chapter 1 - Bentham Science

162 HPFP: Recent Advances in Insects and Other Arthropods Vol. 1 Suzuki and Iwami
by multifamily genes. These multicopy genes are expressed in various tissues including the brain [5],
suggesting that they have multiple functions. In contrast to the multiple copies of the insulin-like genes in
D. melanogaster, a single gene that encodes an insulin receptor homolog has been identified. Mutations of
this receptor result in small and longer-lived flies [9]. Here, we review the molecular aspects and
physiological functions of insulin and insulin-like (-related) peptides in arthropods.
2. MOLECULAR ASPECTS OF INSULIN-LIKE PEPTIDES
In humans, insulin is produced in the pancreatic islets of Langerhans. Insulin is synthesized as its precursor
form, preproinsulin, in the beta cells of the islets. Preproinsulin consists of a signal peptide, B-chain, Cpeptide,
and A-chain and is delivered to the endoplasmic reticulum for proteolytic processing. After
transfer into the endoplasmic reticulum, the signal peptide is excised from the preproinsulin peptide to
become proinsulin. The C-peptide is then excised from proinsulin by two enzymes, endopeptidase I and II,
in the Golgi apparatus. After being processed, the remaining two peptides, the B- and A-chains, are
crosslinked by disulfide bonds into a mature insulin molecule.
In insects, insulin-related molecules are generated from their precursor by proteolytic processing and have
similar structural features to mammalian insulin. The three-dimensional structure of bombyxin, an insulinrelated
molecule, has been solved. This analysis showed that the structure of bombyxin is similar to that of
mammalian insulin and relaxin, a member of the insulin superfamily [10]. The structure of the C-terminal
part of the bombyxin B-chain is more similar to that of relaxin than to that of insulin. The central part of the
bombyxin B-chain is essential for receptor recognition. The hybrid molecule of the bombyxin A-chain and
the human insulin B-chain demonstrates insulin activity but not bombyxin activity. However, the hybrid
molecule of the insulin A-chain and the bombyxin B-chain exhibits bombyxin activity only. The bombyxin
B-chain is therefore essential for bombyxin activity [11].
Since the bombyxin gene was first isolated from the B. mori genome (Table 1; [28]), 32 bombyxin genes
have been identified and 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 distribution patterns in the
cluster have been classified into three categories: gene pairs, gene triplets, and single genes [8]. In the pairs,
two bombyxin genes belonging to families A and B or to families B and C are opposed with opposite
transcriptional orientations. All triplets are arranged in the order of family B, family C, and family A genes
with the transcriptional directions of the family C and family A genes being opposite to the direction of the
family B gene. Equal and unequal crossings-over and duplications may have generated these unique
distribution patterns; however, the mechanism of bombyxin gene evolution remains to be elucidated.
Insulin-related peptides have been identified in many insect and crustacean species (Table 1). Protein
extract from D. melanogaster has been shown to exhibit insulin-like activity in mice [29]. Anti-insulinimmunoreactive
materials were found in the tobacco hornworm, Manduca sexta [30], the blue bottle fly,
Calliphora vomitoria [31], the oriental hornet, Vespa orientalis, the German yellow jacket wasp,
Paravespula germanica, the weaver ant, Polyrhachis simplex [32], the American lobster, Homarus
americanus [33], and many other species. Antiserum against bovine insulin has been used to stain the
median neurosecretory cells of the C. vomitoria brain [21]. In M. sexta, the localization of insulin was
detected by antiserum against the insulin B-chain in the median and lateral neurosecretory cells of the brain
[12] and frontal ganglion cells [13]. After the isolation of bombyxin, a monoclonal antibody against
bombyxin was generated and used for immunostaining to reveal the localization of bombyxin in the four
pairs of mid-dorsal neurosecretory cells [14]. Immunostaining for bombyxin was also observed in the
peripheral region of the corpora allata, indicating that bombyxin is secreted from the brain into hemolymph
through the peripheral region of the corpora allata [14].
Bombyxin producing cells were also identified by in situ hybridization in the four pairs of the median
neurosecretory cells of the larval B. mori brain [15]. Genes homologous to the bombyxin gene were also
isolated from S. cynthia ricini [17] and from the sweet potato hornworm, Agrius convolvuli. The bombyxin
homolog gene transcript in the A. convolvuli brain was also detected in the median neurosecretory cells