Characterization of Drosophila insulin receptor substrate

Biochemical characterization of the Drosophila insulin receptor kinase and longevity-associated mutants

Drosophila melanogaster (fruit fly) insulin receptor (D-IR) is highly homologous to the human counterpart. Like the human pathway, D-IR responds to numerous insulin-like peptides to activate cellular signals that regulate growth, development, and lipid metabolism in fruit flies. Allelic mutations in the D-IR kinase domain elevate life expectancy in fruit flies. We developed a robust heterologous expression system to express and purify wild-type and longevity-associated mutant D-IR kinase domains to investigate enzyme kinetics and substrate specificities. D-IR exhibits remarkable similarities to the human insulin receptor kinase domain but diverges in substrate preferences. We show that longevity-associated mutations reduce D-IR catalytic activity. Deletion of the unique kinase insert domain portion or mutations proximal to activating tyrosines do not influence kinase activity, suggesting their potential role in substrate recruitment and downstream signaling. Through biochemical investigations, this study enhances our comprehension of D-IR's role in Drosophila physiology, complementing genetic studies and expanding our knowledge on the catalytic functions of this conserved signaling pathway.

Identification and characterization of key genes in insulin signaling pathway as molecular targets for controlling the fall webworm, Hyphantria cunea

BACKGROUND

The insulin signaling pathway is closely related to metabolism, growth, reproductive capacity and lifespan of insects. However, the physiological function of the insulin signaling pathway is little known in Hyphantria cunea.

RESULTS

Five insulin signaling pathway genes (HcInR, HcPI3K, HcAKT, HcFOXO and HcTOR) in H. cunea were identified and characterized in this study. The spatiotemporal expression profiles of the genes showed that HcInR, HcAKT, HcPI3K and HcTOR expressions were higher at the egg stage than those in other development stages, whereas HcFOXO was highly expressed in the adult stage; all of these genes were highly expressed in the larval digestive system, especially in the midgut and hindgut. After RNA interference (RNAi) of the five genes in 5^th instar H. cunea larvae, weight gain and survival rate (except in the siHcAKT-injected group) were significantly decreased, and the developmental duration of larval and pupal stages were prolonged. In addition, knockdown of five genes in 7^th instar larvae decreased the pupation rate, survival rate and oviposition capacity, and resulted in abnormal development during larval-pupal transition.

CONCLUSION

Our findings indicate that the insulin signaling pathway plays essential roles in growth and development and the molting process in H. cunea, providing an important basis for developing new potentially molecular targets for RNAi-based pest control and understanding the mechanism of H. cunea outbreak. © 2022 Society of Chemical Industry.

A low-sugar diet enhances Drosophila body size in males and females via sex-specific mechanisms

In Drosophila, changes to dietary protein elicit different body size responses between the sexes. Whether these differential body size effects extend to other macronutrients remains unclear. Here, we show that lowering dietary sugar (0S diet) enhanced body size in male and female larvae. Despite an equivalent phenotypic effect between the sexes, we detected sex-specific changes to signalling pathways, transcription and whole-body glycogen and protein. In males, the low-sugar diet augmented insulin/insulin-like growth factor signalling pathway (IIS) activity by increasing insulin sensitivity, where increased IIS was required for male metabolic and body size responses in 0S. In females reared on low sugar, IIS activity and insulin sensitivity were unaffected, and IIS function did not fully account for metabolic and body size responses. Instead, we identified a female-biased requirement for the Target of rapamycin pathway in regulating metabolic and body size responses. Together, our data suggest the mechanisms underlying the low-sugar-induced increase in body size are not fully shared between the sexes, highlighting the importance of including males and females in larval studies even when similar phenotypic outcomes are observed.

Genetic manipulation of insulin/insulin-like growth factor signaling pathway activity has sex-biased effects on Drosophila body size

In Drosophila raised in nutrient-rich conditions, female body size is approximately 30% larger than male body size due to an increased rate of growth and differential weight loss during the larval period. While the mechanisms that control this sex difference in body size remain incompletely understood, recent studies suggest that the insulin/insulin-like growth factor signaling pathway (IIS) plays a role in the sex-specific regulation of processes that influence body size during development. In larvae, IIS activity differs between the sexes, and there is evidence of sex-specific regulation of IIS ligands. Yet, we lack knowledge of how changes to IIS activity impact body size in each sex, as the majority of studies on IIS and body size use single- or mixed-sex groups of larvae and/or adult flies. The goal of our current study was to clarify the body size requirement for IIS activity in each sex. To achieve this goal, we used established genetic approaches to enhance, or inhibit, IIS activity, and quantified pupal size in males and females. Overall, genotypes that inhibited IIS activity caused a female-biased decrease in body size, whereas genotypes that augmented IIS activity caused a male-specific increase in body size. These data extend our current understanding of body size regulation by showing that most changes to IIS pathway activity have sex-biased effects, and highlights the importance of analyzing body size data according to sex.

Female-biased upregulation of insulin pathway activity mediates the sex difference in Drosophila body size plasticity

Nutrient-dependent body size plasticity differs between the sexes in most species, including mammals. Previous work in Drosophila showed that body size plasticity was higher in females, yet the mechanisms underlying increased female body size plasticity remain unclear. Here, we discover that a protein-rich diet augments body size in females and not males because of a female-biased increase in activity of the conserved insulin/insulin-like growth factor signaling pathway (IIS). This sex-biased upregulation of IIS activity was triggered by a diet-induced increase in stunted mRNA in females, and required Drosophila insulin-like peptide 2, illuminating new sex-specific roles for these genes. Importantly, we show that sex determination gene transformer promotes the diet-induced increase in stunted mRNA via transcriptional coactivator Spargel to regulate the male-female difference in body size plasticity. Together, these findings provide vital insight into conserved mechanisms underlying the sex difference in nutrient-dependent body size plasticity.

Aging Regulated Through a Stability Model of Insulin/Insulin Growth Factor Receptor Function

Mutations of the insulin-like receptor in Drosophila extend lifespan. New research suggests this receptor operates in two modes. The first extends lifespan while slowing reproduction and reducing growth. The second strongly extends lifespan without impairing growth or reproduction; it confers longevity assurance. The mutation that confers longevity assurance resides in the kinase insert domain, which contains a potential SH2 binding site for substrate proteins. We apply a recent model for the function of receptor tyrosine kinases to propose how insulin receptor structure can modulate aging. This concept hypothesizes that strong insulin-like ligands promote phosphorylation of high threshold substrate binding sites to robustly induce reproduction, which impairs survival as a consequence of trade-offs. Lower levels of receptor stimulation provide less kinase dimer stability, which reduces reproduction and extends lifespan by avoiding reproductive costs. Environmental conditions that favor diapause alter the expression of insulin ligands to further repress the stability of the interacting kinase domains, block phosphorylation of low threshold substrates and thus induce a unique molecular program that confers longevity assurance. Mutations of the insulin receptor that block low-phosphorylation site interactions, such as within the kinase insert domain, can extend lifespan while maintaining overall dimer stability. These flies are long-lived while maintaining reproduction and growth. The kinase insert domain of Drosophila provides a novel avenue from which to seek signaling of the insulin/insulin-like growth factor system of humans that modulate aging without impacting reproduction and growth, or incurring insulin resistance pathology.

Insulin signaling represents a gating mechanism between different memory phases in Drosophila larvae

The ability to learn new skills and to store them as memory entities is one of the most impressive features of higher evolved organisms. However, not all memories are created equal; some are short-lived forms, and some are longer lasting. Formation of the latter is energetically costly and by the reason of restricted availability of food or fluctuations in energy expanses, efficient metabolic homeostasis modulating different needs like survival, growth, reproduction, or investment in longer lasting memories is crucial. Whilst equipped with cellular and molecular pre-requisites for formation of a protein synthesis dependent long-term memory (LTM), its existence in the larval stage of Drosophila remains elusive. Considering it from the viewpoint that larval brain structures are completely rebuilt during metamorphosis, and that this process depends completely on accumulated energy stores formed during the larval stage, investing in LTM represents an unnecessary expenditure. However, as an alternative, Drosophila larvae are equipped with the capacity to form a protein synthesis independent so-called larval anaesthesia resistant memory (lARM), which is consolidated in terms of being insensitive to cold-shock treatments. Motivated by the fact that LTM formation causes an increase in energy uptake in Drosophila adults, we tested the idea of whether an energy surplus can induce the formation of LTM in the larval stage. Suprisingly, increasing the metabolic state by feeding Drosophila larvae the disaccharide sucrose directly before aversive olfactory conditioning led to the formation of a protein synthesis dependent longer lasting memory. Moreover, formation of this memory component is accompanied by the suppression of lARM. We ascertained that insulin receptors (InRs) expressed in the mushroom body Kenyon cells suppresses the formation of lARM and induces the formation of a protein synthesis dependent longer lasting memory in Drosophila larvae. Given the numerical simplicity of the larval nervous system this work offers a unique prospect to study the impact of insulin signaling on the formation of protein synthesis dependent memories on a molecular level.

RNA interference against the putative insulin receptor substrate gene chico affects metamorphosis in Leptinotarsa decemlineata

It is noted that insect insulin/insulin-like growth factor/target of rapamycin signaling is critical for the regulation of metamorphosis in holometabolous insects. However, the molecular mechanism remains undetermined. Our previous findings reveal that RNA interference (RNAi)-mediated knockdown of an insulin gene (LdILP2) in Leptinotarsa decemlineata disturbs both 20-hydroxyecdysone (20E) and juvenile hormone (JH) signaling, and impairs pupation. In the present paper, we further observed that the expression of the insulin receptor substrate gene chico (Ldchico) and the phosphoinositide-3-kinase gene pi3k (Ldpi3k92E) was repressed in LdILP2 depleted larvae. Moreover, RNAi of Ldchico or Ldpi3k92E decreased food consumption, affected absorption and metabolism of amino acids and sugars, and reduced expression of several 20E (LdEcR, LdHR3 and LdE75) and JH (LdJHAMT, LdKr-h1 and LdHairy) signaling genes. As a result, larval development was postponed and larval growth was inhibited. Intriguingly, knockdown of Ldchico, rather than Ldpi3k92E, impaired larval-pupal and pupal-adult ecdysis, and specifically repressed transcription of another 20E signaling gene LdUSP. Ingestion of 20E rescued the expression of LdEcR, LdHR3 and LdE75, whereas 20E feeding restored neither the decreased LdUSP mRNA level, nor the reduced pupation and adult emergence rates in Ldchico RNAi larvae. Therefore, Chico is critical for the regulation of larval-pupal-adult transition by a PI3K-independent pathway, perhaps through activation of USP in L. decemlineata.

A Drosophila model of insulin resistance associated with the human TRIB3 Q/R polymorphism

Members of the Tribbles family of proteins are conserved pseudokinases with diverse roles in cell growth and proliferation. Both Drosophila Tribbles (Trbl) and vertebrate Trib3 proteins bind to the kinase Akt (Akt1) to block its phosphorylation activation and reduce downstream insulin-stimulated anabolism. A single nucleotide polymorphism (SNP) variant in human TRIB3, which results in a glutamine (Q) to arginine (R) missense mutation in a conserved motif at position 84, confers stronger Akt binding, resulting in reduced Akt phosphorylation, and is associated with a predisposition to Type 2 diabetes, cardiovascular disease, diabetic nephropathy, chronic kidney disease and leukemogenesis. Here, we used a Drosophila model to understand the importance of the conserved R residue in several Trbl functions. In the fly fat body, misexpression of a site-directed Q mutation at position R141 resulted in weakened binding to Drosophila Akt (dAkt), leading to increased levels of phospho-dAkt, increased cell and tissue size, and increases in the levels of stored glycogen and triglycerides. Consistent with the functional conservation of this arginine in modulating Akt activity, mouse Trib3 R84 misexpressed in the fly fat body blocked dAkt phosphorylation with a strength similar to wild-type Trbl. Limited mutational analysis shows that the R141 site dictates the strength of Akt binding but does not affect other Trbl-dependent developmental processes, suggesting a specificity that could serve as a drug target for metabolic diseases.

Caenorhabditis elegans DAF-2 as a Model for Human Insulin Receptoropathies

Human exome sequencing has dramatically increased the rate of identification of disease-associated polymorphisms. However, examining the functional consequences of those variants has created an analytic bottleneck. Insulin-like signaling in Caenorhabditis elegans has long provided a model to assess consequences of human insulin signaling mutations, but this has not been evaluated in the context of current genetic tools. We have exploited strains derived from the Million Mutation Project (MMP) and gene editing to explore further the evolutionary relationships and conservation between the human and C. elegans insulin receptors. Of 40 MMP alleles analyzed in the C. elegans insulin-like receptor gene DAF-2, 35 exhibited insulin-like signaling indistinguishable from wild-type animals, indicating tolerated mutations. Five MMP alleles proved to be novel dauer-enhancing mutations, including one new allele in the previously uncharacterized C-terminus of DAF-2. CRISPR-Cas9 genome editing was used to confirm the phenotypic consequence of six of these DAF-2 mutations and to replicate an allelic series of known human disease mutations in a highly conserved tyrosine kinase active site residue, demonstrating the utility of C. elegans for directly modeling human disease. Our results illustrate the challenges associated with prediction of the phenotypic consequences of amino acid substitutions, the value of assaying mutant isoform function in vivo, and how recently developed tools and resources afford the opportunity to expand our understanding even of highly conserved regulatory modules such as insulin signaling. This approach may prove generally useful for modeling phenotypic consequences of candidate human pathogenic mutations in conserved signaling and developmental pathways.

The role of insulin/IGF-1 signaling in the longevity of model invertebrates, C. elegans and D. melanogaster

Insulin/insulin-like growth factor (IGF)-1 signaling (IIS) pathway regulates aging in many organisms, ranging from simple invertebrates to mammals, including humans. Many seminal discoveries regarding the roles of IIS in aging and longevity have been made by using the roundworm Caenorhabditis elegans and the fruit fly Drosophila melanogaster. In this review, we describe the mechanisms by which various IIS components regulate aging in C. elegans and D. melanogaster. We also cover systemic and tissue-specific effects of the IIS components on the regulation of lifespan. We further discuss IIS-mediated physiological processes other than aging and their effects on human disease models focusing on C. elegans studies. As both C. elegans and D. melanogaster have been essential for key findings regarding the effects of IIS on organismal aging in general, these invertebrate models will continue to serve as workhorses to help our understanding of mammalian aging. [BMB Reports 2016; 49(2): 81-92]

Learning defects in Drosophila growth restricted chico mutants are caused by attenuated adenylyl cyclase activity

Background

Reduced insulin/insulin-like growth factor signaling (IIS) is a major cause of symmetrical intrauterine growth retardation (IUGR), an impairment in cell proliferation during prenatal development that results in global growth defects and mental retardation. In Drosophila, chico encodes the only insulin receptor substrate. Similar to other animal models of IUGR, chico mutants have defects in global growth and associative learning. However, the physiological and molecular bases of learning defects caused by chico mutations, and by symmetrical IUGR, are not clear.

Results

In this study, we found that chico mutations impair memory-associated synaptic plasticity in the mushroom bodies (MBs), neural centers for olfactory learning. Mutations in chico reduce expression of the rutabaga-type adenylyl cyclase (rut), leading to decreased cAMP synthesis in the MBs. Expressing a rut⁺ transgene in the MBs restores memory-associated plasticity and olfactory associative learning in chico mutants, without affecting growth. Thus chico mutations disrupt olfactory learning, at least in part, by reducing cAMP signaling in the MBs.

Conclusions

Our results suggest that some cognitive defects associated with reduced IIS may occur, independently of developmental defects, from acute reductions in cAMP signaling.

Drosophila Longevity Assurance Conferred by Reduced Insulin Receptor Substrate Chico Partially Requires d4eBP

Mutations of the insulin/IGF signaling (IIS) pathway extend Drosophila lifespan. Based on genetic epistasis analyses, this longevity assurance is attributed to downstream effects of the FOXO transcription factor. However, as reported FOXO accounts for only a portion of the observed longevity benefit, suggesting there are additional outputs of IIS to mediate aging. One candidate is target of rapamycin complex 1 (TORC1). Reduced TORC1 activity is reported to slow aging, whereas reduced IIS is reported to repress TORC1 activity. The eukaryotic translation initiation factor 4E binding protein (4E-BP) is repressed by TORC1, and activated 4E-BP is reported to increase Drosophila lifespan. Here we use genetic epistasis analyses to test whether longevity assurance mutants of chico, the Drosophila insulin receptor substrate homolog, require Drosophila d4eBP to slow aging. In chico heterozygotes, which are robustly long-lived, d4eBP is required but not sufficient to slow aging. Remarkably, d4eBP is not required or sufficient for chico homozygotes to extend longevity. Likewise, chico heterozygote females partially require d4eBP to preserve age-dependent locomotion, and both chico genotypes require d4eBP to improve stress-resistance. Reproduction and most measures of growth affected by either chico genotype are always independent of d4eBP. In females, chico heterozygotes paradoxically produce more rather than less phosphorylated 4E-BP (p4E-BP). Altered IRS function within the IIS pathway of Drosophila appears to have partial, conditional capacity to regulate aging through an unconventional interaction with 4E-BP.

Independent signaling by Drosophila insulin receptor for axon guidance and growth

The Drosophila insulin receptor (DInR) regulates a diverse array of biological processes including growth, axon guidance, and sugar homeostasis. Growth regulation by DInR is mediated by Chico, the Drosophila homolog of vertebrate insulin receptor substrate proteins IRS1-4. In contrast, DInR regulation of photoreceptor axon guidance in the developing visual system is mediated by the SH2-SH3 domain adaptor protein Dreadlocks (Dock). In vitro studies by others identified five NPXY motifs, one in the juxtamembrane region and four in the signaling C-terminal tail (C-tail), important for interaction with Chico. Here we used yeast two-hybrid assays to identify regions in the DInR C-tail that interact with Dock. These Dock binding sites were in separate portions of the C-tail from the previously identified Chico binding sites. To test whether these sites are required for growth or axon guidance in whole animals, a panel of DInR proteins, in which the putative Chico and Dock interaction sites had been mutated individually or in combination, were tested for their ability to rescue viability, growth and axon guidance defects of dinr mutant flies. Sites required for viability were identified. Unexpectedly, mutation of both putative Dock binding sites, either individually or in combination, did not lead to defects in photoreceptor axon guidance. Thus, either sites also required for viability are necessary for DInR function in axon guidance and/or there is redundancy built into the DInR/Dock interaction such that Dock is able to interact with multiple regions of DInR. We also found that simultaneous mutation of all five NPXY motifs implicated in Chico interaction drastically decreased growth in both male and female adult flies. These animals resembled chico mutants, supporting the notion that DInR interacts directly with Chico in vivo to control body size. Mutation of these five NPXY motifs did not affect photoreceptor axon guidance, segregating the roles of DInR in the processes of growth and axon guidance.

Divergent and convergent roles for insulin-like peptides in the worm, fly and mammalian nervous systems

Insulin signaling plays a critical role in coupling external changes to animal physiology and behavior. Despite remarkable conservation in the insulin signaling pathway components across species, divergence in the mechanism and function of the signal is evident. Focusing on recent findings from C. elegans, D. melanogaster and mammals, we discuss the role of insulin signaling in regulating adult neuronal function and behavior. In particular, we describe the transcription-dependent and transcription-independent aspects of insulin signaling across these three species. Interestingly, we find evidence of diverse mechanisms underlying complex networks of peptide action in modulating nervous system function.

Mutations in the Drosophila insulin receptor substrate, CHICO, impair olfactory associative learning

CHICO, the Drosophila homolog of vertebrate insulin receptor substrate (IRS), mediates insulin/insulin-like growth factor signaling (IIS), and reductions in chico severely disrupt cell growth and proliferation. We found extensive expression of chico in various Drosophila brain regions including the mushroom bodies (MBs), critical neural structures for olfactory learning. chico null mutants have significantly reduced brain sizes and perform poorly in an olfactory associative learning task, although their sensitivity to the odors and electric shocks used in this learning paradigm are normal. When initial memory is normalized by training for different amounts of time (short-duration training protocols), memory retention and retrieval in chico flies are indistinguishable from that of wild-type flies, demonstrating that chico mutants are defective specifically for memory formation. Inducing expression of a chico(+) transgene in neurons throughout development restores normal learning in a chico background, while inducing chico(+) specifically at the adult stage does not, suggesting that chico is required for development of a brain region required for forming olfactory associations. Significantly, expressing chico(+) in the MBs restores the number of MB neurons to wild-type amounts and also rescues chico learning defects. Our results suggest that chico-dependent growth of the MBs is essential for development of learning ability.

Insect insulin receptors: insights from sequence and caste expression analyses of two cloned hymenopteran insulin receptor cDNAs from the fire ant

The insulin and insulin-like growth factor (IGF) signalling (IIS) pathway in the honey bee (Apis mellifera) is linked to reproductive division of labour and foraging behaviour. Two insulin receptor genes are present in the released genomes of other social hymenopterans. Limited information is available on the IIS pathway role in ants. The predicted insulin receptor sequences from the recently released draft genome of the fire ant Solenopsis invicta (Hymenoptera: Formicidae) are incomplete and biologically significant data are also lacking. To elucidate the role of the IIS pathway in the fire ant, two putative insulin receptors (SiInR-1 and SiInR-2) were cloned; the first InR cDNAs cloned from social insects. Analyses of putative post-translational modification sites in SiInRs revealed the potential for differential regulation. We investigated the transcriptional expression of both receptors at different developmental stages, castes and queen tissues. In last instar larvae and pharate pupae of workers and reproductive, transcriptional abundance of both receptors was negatively correlated with body size and nutritional status. The expression level of both receptors in different queen tissues appears to correlate with requirements for queen reproductive physiology and behaviours. This study contributes new information to the understanding of social insects because in fire ants juvenile hormone acts as a gonadotropin and workers are fully sterile, contrary to honey bees.

Insect Neuropeptide and Peptide Hormone Receptors: Current Knowledge and Future Directions

Peptides form a very versatile class of extracellular messenger molecules that function as chemical communication signals between the cells of an organism. Molecular diversity is created at different levels of the peptide synthesis scheme. Peptide messengers exert their biological functions via specific signal-transducing membrane receptors. The evolutionary origin of several peptide precursor and receptor gene families precedes the divergence of the important animal Phyla. In this chapter, current knowledge is reviewed with respect to the analysis of peptide receptors from insects, incorporating many recent data that result from the sequencing of different insect genomes. Therefore, detailed information is provided on six different peptide receptor families belonging to two distinct receptor categories (i.e., the heptahelical and the single transmembrane receptors). In addition, the remaining problems, the emerging concepts, and the future prospects in this area of research are discussed.

Insulin reduces apoptosis and increases DNA synthesis and cell size via distinct signalling pathways in Drosophila Kc cells

During development of Drosophila, cell proliferation and size are known to be regulated by insulin. Here we use Drosophila Kc cells to examine the molecular basis for the control of cell growth by insulin. Growing cells in the presence of insulin increased cell number above control levels at 16, 24, 48 and 72 h. We have demonstrated a novel anti-apoptotic effect of insulin (approximately 50%) in these cells, measured by caspase 3-like activity, which contributed to the increase in cell number. The anti-apoptotic effect was observed both in control cells and those in which apoptosis was induced by ultraviolet irradiation. An approximately 2-fold stimulation of bromodeoxyuridine incorporation demonstrated that insulin also increased Kc cell proliferation by stimulating new DNA synthesis. The ability of insulin to increase cell number, stimulate bromodeoxyuridine incorporation and reduce caspase 3-like activity was prevented by PD98059, which inhibits activation of the Drosophila extracellular signal regulated kinase (DERK) pathway, and was unaffected by wortmannin, an inhibitor of Drosophila phosphatidylinositol 3-kinase (DPI3K). Insulin also increased cell size approximately 2-fold and this was prevented by wortmannin and rapamycin, an inhibitor of Drosphilia target of rapamycin (DTOR). In summary, we show that DERK plays an important role in mediating the effect of insulin to reduce apoptosis and increase DNA synthesis whereas the DPI3K/DTOR/Dp70S6 kinase pathway mediates effects of insulin on cell size in Drosophila Kc cells.

Insulin/IGF and target of rapamycin signaling: a TOR de force in growth control

'They come in all sizes.' Apart from its origin and use in the clothing industry, this saying reflects the fact that the size of organisms spans an enormous range. Whether destined to be large or small, species grow in an organized fashion to reach their final specified size. For growth to proceed, food must be metabolized to liberate energy in the form of adenosine triphosphate (ATP) and protein building blocks in the form of amino acids. One major orchestrator of this complex growth process in diverse metazoan species is the insulin/insulin-like growth factor (IGF) system. This review summarizes current studies primarily from Drosophila regarding the function of the insulin/IGF system in the control of growth.

Genetic analysis of insulin signaling in Drosophila

Studies in the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have revealed that components of the insulin signaling pathway have been highly conserved during evolution. Genetic analysis in Drosophila suggests that structural conservation also extends to the functional level. Flies carrying mutations that reduce insulin signaling have a growth deficiency phenotype similar to that seen in mice with disruptions of genes encoding insulin-like growth factors (IGFs) or the IGF-I receptor. Recent studies in flies have demonstrated a role for the insulin signaling pathway in the regulation of metabolism, reproduction and lifespan via modulation of central neuroendocrine pathways. Similarly, mice with loss of brain insulin receptors or insulin receptor substrate 2 deficiency exhibit neuroendocrine defects and female infertility. These parallels suggest that the insulin system has multiple conserved roles, acting directly to modulate growth and indirectly, via the neuroendocrine system, to modulate peripheral physiology in response to changes in nutrient availability.

Insulin-related peptides and their conserved signal transduction pathway

The 'insulin superfamily' is an ancient category of small, structurally related proteins, such as insulin, insulin-like growth factors (IGF) and relaxin. Insulin-like signaling molecules have also been described in different invertebrates, including nematodes, mollusks, and insects. They initiate an evolutionary conserved signal transduction mechanism by binding to a heterotetrameric, membrane-spanning receptor tyrosine kinase. Recent physiological and genetic studies have revealed that, in different metazoans, the insulin signaling pathway plays a pivotal role in the regulation of a variety of interrelated, fundamental processes, such as metabolism, growth, reproduction and aging.