Bombyxin: An Insect Brain Peptide that Belongs to the Insulin Family

Abstract

ABSTRACT-Bombyxin is a 5 kDa secretory brain peptide that belongs to the insulin family. Bombyxin of the silkmoth Bombyx mori can induce adult development when injected into brain-removed dormant pupae of the saturniid moth Samia cynthia ricini by activating the prothoracic glands to synthesize and release ecdysone. Bombyx bombyxin has been shown to lower the concentration of the major haemolymph sugar, trehalose, and to elevate the trehalase activity in the midgut and muscles in Bombyx, but the doses required to be effective are higher than the amounts in the feeding larvae. The exact physiological function of bombyxin in Bombyx itself is therefore still obscure, but its insulin-like structure suggests it has important roles. Bombyxin comprises a mixture of highly heterogeneous molecular forms whose amino acid sequences have 40% identity with human insulin. The Bombyx bombyxin gene encodes a precursor consisting of the signal peptide, B chain, C peptide, and A chain, in that order from the N terminus. So far, 32 bombyxin genes have been identified in Bombyx, and they are classified into 7 families, A to G, according to their sequence similarity. The bombyxin genes have no introns and cluster in unique distribution patterns. The gene arrangement in the cluster has been classified into three categories: gene pairs, gene triplets, and single genes. Nucleotide sequence analysis indicates that equal and unequal crossings-over and duplications may have generated these unique distribution patterns. The Bombyx bombyxin genes are expressed predominantly in the brain and at low levels in a number of other tissues. Genes of all 7 families are expressed in four pairs of the medial neurosecretory cells of the brain. Detailed examination indicated that only a limited number of genes in the A, B and C family members are expressed and that their expression shows a gene-arrangement-dependent pattern.

Transcription Element Responsible for the Brain Cell-Specific Expression of the Bombyxin Gene that Encodes an Insect Insulin-related Peptide

“…As shown in Fig. 1B, strong green fluorescence was detected under a UV microscope with U-MNIBA cube in four pairs of the neurosecretory cells in pars intercerebralis of brain that had previously been identified as BPCs (Iwami, 1990(Iwami, , 2000Ichikawa, 1991). When the fluorescence was detected with U-MNUA cube to differentiate the real GFP expression from the tissue autofluorescence, very weak autofluorescence was observed in the brain (Fig.…”

“…Thirty-two copies of bombyxin gene are present in the Bombyx genome and classified into 7 families, A to G, according to the sequence similarities Iwami, 1990Iwami, , 2000Kondo et al, 1996;Yoshida et al, 1997). These genes are clustered on at least two genomic segments and arranged into three categories: a gene exists singly (single gene), makes a pair with an another gene (gene pair), or makes a triplet with two other genes (gene triplet).…”

“…These genes are clustered on at least two genomic segments and arranged into three categories: a gene exists singly (single gene), makes a pair with an another gene (gene pair), or makes a triplet with two other genes (gene triplet). All bombyxin genes lack introns Iwami, 1990Iwami, , 2000Kondo et al, 1996;Yoshida et al, 1997). Multiple bombyxin gene copies in the Bombyx genome and the lack of introns are in sharp contrast to vertebrate insulin genes that exist in one or two copies per haploid genome and have one or two introns (Steiner et al, 1985).…”

“…The vertebrate insulin genes are expressed in cells of the gasteroenteromic organ, whereas the invertebrate insulin-related genes are expressed predominantly in neurosecretory cells. In particular, bombyxin genes of all 7 families are expressed exclusively in four pairs of the neurosecretory cells in the brain (Iwami, 1990(Iwami, , 2000Yoshida et al, 1997). Bombyxin-producing cells (BPCs) are located in the pars intercerebralis (Ichikawa, 1991).…”

Studies of the Structure-Activity Relationships of Peptides and Proteins Involved in Growth and Development Based on Their Three-Dimensional Structures

“…33) The genes for the bombyxin homologs of S. cynthia ricini (SBRPs) were cloned and found to be expressed in the brain 29) in a manner similar to bombyxin. 34,35) Because of the similar molecular sizes of SBRPs (5 kDa) and S. cynthia ricini small PTTH (4 kDa), SBRPs are considered to be the most probable candidates for the small PTTH of S. cynthia ricini.…”