InteractiveFly: GeneBrief

Secreted decoy of InR: Biological Overview | References

BIOLOGICAL OVERVIEW

Members of the insulin peptide family have conserved roles in the regulation of growth and metabolism in a wide variety of metazoans. Drosophila insulin-like peptides (Dilps) promote tissue growth through the single insulin-like receptor (InR). Despite the important role of Dilps in nutrient-dependent growth control, the molecular mechanism that regulates the activity of circulating Dilps is not well understood. This study reports the function of a novel secreted decoy of InR (SDR) as a negative regulator of insulin signaling. SDR is predominantly expressed in surface glia of the larval CNS and is secreted into the hemolymph. Larvae lacking SDR grow at a faster rate, thereby increasing adult body size. Conversely, overexpression of SDR reduces body growth non-cell-autonomously. SDR is structurally similar to the extracellular domain of InR and interacts with several Dilps in vitro independent of Imp-L2, the ortholog of the mammalian insulin-like growth factor-binding protein 7 (IGFBP7). It was further demonstrated that SDR is constantly secreted into the hemolymph independent of nutritional status and is essential for adjusting insulin signaling under adverse food conditions. It is proposed that Drosophila uses a secreted decoy to fine-tune systemic growth against fluctuations of circulating insulin levels (Okamoto, 2013).

The insulin/insulin-like growth factor (IGF) signaling (IIS) pathway is an evolutionarily conserved endocrine signaling pathway that controls a wide variety of processes, including growth and development. The central players in this pathway are insulin-like peptides, which include insulin, IGF-1, and IGF-II in mammals; 40 insulin-like peptides in worms and the seven canonical Drosophila insulin-like peptides (Dilps) in flies that can promote body growth. These secreted ligands transmit intercellular signals through the activation of insulin receptor tyrosine kinase (insulin-like receptor [InR] in Drosophila), leading to the activation of the PI3-kinase (PI3K) signaling pathway (Okamoto, 2013).

In mammals, six classic IGF-binding proteins (IGFBPs) bind to IGF-I and IGF-II with high affinity in serum and modulate IGF activity. Less than 5% of the IGFs in the circulation are free, and most IGFs are bound in the complex, which consists of IGF-I or IGF-II, IGFBP3, and an acid-labile subunit (ALS). This complex is believed to be the principle carrier form of IGFs. These proteins either enhance or dampen the IIS pathway by extending the half-life of IGFs, by altering the local and systemic availability of IGFs, or by preventing them from binding to the receptor (Hwa, 1999). In addition, an IGFBP-related protein, IGFBP7, binds to IGFs with comparatively low affinity and has been demonstrated to be a potent tumor suppressor in a wide variety of cancers (Okamoto, 2013).

Insects also express an IGFBP-like protein, referred to as Imp-L2, that resembles IGFBP7. Imp-L2 binds to Dilp-2 and Dilp-5 and acts as a non-cell-autonomous inhibitor of IIS during development. However, it remains unknown whether other factors besides Imp-L2 regulate the seven Dilps in the extracellular space (Okamoto, 2013).

This study characterizes a secreted decoy of InR (SDR) that binds to Dilps and antagonizes IIS during development. Biochemical and genetic analyses suggest that SDR belongs to a novel class of functional insulin-binding proteins and that it acts in a manner complementary to Imp-L2 in Drosophila (Okamoto, 2013).

Drosophila has seven Dilps and one IGFBP-type protein (Imp-L2), whereas mammals have seven IGFBPs. An ongoing challenge has been to resolve how these proteins cooperate in the control of systemic growth via IIS. The biochemical experiments described in this study revealed that Imp-L2 binds to several Dilps, including Dilp1, Dilp2, Dilp4, Dilp5, and Dilp6. In contrast, SDR binds most strongly to Dilp3, indicating that each Dilp has distinct binding preferences for either Imp-L2 or SDR. Interestingly, in addition to expression in brain insulin-producing cells (IPCs), dilp3 is expressed in a subset of glia and midgut muscles where SDR is highly expressed (Veenstra, 2008; Sousa-Nunes, 2011). Although SDR can be detected in the hemolymph and regulates systemic growth, SDR may also act locally in the tissues where it is expressed. The slight up-regulation of the SDR transcripts during the third instar likely reflects SDR expression in imaginal discs. It is possible that SDR expression is actively regulated in a stage- and tissue-specific manner to fulfill such a local function (Okamoto, 2013).

Recent reports have revealed that fluctuations in ligand levels have more significant biological impacts on downstream signaling events than was previously appreciated. The dynamics of insulin levels and their specific temporal pattern can elicit a unique physiological response through different kinetic behavior and network connectivity of Akt. Therefore, the function of SDR and Imp-L2 in the regulation of Dilps may be more complex than the interference of a ligand-receptor interaction. The ability to maintain constant Dilp-binding protein levels in the hemolymph would provide a robust system for growth regulation in combination with dynamic Imp-L2 levels (Okamoto, 2013).

There are functional similarities between SDR and Imp- L2 in the sense that both act as negative regulators of IIS. However, phenotypic and biochemical analyses revealed important differences between SDR and the Imp-L2-ALS complex (Arquier, 2008; Honegger, 2008). First, heterozygous SDR mutants exhibit approximately normal body size, whereas loss of one copy of Imp-L2 leads to a moderate increase in body size. The partial knockdown of SDR by a weak ubiquitous Gal4 driver, arm-Gal4, consistently showed no phenotype. Second, overexpression of Imp-L2 causes lethality, whereas moderate overexpression of Imp-L2 significantly impairs body growth, resulting in a delay of larval development. In contrast, SDR overexpression leads to moderate reduction in body weight with no apparent developmental delay or lethality, even though the vast excess of SDR proteins was achieved compared with endogenous levels. Consistently, the lethality induced by ectopic Dilp2 expression can be rescued by overexpression of Imp-L2 but not by overexpression of SDR. Third, both Imp-L2 and ALS are widely expressed in a number of different tissues. Fat body-derived Imp-L2-ALS, however, seems to be critical for the systemic regulation of IIS, whereas glia-derived SDR is important in this respect. Last, Imp-L2 and ALS expression responds to nutritional status, whereas the production of SDR is constant (Okamoto, 2013).

It is interesting to consider the analogous case in mammals, which exhibit distinct alterations in IGFBP protein levels after fasting; IGFBP1 is up-regulated by fasting, whereas IGFBP2 and IGFBP3 remain constant in circulating blood. SDR seems to act as a constitutive regulator of Dilps in the hemolymph, whereas Imp-L2 is a dynamic regulator that inhibits IIS in response to nutrient levels. It is equally possible, however, that the function of SDR is regulated post-translationally in the hemolymph; secreted SDR is inactive, and modifications and/or binding partners allow SDR to bind to Dilps. In contrast, Imp-L2 is likely active once secreted into the hemolymph. Further analysis will be required to understand the regulatory mechanism of SDR in the hemolymph and the functional relationship between SDR and the Imp-L2-ALS complex. Together, these observations suggest that Drosophila uses two different regulators that have distinct molecular activities to fine-tune the activity of circulating Dilps (Okamoto, 2013).

In mammals, antagonistic soluble decoys have been described for many receptors, including receptor tyrosine kinases, immune receptors, and seven-pass transmembrane receptors. Although the SDR-like gene is found only in dipterous insects, including flies and mosquitoes, similar decoy systems for IIS are likely present in other species. The C. elegans insulin receptor Daf-2 contains an alternative splice variant that encodes a putative secreted protein. Similarly, the mammalian insulin receptor can potentially produce a soluble decoy by alternative intronic polyadenylation (Vorlová, 2011). In both cases, the physiological function of the putative secreted protein has not been addressed. The type II IGF receptor, also known as mannose-6-phosphate receptor, is thought to be cleaved to produce a soluble form (sIGF2R) that binds to IGF-II with high affinity in vivo. Indeed, ectopic expression of sIGF2R inhibits cellular growth and reduces organ size. In Drosophila, however, a soluble form of InR that is produced by alternative splicing or ectodomain shedding has not been described. Instead, SDR may have arisen by a gene duplication event in Drosophila (Okamoto, 2013).

It remains unknown whether SDR can form a nonfunctional heterodimer with InR on the plasma membrane and thereby directly antagonize signaling through InR. Based on sequence similarity, SDR and InR are expected to show similar binding affinities for each Dilp. It is therefore hypothesized that receptor-like decoy molecules function to fine-tune receptor signaling by sequestering multiple ligands. The constitutive production of such decoys may be beneficial to adapt endocrine signals in response to environmental changes, including the availability of food (Okamoto, 2013).

Search PubMed for articles about Drosophila Secreted decoy of InR

Arquier, N., Geminard, C., Bourouis, M., Jarretou, G., Honegger, B., Paix, A. and Leopold, P. (2008). Drosophila ALS regulates growth and metabolism through functional interaction with insulin-like peptides. Cell Metab 7: 333-338. PubMed ID: 18396139

Honegger, B., Galic, M., Kohler, K., Wittwer, F., Brogiolo, W., Hafen, E. and Stocker, H. (2008). Imp-L2, a putative homolog of vertebrate IGF-binding protein 7, counteracts insulin signaling in Drosophila and is essential for starvation resistance. J Biol 7: 10. PubMed ID: 18412985

Hwa, V., Oh, Y. and Rosenfeld, R. G. (1999). The insulin-like growth factor-binding protein (IGFBP) superfamily. Endocr Rev 20: 761-787. PubMed ID: 10605625

Okamoto, N., Nakamori, R., Murai, T., Yamauchi, Y., Masuda, A. and Nishimura, T. (2013). A secreted decoy of InR antagonizes insulin/IGF signaling to restrict body growth in Drosophila. Genes Dev 27: 87-97. PubMed ID: 23307869

Sousa-Nunes, R., Yee, L. L. and Gould, A. P. (2011). Fat cells reactivate quiescent neuroblasts via TOR and glial insulin relays in Drosophila. Nature 471: 508-512. PubMed ID: 21346761

Veenstra, J. A., Agricola, H. J. and Sellami, A. (2008). Regulatory peptides in fruit fly midgut. Cell Tissue Res 334: 499-516. PubMed ID: 18972134

Vorlova, S., Rocco, G., Lefave, C. V., Jodelka, F. M., Hess, K., Hastings, M. L., Henke, E. and Cartegni, L. (2011). Induction of antagonistic soluble decoy receptor tyrosine kinases by intronic polyA activation. Mol Cell 43: 927-939. PubMed ID: 21925381

date revised: 7 September 2013

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