An insulin-like peptide (ILP) was first isolated from the silkmoth Bombyx moriand designated Bombyxin (Nagasawa et al., 1986). A single ILP was also found in the locust Locusta migratoria (Hetru et al., 1991). Now ILPs have been identified in all investigated insect species and the number of ILP encoding genes in each species range between 1 (e. g. locusts) and 38 (Bombyx mori) (Badisco et al., 2008; Mizoguchi and Okamoto, 2013). In Drosophila 8 ILP genes are known (see Grönke et al., 2010; Nässel and Vanden Broeck, 2016). Two of these are considered relaxin-like, one is IGF-like, and the others insulin-like.
Insulins and ILPs typically consist of heterodimers of A and B chains of about 20 and 30 amino acids, respectively. The ILP precursor may also generate a C-peptide. In the A- and B-chains six cysteine residues are located at highly conserved sites and external and internal disulfide bridges contribute to a conserved folding structure. In contrast to insulins, the IGFs are single chain peptides with internal cysteine bridges (A and B chains with C-peptide attached).
The ILPs of insulin- and IGF-type act on receptor tyrosin kinases (typically one receptor type in each species) and the relaxin-like peptide, DILP8, in Drosophila activates a GPCR type receptor, Lgr3 (Colombani et al., 2015).
In all insects studied ILPs of one or more types are produced by a set of median neurosecretory cells with axon terminations in the corpora cardiaca and associated neurohemal areas. In some species, such as Drosophila, a few ILPs are produced by other neuron types, as well as intestinal cells or imaginal discs (see Nässel and Vanden Broeck, 2016). The IGF-like peptides are commonly expressed in the adipocytes of the fat body.
ILPs have a broad range of functions in insects, including regulation of growth, metabolism, reproduction, stress responses and lifespan. At least in Drosophila the major different ILP gene products (insulin-like, IGF-like and relaxin-like) appear to have distinct functions. The entire insulin signaling cascade downstream the insulin receptor (tyrosine kinase) is well conserved over evolution.
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