addition to AT is not limited to M. sexta.
Biological function
Target cells/tissues and functions Manse-AT stimulates JH biosynthesis by adult female CA from many lepidopteran insects, and larval CA from some lepidopteran species.Manse-AT is also reported to stimulate the CA of nonlepidopteran insects, such as the honey bee and blow fly, although no allatotropic peptide has been isolated from these insects.
The expression of ATR in the CA observed in some insects readily explains the action of AT on the CA, but its expression in the CC rather than the CA in B. mori does not. In this species, because the ATR-expressing cells in the CC produce short neuropeptide-F (sNPF) and this peptide inhibits the JH synthetic activity of the CA in vitro, it is proposed that ATR may indirectly mediate AT activity on the CA by suppressing sNPF action on the CA.
Besides the action on the CA, AT accelerates the heart rate, inhibits ion transport across the midgut epithelium, and controls the release of digestive enzymes in the midgut as well as muscle contraction, eventually affecting feeding behavior.AT family peptides from hemimetabolous insects such as locust myotropin also show cardiostimulatory and myoregulatory activities.The effects of AT on noninsect invertebrates have been examined using synthetic mosquito AT. This AT showed myoregulatory activity on the turbellarians and hydra.A widespread role of AT in myoregulation and the presence of AT in organisms that do not have JH suggest that the primary role of AT is myoregulation and that the control of JH synthesis is secondary.
