A comparative structural bioinformatics analysis of the insulin receptor family ectodomain based on phylogenetic information

Insulin Receptor Isoforms and Insulin Growth Factor-like Receptors: Implications in Cell Signaling, Carcinogenesis, and Chemoresistance

This comprehensive review thoroughly explores the intricate involvement of insulin receptor (IR) isoforms and insulin-like growth factor receptors (IGFRs) in the context of the insulin and insulin-like growth factor (IGF) signaling (IIS) pathway. This elaborate system encompasses ligands, receptors, and binding proteins, giving rise to a wide array of functions, including aspects such as carcinogenesis and chemoresistance. Detailed genetic analysis of IR and IGFR structures highlights their distinct isoforms, which arise from alternative splicing and exhibit diverse affinities for ligands. Notably, the overexpression of the IR-A isoform is linked to cancer stemness, tumor development, and resistance to targeted therapies. Similarly, elevated IGFR expression accelerates tumor progression and fosters chemoresistance. The review underscores the intricate interplay between IRs and IGFRs, contributing to resistance against anti-IGFR drugs. Consequently, the dual targeting of both receptors could present a more effective strategy for surmounting chemoresistance. To conclude, this review brings to light the pivotal roles played by IRs and IGFRs in cellular signaling, carcinogenesis, and therapy resistance. By precisely modulating these receptors and their complex signaling pathways, the potential emerges for developing enhanced anti-cancer interventions, ultimately leading to improved patient outcomes.

Insulin Receptor-Related Receptor Regulates the Rate of Early Development in Xenopus laevis

The orphan insulin receptor-related receptor (IRR) encoded by insrr gene is the third member of the insulin receptor family, also including the insulin receptor (IR) and the insulin-like growth factor receptor (IGF-1R). IRR is the extracellular alkaline medium sensor. In mice, insrr is expressed only in small populations of cells in specific tissues, which contain extracorporeal liquids of extreme pH. In particular, IRR regulates the metabolic bicarbonate excess in the kidney. In contrast, the role of IRR during Xenopus laevis embryogenesis is unknown, although insrr is highly expressed in frog embryos. Here, we examined the insrr function during the Xenopus laevis early development by the morpholino-induced knockdown. We demonstrated that insrr downregulation leads to development retardation, which can be restored by the incubation of embryos in an alkaline medium. Using bulk RNA-seq of embryos at the middle neurula stage, we showed that insrr downregulation elicited a general shift of expression towards genes specifically expressed before and at the onset of gastrulation. At the same time, alkali treatment partially restored the expression of the neurula-specific genes. Thus, our results demonstrate the critical role of insrr in the regulation of the early development rate in Xenopus laevis.

Structures and interactions of insulin-like peptides from cone snail venom

The venomous insulin-like peptides released by certain cone snails stimulate hypoglycemic shock to immobilize fish and catch the prey. Compared to human insulin (hIns), the cone snail insulins (Con-Ins) are typically monomeric and shorter in sequence, yet they exhibit moderate hIns-like biological activity. We have modeled six variants of Con-Ins (G3, K1, K2, T1A, T1B, and T2) and carried out explicit-solvent molecular dynamics (MD) simulations of eight types of insulins, two with known structures (hIns and Con-Ins-G1) and six Con-Ins with modeled structures, to characterize key residues of each insulin that interact with the truncated human insulin receptor (μIR). We show that each insulin/μIR complex is stable during explicit-solvent MD simulations and hIns interactions indicate the highest affinity for the "site 1" of IR. The residue contact maps reveal that each insulin preferably interacts with the αCT peptide than the L1 domain of IR. Through analysis of the average nonbonded interaction energy contribution of every residue of each insulin for the μIR, we probe the residues establishing favorable interactions with the receptor. We compared the interaction energy of each residue of every Con-Ins to the μIR and observed that γ-carboxylated glutamate (Gla), His, Thr, Tyr, Tyr/His, and Asn in Con-Ins are favorable substitutions for GluA4, AsnA21, ValB12, LeuB15, GlyB20, and ArgB22 in hIns, respectively. The identified insulin analogs, although lacking the last eight residues of the B-chain of hIns, bind strongly to μIR. Our findings are potentially useful in designing potent fast-acting therapeutic insulin.

Characterization of Insulin-Like Growth Factor Binding Protein-5 (IGFBP-5) Gene and Its Potential Roles in Ontogenesis in the Pacific Abalone, Haliotis discus hannai

Insulin-like growth factor binding protein family is known to be involved in regulating biological actions of insulin-like growth factors (IGFs). In the present study, a full-length cDNA encoding the IGFBP-5 gene was cloned and characterized from the cerebral ganglion of Haliotis discus hannai. The 921-bp full-length sequence of Hdh IGFBP-5 cDNA transcript had an open reading frame of 411 bp encoding a predicted polypeptide of 136 amino acids, sharing high sequence identities with IGFBP-5 of H. diversicolor. The deduced Hdh IGFBP-5 protein contained a putative transmembrane domain (13-35 aa) in the N-terminal region. It also possessed a signature domain of IGFBP protein family (IB domain, 45-120 aa). Six cysteine residues (Cys-47, Cys-55, Cys-73, Cys-85, Cys-98, and Cys-118) in this cloned sequence could potentially form an intrachain disulfide bond. Phylogenetic analysis indicated that the Hdh IGFBP-5 gene was robustly clustered with IGFBP-5 of H. diversicolor. Tissue distribution analysis based on qPCR assay showed that Hdh IGFBP-5 was widely expressed in all examined tissues, with significantly (p < 0.05) higher expression in the cerebral ganglion. In male and female gametogenetic cycles, Hdh IGFBP-5 mRNA was expressed at all stages, showing significantly higher level at ripening stage. The expression level of Hdh IGFBP-5 mRNA was significantly higher in the polar body stage than in other ontogenic stages. In situ hybridization revealed that Hdh IGFBP-5 mRNA was present in the neurosecretory cells of the cerebral ganglion. This is the first study describing IGFBP-5 in H. discus hannai that might be synthesized in the neural ganglia. Our results demonstrate Hdh IGFBP-5 is involved in regulating ontogenic development and reproductive regulation of H. discus hannai.

Cryo-EM structure of the complete and ligand-saturated insulin receptor ectodomain

Glucose homeostasis and growth essentially depend on the hormone insulin engaging its receptor. Despite biochemical and structural advances, a fundamental contradiction has persisted in the current understanding of insulin ligand-receptor interactions. While biochemistry predicts two distinct insulin binding sites, 1 and 2, recent structural analyses have resolved only site 1. Using a combined approach of cryo-EM and atomistic molecular dynamics simulation, we present the structure of the entire dimeric insulin receptor ectodomain saturated with four insulin molecules. Complementing the previously described insulin-site 1 interaction, we present the first view of insulin bound to the discrete insulin receptor site 2. Insulin binding stabilizes the receptor ectodomain in a T-shaped conformation wherein the membrane-proximal domains converge and contact each other. These findings expand the current models of insulin binding to its receptor and of its regulation. In summary, we provide the structural basis for a comprehensive description of ligand-receptor interactions that ultimately will inform new approaches to structure-based drug design.

Abundance of insulin-like growth factors 1 and 2, and type 1 insulin-like growth factor receptor in placentas of dogs

Insulin-like growth factors (IGFs) are among the primary compounds regulating placental development. In bitches, relative abundance of IGF1, IGF2 and IGFR1 mRNA transcripts have been studied in the pre-implantation uterus and early endotheliochorial placentas. The IGF2 and IGFR1 distribution has also been previously described in the uterus before embryo implantation. The aim of this study was to detect, characterize, and localize the presence of IGF1, IGF2, and IGFR1 in early-developing and mature placentas of dogs. Placentas of 15 bitches were analyzed using immunohistochemistry. The IGFs were located in endometrial epithelium and glands, with the staining pattern and intensity being less in mature placentas. Cytotrophoblast cells (CTB) and syncytiotrophoblast (STB) cells contained both IGFs; the labeling was greater in CTB of the early-developing than mature placentas. The maternal endothelium was positively stained for both IGFs, while the vascular endothelium of the chorioallantoic membrane were only stained for IGF2. The IGFR1 was detected in all cell populations evaluated. Results regarding trophoblastic IGF are quite consistent with those reported in human placentas. Spatiotemporal IGFs/IGFR1 pattern might reflect the occurrence of autocrine and paracrine signaling during placentation in bitches, and the involvement in early placental developmental processes. Furthermore, it is hypothesized that, besides hemotrophic actions of plasma IGFs, endometrial secreted IGFs may promote early placental development through histotrophic signaling.

Roles of the insulin signaling pathway in insect development and organ growth

Organismal development is a complex process as it requires coordination of many aspects to grow into fit individuals, such as the control of body size and organ growth. Therefore, the mechanisms of precise control of growth are essential for ensuring the growth of organisms at a correct body size and proper organ proportions during development. The control of the growth rate and the duration of growth (or the cessation of growth) are required in size control. The insulin signaling pathway and the elements involved are essential in the control of growth. On the other hand, the ecdysteroid molting hormone determines the duration of growth. The secretion of these hormones is controlled by environmental factors such as nutrition. Moreover, the target of rapamycin (TOR) pathway is considered as a nutrient sensing pathway. Important cross-talks have been shown to exist among these pathways. In this review, we outline the control of body and organ growth by the insulin/TOR signaling pathway, and also the interaction between nutrition via insulin/TOR signaling and ecdysteroids at the coordination of organismal development and organ growth in insects, mainly focusing on the well-studied fruit fly Drosophila melanogaster.

Understanding Insulin Endocrinology in Decapod Crustacea: Molecular Modelling Characterization of an Insulin-Binding Protein and Insulin-Like Peptides in the Eastern Spiny Lobster, Sagmariasus verreauxi

The insulin signalling system is one of the most conserved endocrine systems of Animalia from mollusc to man. In decapod Crustacea, such as the Eastern spiny lobster, Sagmariasus verreauxi (Sv) and the red-claw crayfish, Cherax quadricarinatus (Cq), insulin endocrinology governs male sexual differentiation through the action of a male-specific, insulin-like androgenic gland peptide (IAG). To understand the bioactivity of IAG it is necessary to consider its bio-regulators such as the insulin-like growth factor binding protein (IGFBP). This work has employed various molecular modelling approaches to represent S. verreauxi IGFBP and IAG, along with additional Sv-ILP ligands, in order to characterise their binding interactions. Firstly, we present Sv- and Cq-ILP2: neuroendocrine factors that share closest homology with Drosophila ILP8 (Dilp8). We then describe the binding interaction of the N-terminal domain of Sv-IGFBP and each ILP through a synergy of computational analyses. In-depth interaction mapping and computational alanine scanning of IGFBP_N' highlight the conserved involvement of the hotspot residues Q₆₇, G₇₀, D₇₁, S₇₂, G₉₁, G₉₂, T₉₃ and D₉₄. The significance of the negatively charged residues D₇₁ and D₉₄ was then further exemplified by structural electrostatics. The functional importance of the negative surface charge of IGFBP is exemplified in the complementary electropositive charge on the reciprocal binding interface of all three ILP ligands. When examined, this electrostatic complementarity is the inverse of vertebrate homologues; such physicochemical divergences elucidate towards ligand-binding specificity between Phyla.

Evolution of the vertebrate insulin receptor substrate (Irs) gene family

BACKGROUND

Insulin receptor substrate (Irs) proteins are essential for insulin signaling as they allow downstream effectors to dock with, and be activated by, the insulin receptor. A family of four Irs proteins have been identified in mice, however the gene for one of these, IRS3, has been pseudogenized in humans. While it is known that the Irs gene family originated in vertebrates, it is not known when it originated and which members are most closely related to each other. A better understanding of the evolution of Irs genes and proteins should provide insight into the regulation of metabolism by insulin.

RESULTS

Multiple genes for Irs proteins were identified in a wide variety of vertebrate species. Phylogenetic and genomic neighborhood analyses indicate that this gene family originated very early in vertebrae evolution. Most Irs genes were duplicated and retained in fish after the fish-specific genome duplication. Irs genes have been lost of various lineages, including Irs3 in primates and birds and Irs1 in most fish. Irs3 and Irs4 experienced an episode of more rapid protein sequence evolution on the ancestral mammalian lineage. Comparisons of the conservation of the proteins sequences among Irs paralogs show that domains involved in binding to the plasma membrane and insulin receptors are most strongly conserved, while divergence has occurred in sequences involved in interacting with downstream effector proteins.

CONCLUSIONS

The Irs gene family originated very early in vertebrate evolution, likely through genome duplications, and in parallel with duplications of other components of the insulin signaling pathway, including insulin and the insulin receptor. While the N-terminal sequences of these proteins are conserved among the paralogs, changes in the C-terminal sequences likely allowed changes in biological function.

Structure and dynamics of the insulin receptor: implications for receptor activation and drug discovery

Recently, major progress has been made in uncovering the mechanisms of how insulin engages its receptor and modulates downstream signal transduction. Here, we present in detail the current structural knowledge surrounding the individual components of the complex, binding sites, and dynamics during the activation process. A novel kinase triggering mechanism, the 'bow-arrow model', is proposed based on current knowledge and computational simulations of this system, in which insulin, after its initial interaction with binding site 1, engages with site 2 between the fibronectin type III (FnIII)-1 and -2 domains, which changes the conformation of FnIII-3 and eventually translates into structural changes across the membrane. This model provides a new perspective on the process of insulin binding to its receptor and, thus, could lead to future novel drug discovery efforts.

Prediction of a novel internal rearrangement of the insulin receptor

The insulin receptor (IR) plays critical roles in metabolism and growth, directed by the binding of insulin. Decades of research to understand the mechanism of insulin binding and activation of the IR have identified a region of the receptor, the C-terminal (CT) peptide, to be crucial for insulin binding. In particular, a truncated IR consisting of the first three domains fused to the CT peptide was found to bind insulin with nanomolar affinity, with undetectable binding in the absence of fused or soluble CT peptide. Problematically, all current crystal structures of the IR indicate the fusion point of the CT peptide to the three domains is located far from the position of the CT peptide as resolved in such structures. We have attempted to address this problem using molecular modelling and dynamics simulations. The results led to the identification of a potential inter-domain interaction between the L2 domain and the CT peptide that is not observed in any of the crystal structures of the IR. Investigations into this new interaction found a conformational change that could potentially be in response to insulin binding. Additionally, further simulation work with the new conformation demonstrated its compatibility with the position and orientation of insulin from the latest insulin-bound IR crystal structure.

Structural Dynamics of Insulin Receptor and Transmembrane Signaling

The insulin receptor (IR) is a (αβ)2-type transmembrane tyrosine kinase that plays a central role in cell metabolism. Each αβ heterodimer consists of an extracellular ligand-binding α-subunit and a membrane-spanning β-subunit that comprises the cytoplasmic tyrosine kinase (TK) domain and the phosphorylation sites. The α- and β-subunits are linked via a single disulfide bridge, and the (αβ)2 tetramer is formed by disulfide bonds between the α-chains. Insulin binding induces conformational changes in IR that reach the intracellular β-subunit followed by a protein phosphorylation and activation cascade. Defects in this signaling process, including IR dysfunction caused by mutations, result in type 2 diabetes. Rational drug design aimed at treatment of diabetes relies on knowledge of the detailed structure of IR and the dynamic structural transformations during transmembrane signaling. Recent X-ray crystallographic studies have provided important clues about the mode of binding of insulin to IR, the resulting structural changes and their transmission to the TK domain, but a complete understanding of the structural basis underlying insulin signaling has not been achieved. This review presents a critical analysis of the current status of the structure-function relationship of IR, with a comparative assessment of the other IR family receptors, and discusses potential advancements that may provide insight into the molecular mechanism of insulin signaling.

Activation of transmembrane cell-surface receptors via a common mechanism? The "rotation model"

It has long been thought that transmembrane cell-surface receptors, such as receptor tyrosine kinases and cytokine receptors, among others, are activated by ligand binding through ligand-induced dimerization of the receptors. However, there is growing evidence that prior to ligand binding, various transmembrane receptors have a preformed, yet inactive, dimeric structure on the cell surface. Various studies also demonstrate that during transmembrane signaling, ligand binding to the extracellular domain of receptor dimers induces a rotation of transmembrane domains, followed by rearrangement and/or activation of intracellular domains. The paper here describes transmembrane cell-surface receptors that are known or proposed to exist in dimeric form prior to ligand binding, and discusses how these preformed dimers are activated by ligand binding.

Theoretical and computational studies of peptides and receptors of the insulin family

Synergistic interactions among peptides and receptors of the insulin family are required for glucose homeostasis, normal cellular growth and development, proliferation, differentiation and other metabolic processes. The peptides of the insulin family are disulfide-linked single or dual-chain proteins, while receptors are ligand-activated transmembrane glycoproteins of the receptor tyrosine kinase (RTK) superfamily. Binding of ligands to the extracellular domains of receptors is known to initiate signaling via activation of intracellular kinase domains. While the structure of insulin has been known since 1969, recent decades have seen remarkable progress on the structural biology of apo and liganded receptor fragments. Here, we review how this useful structural information (on ligands and receptors) has enabled large-scale atomically-resolved simulations to elucidate the conformational dynamics of these biomolecules. Particularly, applications of molecular dynamics (MD) and Monte Carlo (MC) simulation methods are discussed in various contexts, including studies of isolated ligands, apo-receptors, ligand/receptor complexes and intracellular kinase domains. The review concludes with a brief overview and future outlook for modeling and computational studies in this family of proteins.

Determination of Alkali-Sensing Parts of the Insulin Receptor-Related Receptor Using the Bioinformatic Approach

IRR (insulin receptor-related receptor) is a receptor tyrosine kinase belonging to the insulin receptor family, which also includes insulin receptor and IGF-IR receptor. We have previously shown that IRR is activated by extracellular fluid with pH > 7.9 and regulates excess alkali excretion in the body. We performed a bioinformatic analysis of the pH-sensitive potential of all three members of the insulin receptor family of various animal species (from frog to man) and their chimeras with swapping of different domains in the extracellular region. An analysis using the AcalPred program showed that insulin receptor family proteins are divided into two classes: one class with the optimal working pH in the acidic medium (virtually all insulin receptor and insulin-like growth factor receptor orthologs, except for the IGF-IR ortholog from Xenopus laevis) and the second class with the optimal working pH in the alkaline medium (all IRR orthologs). The program had predicted that the most noticeable effect on the pH-sensitive property of IRR would be caused by the replacement of the L1 and C domains in its extracellular region, as well as the replacement of the second and third fibronectin repeats. It had also been assumed that replacement of the L2 domain would have the least significant effect on the alkaline sensitivity of IRR. To test the in silico predictions, we obtained three constructs with swapping of the L1C domains, the third L2 domain, and all three domains L1CL2 of IRR with similar domains of the insulin-like growth factor receptor. We found that replacement of the L1C and L1CL2 domains reduces the receptor's ability to be activated with alkaline pH, thus increasing the half-maximal effective concentration by about 100%. Replacement of the L2 domain increased the half-maximal effective concentration by 40%. Thus, our results indicate the high predictive potential of the AcalPred algorithm, not only for the pH-sensitive enzymes, but also for pH-sensitive receptors.

Flexibility in the insulin receptor ectodomain enables docking of insulin in crystallographic conformation observed in a hormone-bound microreceptor

Insulin binding to the insulin receptor (IR) is the first key step in initiating downstream signaling cascades for glucose homeostasis in higher organisms. The molecular details of insulin recognition by IR are not yet completely understood, but a picture of hormone/receptor interactions at one of the epitopes (Site 1) is beginning to emerge from recent structural evidence. However, insulin-bound structures of truncated IR suggest that crystallographic conformation of insulin cannot be accommodated in the full IR ectodomain due to steric overlap of insulin with the first two type III fibronectin domains (F1 and F2), which are contributed to the insulin binding-pocket by the second subunit in the IR homodimer. A conformational change in the F1-F2 pair has thus been suggested. In this work, we present an all-atom structural model of complex of insulin and the IR ectodomain, where no structural overlap of insulin with the receptor domains (F1 and F2) is observed. This structural model was arrived at by flexibly fitting parts of our earlier insulin/IR all-atom model into the simulated density maps of crystallized constructs combined with conformational sampling from apo-IR solution conformations. Importantly, our experimentally-consistent model helps rationalize yet unresolved Site.

Retinal injury, growth factors, and cytokines converge on β-catenin and pStat3 signaling to stimulate retina regeneration

Müller glia (MG) in the zebrafish retina respond to retinal injury by generating multipotent progenitors for retinal repair. Here, we show that Insulin, Igf-1, and fibroblast growth factor (FGF) signaling components are necessary for retina regeneration. Interestingly, these factors synergize with each other and with heparin-binding EGF-like growth factor (HB-EGF) and cytokines to stimulate MG to generate multipotent progenitors in the uninjured retina. These factors act by stimulating a core set of signaling cascades (Mapk/Erk, phosphatidylinositol 3-kinase [PI3K], β-catenin, and pStat3) that are also shared with retinal injury and exhibit a remarkable amount of crosstalk. Our studies suggest that MG both produce and respond to factors that stimulate MG reprogramming and proliferation following retinal injury. The identification of a core set of regeneration-associated signaling pathways required for MG reprogramming not only furthers our understanding of retina regeneration in fish but also suggests targets for enhancing regeneration in mammals.

Characterization of insulin-like peptides (ILPs) in the sea urchin Strongylocentrotus purpuratus: insights on the evolution of the insulin family

The evolutionary history of the insulin-like peptides (ILPs), members of the insulin family, is still a matter of debate. Although ILPs structure and expression have been described in different metazoans, little is known about these molecules in non-chordate deuterostomes, such as the echinoderms. In order to fill this gap in the current literature, we have characterized two members of the insulin family found in the sea urchin Strongylocentrotus purpuratus genome (SpIgf1 and SpIgf2 that, after our analysis, we suggest to rename SpILP1 and SpILP2, respectively) together with their putative receptor (SpInsr). We found that SpILP1 gene structure is more similar to the cephalochordate amphioxus ILP, while the SpILP2 gene shows a completely different organization. In addition, we have revealed that SpILP1 and SpILP2 transcripts are expressed in different compartments during embryo/larva development and that the SpILP1 protein mature form differs in the egg and the larva, suggesting different biological roles. Finally, we have analyzed SpILP1 transcript and protein expression in response to different feeding regimes through real-time quantitative PCR, Western blot and immunohistochemistry methodologies, and found that its expression and localization are feeding-dependent. We discuss our findings in a comparative evolutionary perspective including data available in other animal models and provide new insights into the evolution of the insulin family molecules. In the model we put forward, the last common ancestor of all deuterostomes presented an ILP composed of the B-C-A-D-E domains, and successive lineage specific independent gene duplication events resulted in the presence of several ILPs in vertebrates and in echinoderms.

Insulin receptor-like ectodomain genes and splice variants are found in both arthropods and human brain cDNA

Truncated receptor ectodomains have been described for several classes of cell surface receptors, including those that bind to growth factors, cytokines, immunoglobulins, and adhesion molecules. Soluble receptor isoforms are typically generated by proteolytic cleavage of the cell surface receptor or by alternative splicing of RNA transcripts arising from the same gene encoding the full-length receptor. Both the epidermal growth factor receptor (EGFR) and the insulin receptor (INSR) families produce soluble receptor splice variants in vertebrates and truncated forms of insulin receptor-like sequences have previously been described in Drosophila. The EGFR and INSR ectodomains share significant sequence homology with each other suggestive of a common evolutionary origin. We discovered novel truncated insulin receptor-like variants in several arthropod species. We performed a phylogenetic analysis of the conserved extracellular receptor L1 and L2 subdomains in invertebrate species. While the segregation of insulin receptor-like L1 and L2 domains indicated that an internal domain duplication had occurred only once, the generation of truncated insulin receptor-like sequences has occurred multiple times. The significance of this work is the previously unknown and widespread occurrence of truncated isoforms in arthropods, signifying that these isoforms play an important functional role, potentially related to such isoforms in mammals.

Structural determinants of the insulin receptor-related receptor activation by alkali

IRR is a member of the insulin receptor (IR) family that does not have any known agonist of a peptide nature but can be activated by mildly alkaline medium and was thus proposed to function as an extracellular pH sensor. IRR activation by alkali is defined by its N-terminal extracellular region. To reveal key structural elements involved in alkali sensing, we developed an in vitro method to quantify activity of IRR and its mutants. Replacing the IRR L1C domains (residues 1-333) or L2 domain (residues 334-462) or both with the homologous fragments of IR reduced the receptor activity to 35, 64, and 7% percent, respectively. Within L1C domains, five amino acid residues (Leu-135, Gly-188, Arg-244, and vicinal His-318 and Lys-319) were identified as IRR-specific by species conservation analysis of the IR family. These residues are exposed and located in junctions between secondary structure folds. The quintuple mutation of these residues to alanine had the same negative effect as the entire L1C domain replacement, whereas none of the single mutations was as effective. Separate mutations of these five residues and of L2 produced partial negative effects that were additive. The pH dependence of cell-expressed mutants (L1C and L2 swap, L2 plus triple LGR mutation, and L2 plus quintuple LGRHK mutation) was shifted toward alkalinity and, in contrast with IRR, did not show significant positive cooperativity. Our data suggest that IRR activation is not based on a single residue deprotonation in the IRR ectodomain but rather involves synergistic conformational changes at multiple points.

Structural and functional analysis of the amphioxus IGFBP gene uncovers ancient origin of IGF-independent functions

IGFs play key roles in regulating vertebrate development, growth, reproduction, and aging. In extracellular fluids, IGFs are bound and regulated by a family of IGF-binding proteins (IGFBPs). Although all known IGFBPs are secreted proteins, some are also found in the nucleus and possess IGF-independent activities. When and how these distinct modes of biological actions have evolved is unknown. In this study, we identified and analyzed an IGFBP gene from amphioxus. Amphioxus shares a common ancestor with the modern vertebrate lineage that dates back to more than 520 million years ago. The amphioxus IGFBP shares all major structural characteristics of vertebrate IGFBPs. Phylogenetic analyses place it in a basal position in the IGFBP lineage. Ligand blot analysis reveals that amphioxus IGFBP does not bind to IGF-I or -II. Changing its Phe70 into Leu, however, is sufficient to convert it into a functional IGF binder. When tested in cultured cells, amphioxus IGFBP is localized in the nucleus, and this is attributed to 2 redundant nuclear localization sequences in its L domain. Furthermore, the amphioxus IGFBP N-terminal domain has strong transcriptional activation activity. Forced expression of amphioxus IGFBP in zebrafish embryos results in dorsalized phenotypes. This action requires nuclear localization. These results suggest that the nuclear localization and transcription activation activity of IGFBPs are ancient functions and the IGF-binding function may have been acquired by opportunistic gain-of-functional mutations later in evolution.

Strongyloides stercoralis daf-2 encodes a divergent ortholog of Caenorhabditis elegans DAF-2

We hypothesise that developmental arrest in infectious larvae of parasitic nematodes is regulated by signalling pathways homologous to Caenorhabditis elegans DAF (dauer formation) pathways. Alignment of Strongyloides stercoralis (Ss) DAF-2 with DAF-2 of C. elegans and homologs of other species shows that most structural motifs in these insulin-like receptors are conserved. However, the catalytic domain of Ss-DAF-2 contains two substitutions (Q1242 and Q1256), that would result in constitutive dauer formation in C. elegans or diabetes in vertebrate animals. Ss-daf-2 also shows two alternately spliced isoforms, the constitutively expressed Ss-daf-2a, and Ss-daf-2b, which is only expressed in stages leading to parasitism.

The insulin receptor: both a prototypical and atypical receptor tyrosine kinase

Unlike prototypical receptor tyrosine kinases (RTKs), which are single-chain polypeptides, the insulin receptor (InsR) is a preformed, covalently linked tetramer with two extracellular α subunits and two membrane-spanning, tyrosine kinase-containing β subunits. A single molecule of insulin binds asymmetrically to the ectodomain, triggering a conformational change that is transmitted to the cytoplasmic kinase domains, which facilitates their trans-phosphorylation. As in prototypical RTKs, tyrosine phosphorylation in the juxtamembrane region of InsR creates recruitment sites for downstream signaling proteins (IRS [InsR substrate] proteins, Shc) containing a phosphotyrosine-binding (PTB) domain, and tyrosine phosphorylation in the kinase activation loop stimulates InsR's catalytic activity. For InsR, phosphorylation of the activation loop, which contains three tyrosine residues, also creates docking sites for adaptor proteins (Grb10/14, SH2B2) that possess specialized Src homology-2 (SH2) domains, which are dimeric and engage two phosphotyrosines in the activation loop.

Similar but different: ligand-induced activation of the insulin and epidermal growth factor receptor families

The insulin and epidermal growth factor receptor families are among the most intensively studied proteins in biology. They are closely related members of the receptor tyrosine kinase superfamily and deregulated signaling by members of either receptor family has been implicated in the progression of a variety of cancers. These receptors have thus emerged as validated therapeutic targets for the development of anti-tumour agents. Recent studies have revealed detail of the ligand-binding sites in the insulin receptor family, as well as detail of conformational change upon ligand binding in the epidermal growth factor receptor family. Taken together, these findings and further data relating to kinase activation highlight the fact that while the receptor families share common structural elements, the structural detail of their functioning is remarkably different.

Insulin receptor-related receptor as an extracellular alkali sensor

The insulin receptor-related receptor (IRR), an orphan receptor tyrosine kinase of the insulin receptor family, can be activated by alkaline media both in vitro and in vivo at pH >7.9. The alkali-sensing property of IRR is conserved in frog, mouse, and human. IRR activation is specific, dose-dependent and quickly reversible and demonstrates positive cooperativity. It also triggers receptor conformational changes and elicits intracellular signaling. The pH sensitivity of IRR is primarily defined by its L1F extracellular domains. IRR is predominantly expressed in organs that come in contact with mildly alkaline media. In particular, IRR is expressed in the cell subsets of the kidney that secrete bicarbonate into urine. Disruption of IRR in mice impairs the renal response to alkali loading attested by development of metabolic alkalosis and decreased urinary bicarbonate excretion in response to this challenge. We therefore postulate that IRR is an alkali sensor that functions in the kidney to manage metabolic bicarbonate excess.

IGF1R variants associated with isolated single suture craniosynostosis

The genetic contribution to the pathogenesis of isolated single suture craniosynostosis is poorly understood. The role of mutations in genes known to be associated with syndromic synostosis appears to be limited. We present our findings of a candidate gene resequencing approach to identify rare variants associated with the most common forms of isolated craniosynostosis. Resequencing of the coding regions, splice junction sites, and 5' and 3' untranslated regions of 27 candidate genes in 186 cases of isolated non-syndromic single suture synostosis revealed three novel and two rare sequence variants (R406H, R595H, N857S, P190S, M446V) in insulin-like growth factor I receptor (IGF1R) that are enriched relative to control samples. Mapping the resultant amino acid changes to the modeled homodimer protein structure suggests a structural basis for segregation between these and other disease-associated mutations found in IGF1R. These data suggest that IGF1R mutations may contribute to the risk and in some cases cause single suture craniosynostosis.

Exploring the evolutionary relationship of insulin receptor substrate family using computational biology

Insulin receptor substrate (IRS) harbors proteins such as IRS1, IRS2, IRS3, IRS4, IRS5 and IRS6. These key proteins act as vital downstream regulators in the insulin signaling pathway. However, little is known about the evolutionary relationship among the IRS family members. This study explores the potential to depict the evolutionary relationship among the IRS family using bioinformatics, algorithm analysis and mathematical models.

Structural and biological properties of the Drosophila insulin-like peptide 5 show evolutionary conservation

We report the crystal structure of two variants of Drosophila melanogaster insulin-like peptide 5 (DILP5) at a resolution of 1.85 Å. DILP5 shares the basic fold of the insulin peptide family (T conformation) but with a disordered B-chain C terminus. DILP5 dimerizes in the crystal and in solution. The dimer interface is not similar to that observed in vertebrates, i.e. through an anti-parallel β-sheet involving the B-chain C termini but, in contrast, is formed through an anti-parallel β-sheet involving the B-chain N termini. DILP5 binds to and activates the human insulin receptor and lowers blood glucose in rats. It also lowers trehalose levels in Drosophila. Reciprocally, human insulin binds to the Drosophila insulin receptor and induces negative cooperativity as in the human receptor. DILP5 also binds to insect insulin-binding proteins. These results show high evolutionary conservation of the insulin receptor binding properties despite divergent insulin dimerization mechanisms.

Activation of Erk1/2 phosphorylation but not of Akt/Pkb through an inducible CSF1R/IRR-receptor construct in INS-1E beta-cells

CONTEXT

The insulin receptor-related receptor (IRR) is an orphan receptor belonging to the insulin receptor (IR) family. Despite its unknown function, the specific tissue expression and the high sequence homology with the IR and the insulin-like growth factor 1 receptor (IGF1R) suggest a biological role in beta-cells.

OBJECTIVES

In this study we investigated the influence of a stimulatable IRR-tyrosine kinase on major IR/IGF1R signaling pathways and on proliferation and apoptosis of INS-1E beta-cells.

METHODS

INS-1E cells were stably transfected with a colony stimulating factor 1 receptor (CSF1R)/IRR construct activated by a macrophage colony stimulating factor.

RESULTS AND CONCLUSION

After stimulation the construct showed time and dose dependent autophosphorylation and transient extracellular signal regulated kinase 1/2 activation. Protein kinase b was not phosphorylated and also an effect on proliferation and apoptosis of INS-1E could not be demonstrated. Thus, no obvious biologic function of the IRR is present in INS-1E beta-cells.

Ligand-induced activation of the insulin receptor: a multi-step process involving structural changes in both the ligand and the receptor

Current models of insulin binding to the insulin receptor (IR) propose (i) that there are two binding sites on the surface of insulin which engage with two binding sites on the receptor and (ii) that ligand binding involves structural changes in both the ligand and the receptor. Many of the features of insulin binding to its receptor, namely B-chain helix interactions with the leucine-rich repeat domain and A-chain residue interactions with peptide loops from another part of the receptor, are also seen in models of relaxin and insulin-like peptide 3 binding to their receptors. We show that these principles can likely be extended to the group of mimetic peptides described by Schäffer and coworkers, which are reported to have no sequence identity with insulin. This review summarizes our current understanding of ligand-induced activation of the IR and highlights the key issues that remain to be addressed.