The insulin receptor and insulin of the Atlantic hagfish. Extraordinary conservation of binding specificity and negative cooperativity in the most primitive vertebrate

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.

New insights into the signaling system and function of insulin in fish

Fish have provided essential information about the structure, biosynthesis, evolution, and function of insulin (INS) as well as about the structure, evolution, and mechanism of action of insulin receptors (IR). INS, insulin-like growth factor (IGF)-1, and IGF-2 share a common ancestor; INS and a single IGF occur in Agnathans, whereas INS and distinct IGF-1 and IGF-2s appear in Chondrichthyes. Some but not all teleost fish possess multiple INS genes, but it is not clear if they arose from a common gene duplication event or from multiple separate gene duplications. INS is produced by the endocrine pancreas of fish as well as by several other tissues, including brain, pituitary, gastrointestinal tract, and adipose tissue. INS regulates various aspects of feeding, growth, development, and intermediary metabolism in fish. The actions of INS are mediated through the insulin receptor (IR), a member of the receptor tyrosine kinase family. IRs are widely distributed in peripheral tissues of fish, and multiple IR subtypes that derive from distinct mRNAs have been described. The IRs of fish link to several cellular effector systems, including the ERK and IRS-PI3k-Akt pathways. The diverse effects of INS can be modulated by altering the production and release of INS as well as by adjusting the production/surface expression of IR. The diverse actions of INS in fish as well as the diverse nature of the neural, hormonal, and environmental factors known to affect the INS signaling system reflects the various life history patterns that have evolved to enable fish to occupy a wide range of aquatic habitats.

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.

Structural basis of the aberrant receptor binding properties of hagfish and lamprey insulins

The insulin from the Atlantic hagfish (Myxine glutinosa) has been one of the most studied insulins from both a structural and a biological viewpoint; however, some aspects of its biology remain controversial, and there has been no satisfying structural explanation for its low biological potency. We have re-examined the receptor binding kinetics, as well as the metabolic and mitogenic properties, of this phylogenetically ancient insulin, as well as that from another extant representative of the ancient chordates, the river lamprey (Lampetra fluviatilis). Both insulins share unusual binding kinetics and biological properties with insulin analogues that have single mutations at residues that contribute to the hexamerization surface. We propose and demonstrate by reciprocal amino acid substitutions between hagfish and human insulins that the reduced biological activity of hagfish insulin results from unfavorable substitutions, namely, A10 (Ile to Arg), B4 (Glu to Gly), B13 (Glu to Asn), and B21 (Glu to Val). We likewise suggest that the altered biological activity of lamprey insulin may reflect substitutions at A10 (Ile to Lys), B4 (Glu to Thr), and B17 (Leu to Val). The substitution of Asp at residue B10 in hagfish insulin and of His at residue A8 in both hagfish and lamprey insulins may help compensate for unfavorable changes in other regions of the molecules. The data support the concept that the set of unusual properties of insulins bearing certain mutations in the hexamerization surface may reflect those of the insulins evolutionarily closer to the ancestral insulin gene product.

Insulin and its receptor: structure, function and evolution

I present here a personal perspective on more than three decades of research into the structural biology of the insulin-receptor interaction. The solution of the three-dimensional structure of insulin in 1969 provided a detailed understanding of the insulin surfaces involved in self-assembly. In subsequent years, hundreds of insulin analogues were prepared by insulin chemists and molecular biologists, with the goal of relating the structure to the biological function of the molecule. The design of methods for direct receptor-binding studies in the 1970s, and the cloning of the receptor in the mid 1980s, provided the required tools for detailed structure-function studies. In the absence of a full three-dimensional structure of the insulin-receptor complex, I attempt to assemble the existing pieces of the puzzle generated by our and other laboratories, in order to generate a plausible mechanistic model of the insulin-receptor interaction that explains its kinetics and negative cooperativity.

Insulin, insulin-like growth factor-I (IGF-I) and glucagon: the evolution of their receptors

Insulin and glucagon, two of the most studied pancreatic hormones bind to specific membrane receptors to exert their biological actions. Insulin-like growth factors IGF-I and IGF-II are structurally related to insulin, although they are expressed ubiquitously. The biological functions of the IGFs are mediated by different transmembrane receptors, which includes the insulin, IGF-I and IGF-II receptors. The interaction of insulin, insulin related peptides and glucagon with the corresponding receptors has been studied extensively in mammals and continues to be so. At the same time, research on ectothermic animals has made enormous progress in the recent years. This paper summarizes current knowledge on insulin, IGF-I and glucagon receptors, from a comparative point of view with special attention to non-mammalian vertebrates. The review covers adult and mostly typical target tissues, and with very few exceptions, developmental aspects are not considered. Binding characteristics, tissue distribution and structure of insulin and IGF-I receptors will be considered first, because both ligands and receptors are structurally related and have overlapping functions. These sections will be followed by similar distribution of information on glucagon receptors. Readers interested in either structure or functions of insulin, IGFs and glucagon in nonmammalian vertebrates are referred to other reviews (Mommsen TP, Plisetskaya EM. Insulin in fishes and agnathans: history, structure and metabolic regulation. Rev Aquat Sci 1991;4:225-259; Mommsen TP, Plisetskaya EM. Metabolic and endocrine functions of glucagon-like peptides: evolutionary and biochemical perspectives. Fish Physiol Biochem 1993;11:429-438; Duguay SJ, Mommsen TP. Molecular aspects of pancreatic peptides. In: Sherwood NM, Hew CL, editors, Fish Physiology. vol 13. 1994:225-271; Plisetskaya EM, Mommsen TP. Glucagon and glucagon-like peptides in fishes. Int Rev Citol 1996;168:187-257.).

Insulin-family peptide-receptor interaction at the early stage of vertebrate evolution

This is an overview of our studies on insulin and insulin-like growth factor-I (IGF-I) interactions with their own and each other's receptors in the lamprey (Lampetra fluviatilis L.), an extant representative of the ancient vertebrate group of Agnathans as compared to mammal (rat). Lamprey insulin receptor shows species specificity, namely, it binds its own insulin with higher affinity than mammalian hormone. Nevertheless, and unlike mammalian insulin receptor, lamprey receptor discriminates relatively poorly between insulin and IGF-I. Autophosphorylation patterns are identical for both receptors. In contrast, IGF-I receptors in lamprey tissues are very similar to mammalian IGF-I receptors confirming known evolutionary conservatism of IGF receptor system. Presumed common evolutionary origin of insulin and IGF-I receptors and poor ability of lamprey insulin receptor to discriminate between two ligands, implies that lamprey insulin receptor is closer to putative ancestral protein that IGF-I receptor. Contrary to the common belief, ambient temperatures for lampreys (4-15 degrees C) put no constraints on either downregulation of receptors or the endocytosis of hormone-receptor complexes.

Lamprey but not porcine insulin binds with different affinity to lamprey and rat hepatocytes

The displacement of porcine [125I] insulin bound to rat and lamprey isolated hepatocytes with unlabeled lamprey and porcine insulins was investigated. Binding affinity of lamprey insulin for insulin receptor of rat was similar to that of porcine insulin. In contrast, the binding affinity of lamprey insulin for its own insulin receptor was higher than for a rat receptor. To determine the binding affinity constants of lamprey insulin receptor, the competition binding experiments were carried out on isolated lamprey hepatocytes using lamprey insulin as unlabeled ligand and tracer. The affinity of the same binding sites on lamprey hepatocytes was assessed in similar experiments but employing porcine insulin as unlabeled ligand and tracer. It was found that while Kd of low affinity binding sites on lamprey hepatocytes were similar for lamprey and porcine insulins, the Kd of high affinity binding sites was different: the displacement curve for lamprey insulin being shifted to the left as compared to the curve for porcine insulin. The number of high and low affinity binding sites, calculated independently in Scatchard plots, was equal. We conclude that the high affinity insulin binding sites of lamprey but not of rat hepatocytes reveal some species specificity in ligand-receptor interaction.

Isolation and characterization of Muscovy (Cairna moschata) duck insulin

Ducks (Anatidae Family, Anseriform order) are divided in two genera: Pekin duck (Anasplatyrhynchos genus) and Muscovy duck (Cairina moschata genus) and differ for their number of liver insulin receptors (despite rather similar plasma insulin levels). The possibility that the presence of different endogenous insulins account for the difference in insulin receptor number between the two duck species led us to purify, sequence and characterize the binding properties of Muscovy duck insulin. The sequence of Muscovy duck insulin (measured mass: 5729.11) was identical to that described in two other species from the Anseriforme order: Pekin duck or goose. The binding affinity of Muscovy duck insulin for rat liver insulin receptors (either membrane bound or solubilized receptors) was lower than that of porcine insulin (0.3), which most likely accounts for the low biological potency of Pekin duck insulin previously described. In contrast, liver receptors from chicken and both duck species exhibited the same affinity for duck and porcine insulin suggesting the presence of specific changes in the structure of binding sites of bird liver insulin receptors. The decrease in the number of insulin receptors in Muscovy duck liver is not therefore the consequence of a change at the level of the insulin molecule itself. As discussed, among bird insulins, the hypoactive "duck type" insulin would have appeared after the hyperactive "chicken type" insulin during the evolution of Aves.

Molecular cloning and sequencing of a guinea-pig pro-opiomelanocortin cDNA

The guinea-pig has high levels of circulating cortisol. Though adrenocorticotropin (ACTH) levels are similar to those in other mammals, guinea-pig ACTH has been reported to have a single amino-acid substitution which results in increased bioactivity of the peptide. Pro-opiomelanocortin (POMC) is the precursor for ACTH, gamma-melanocyte-stimulating hormone (gamma-MSH) and the endogenous opioid peptide beta-endorphin. Both to confirm this substitution in guinea-pig ACTH and to establish whether other non-conservative substitutions occur elsewhere in the precursor we cloned guinea-pig POMC. The guinea-pig alanine for proline substitution at position 24 of ACTH was confirmed. Potentially significant mutations were also identified in gamma-MSH and beta-endorphin. A similar pattern of POMC mRNA expression was obtained for guinea-pig and rat as determined by Northern analysis and in situ hybridization. Southern blot analysis indicated that guinea-pig POMC is a single-copy gene. Cloning and sequencing of guinea-pig POMC thus further demonstrate the divergence of the New World hystricomorph peptides from those in New World primates, and underscore the differences observed in other endocrine axes in the guinea-pig.

Insulin binding to liver plasma membranes of coho salmon during smoltification

Smoltification of coho salmon (Oncorhynchus kisutch) is accompanied by characteristic changes in plasma insulin levels, with peak values attained at the beginning of the transformation period (the parr to transitional stage). To relate these changes to responsiveness of target tissues a study was made of binding of homologous insulin to liver plasma membranes in coho salmon undergoing the parr-to-smolt transformation. In general, parameters of specific insulin binding to the preparations of liver plasma membranes were similar to same parameters of insulin binding in other teleost fishes investigated so far and in mammals. The total binding capacity of insulin, a value that reflects the total number of binding sites, was higher in smolts than in parrs. The number of high affinity, low capacity binding sites in the liver membranes steadily increased as smoltification progressed, reached a maximum in early smolts, and declined thereafter. Another increase in the numbers of high affinity binding sites was observed in smolts transferred from fresh water to seawater. The specific binding of insulin in salmon at particular developmental stages was probably related to, and determined by, both the quantity of binding sites and their relative affinities. In smolts the highest levels of specific binding of insulin to the liver plasma membranes were coincident with the low levels of insulin in the peripheral blood that passed through the liver.

Comparative studies on the major features of insulin receptors in mammalian and non-mammalian liver membranes

1. The camel has insulin receptors that by multiple function criteria are very similar to those of the other mammals (rabbit and rat) and non-mammals (chicken and pigeon), with sharp pH dependence to insulin binding at pH 7.2-7.6. 2. Equilibrium binding was faster at higher temperatures (24-37 degrees C) than at lower (4 degrees C). 3. Binding data yielded curvilinear Scatchard plots with half maximal displacement of 125I-insulin at 9 x 10(-9) M, 2.5 x 10(-9) M, 6.3 x 10(-10) M for camel, rabbit, pigeon and chicken respectively, suggesting differences in mammalian and non-mammalian liver membranes. 4. Autoradiogram patterns showed the presence of an identical subunit structure with Mr 74,000 for all membranes studied. Pigeon membrane showed a band with Mr 110,000, the absence of which in other membranes could be due to the degradation factor or the concentration of disuccinimidyl suberate (DSS).

Insulin receptors of chicken liver and brain. Characterization of alpha and beta subunit properties

Receptors on membranes of chicken liver and brain bound porcine 125I-insulin in a specific and temperature-dependent manner. Competition with unlabeled insulin derivatives exhibited typical insulin potency ratios, i.e. chicken greater than porcine insulin greater than human proinsulin (2.1/1/0.02). Apparent binding affinity was higher in brain with a 50% inhibition of tracer binding of 1.3 +/- 0.2 nM porcine insulin as compared to 2.8 +/- 0.3 nM in liver. The apparent molecular mass of the 125I-insulin cross-linked alpha subunit of the insulin receptor was 139 +/- 2 kDa for chicken liver and 127 +/- 2 kDa for chicken brain. These molecular masses were similar to those of rat liver and brain insulin receptors. Neuraminidase treatment of the cross-linked insulin receptor increased the mobility of the alpha subunit from liver but did not affect that from brain, suggesting a difference in the glycosylation of the chicken brain alpha subunit as previously described in the rat. Despite this change, both receptors could be purified on wheat germ agglutinin (WGA) chromatography after Triton solubilization. In the presence of CTP and vanadate (phosphatase inhibitors) insulin-stimulatable tyrosine-specific phosphorylation of exogenous substrates was demonstrated with chicken liver and brain receptors. The reaction was dependent on Mg2+ and Mn2+. As noted with other insulin receptors, the best artificial substrate for phosphorylation was poly(Glu,Tyr)4:1. In both chicken liver and brain the smallest effective insulin dose as well as maximal stimulation of phosphorylation of the substrate was similar to that seen with rat liver, and in all three tissues chicken insulin was more potent than porcine insulin. In chicken liver an active ATP hydrolytic activity copurified with the insulin receptors during WGA chromatography. Further purification using S-300 Sephacryl filtration or affinity (insulin-biotin-avidin) chromatography could dissociate the phosphorylation and the hydrolytic activities. Gel electrophoresis, under reducing conditions revealed beta subunits with apparent Mr of 97-99 kDa in chicken liver and brain, which were phosphorylated in the presence of insulin. Similar apparent molecular masses have been described for the beta subunit of rat liver receptors. These studies suggest that both chicken brain and liver insulin receptors exhibit coupling of alpha and beta subunits with fully active tyrosine kinase and that the structural difference of the brain insulin receptor is widespread and phylogenetically old.

Insulin receptors in lizard brain and liver: structural and functional studies of alpha and beta subunits demonstrate evolutionary conservation

Specific insulin receptors are present in the liver and brain of the lizard Anolis carolinesis. In this study, the specific binding of 125I-insulin to the receptors showed time, temperature and pH dependency. Specific binding to crude membranes prepared from brain was 1-2% of the total radioactivity added compared to 4-5% in the crude membranes prepared from liver. Solubilization and wheat germ agglutinin purification of the membranes resulted in an increase in the specific binding (per mg of protein) between 6 and 32 times for liver membranes and 13-186 for brain membranes. Binding inhibition of tracer insulin by unlabeled porcine insulin was characteristic for insulin receptors with 50% inhibition for liver crude membranes at 60 ng/ml of porcine insulin and 0.7 ng/ml for purified brain insulin receptors. Chicken insulin was 2- to 3-fold more potent and proinsulin about 100 times less potent than porcine insulin. The alpha-subunits of liver and brain had apparent molecular weights on sodium dodecyl sulfate polyacrylamide gel electrophoresis of 135 kDa and 120 kDa respectively. Apparent molecular weights of beta subunits were 92 kDa for both tissues. Insulin stimulated phosphorylation of the beta subunit of both brain and liver receptors. Both tissues demonstrated tyrosine-specific phosphorylation, which was stimulated by insulin, of exogenously added artificial substrates. In addition, purified brain insulin receptor preparations contained an endogenous protein with apparent molecular weight of 105 kDa, whose phosphorylation was stimulated by insulin (10(-7) mol/l). This phosphoprotein was not immunoprecipitated by anti-insulin receptor antibodies. These studies suggest that the structural differences between brain and liver receptors previously demonstrated in the rat are also present in the lizard, which is about 300,000,000 years older than the mammalian species. Thus, there is strong evolutionary conservation of the brain insulin receptor.

Characterization of coho salmon (Oncorhynchus kisutch) insulin

Insulin has been isolated from islet tissue of coho salmon (Oncorhynchus kisutch) by gel filtration and HPLC and the complete amino acid sequence has been determined. The sequence differs from bovine insulin at 14 sites but all interchanges are conservative from the viewpoint of preservation of conformation. A comparison of insulin sequences from other fish is presented. Salmon insulin cross-reacts very weakly with antiserum to bovine insulin and vice versa. A completely homologous radioimmunoassay has been developed and used to estimate the insulin in salmon islet tissue and in plasma. The hypoglycemic effect of salmon insulin in salmon was more pronounced and persisted longer than that caused by identical doses of bovine insulin.

The effect of insulin on amino acid metabolism and glycogen content in isolated liver cells of juvenile coho salmon, Oncorhynchus kisutch

Amino acid transport and [14C]leucine incorporation into liver proteins as well as the secretion of proteins into incubation medium were studied in liver cells isolated from coho salmon (Oncorhynchus kisutch) parr. Pink salmon (Oncorhynchus gorbuscha) or mammalian (bovine) insulin caused a significant increase in TCA-precipitable radioactivity from both cells and incubation medium. The effects appeared at insulin concentration of 10(-8) M with a maximal response at 5 X 10(-8) M. The radioactivity of the TCA-soluble fraction was not changed by insulin. Insulin increased the amount of the non-metabolized amino acid [14C]cycloleucine, in the TCA-soluble fraction of hepatocytes. The glycogen content of hepatocytes was increased in the presence of insulin at 10(-9) M but was not changed from the control value in the presence of insulin at 10(-8) M.

Comparative biochemistry of the guinea-pig: a partial checklist

A great deal is known about guinea-pig biochemistry, but the information is scattered and difficult to assemble. The guinea-pig also possesses a number of unusual biochemical features which add to its interest. For these reasons we have compiled a list of biochemical characteristics of the guinea-pig, organized in a series of tables, with brief discussions of some of the entries.

The effect of high-protein and high-carbohydrate diets on [125I]iodoinsulin binding in skeletal muscle plasma membranes and isolated hepatocytes of rainbow trout (Salmo gairdneri)

[125I]iodoinsulin-binding studies in the presence of a concentration range of bovine insulin were conducted to establish specific insulin-binding levels in skeletal muscle plasma membranes and isolated hepatocytes of rainbow trout (Salmo gairdneri) reared on control, high-protein or high-carbohydrate diets. Negative co-operativity was observed and receptor concentrations and apparent dissociation constants established for each preparation. No differences of specific binding attributed to diet were detected in skeletal muscle plasma membrane preparations; however, the receptor concentration of isolated hepatocytes from high-carbohydrate-reared trout was increased. This contrasted to comparable mammalian studies.

Role of follicle wall in meiosis reinitiation induced by insulin in Rana pipiens oocytes

The involvement of the ovarian follicle wall in insulin induction of Rana pipiens oocyte maturation in vitro was examined. Complete removal of the follicle wall significantly decreased, but did not obliterate, oocyte maturation (i.e., germinal vesicle breakdown, GVBD) induced by insulin. Dose-response studies of GVBD induction revealed that oocytes within intact follicles were at least 100 times more sensitive to insulin than denuded oocytes. Addition of cyanoketone, a steroid biosynthesis inhibitor, to intact follicles also suppressed insulin-induced GVBD. Inhibitory effects of either follicle wall removal or cyanoketone were not observed when denuded oocytes were treated with progesterone. Addition of either progesterone or pregnenolone to insulin-treated denuded oocytes augmented the oocyte GVBD response compared to either steroid alone and essentially replaced the effect of the follicle wall. In summary, steroidogenesis in the follicle wall appears to be a major factor contributing to the ability of insulin to induce GVBD. However, whether insulin stimulates follicle wall steroidogenesis or simply augments the biological activity of endogenous basal steroid levels is unresolved. The in vitro results show that oocyte maturation can be modulated by the combined actions of several hormones. Such steroid-insulin interactions may also be relevant to understanding the control of oocyte maturation in amphibians and other vertebrates, including mammals, under physiological conditions in vivo.