Structural and crystallographic observations on hagfish insulin

Novel Human Insulin Isoforms and Cα-Peptide Product in Islets of Langerhans and Choroid Plexus

Human insulin (INS) gene diverged from the ancestral genes of invertebrate and mammalian species millions of years ago. We previously found that mouse insulin gene (Ins2) isoforms are expressed in brain choroid plexus (ChP) epithelium cells, where insulin secretion is regulated by serotonin and not by glucose. We further compared human INS isoform expression in postmortem ChP and islets of Langerhans. We uncovered novel INS upstream open reading frame isoforms and their protein products. In addition, we found a novel alternatively spliced isoform that translates to a 74-amino acid (AA) proinsulin containing a shorter 19-AA C-peptide sequence, herein designated Cα-peptide. The middle portion of the conventional C-peptide contains β-sheet (GQVEL) and hairpin (GGGPG) motifs that are not present in Cα-peptide. Islet amyloid polypeptide (IAPP) is not expressed in ChP, and its amyloid formation was inhibited in vitro more efficiently by Cα-peptide than by C-peptide. Of clinical relevance, the ratio of the 74-AA proinsulin to proconvertase-processed Cα-peptide was significantly increased in islets from type 2 diabetes mellitus autopsy donors. Intriguingly, 100 years after the discovery of insulin, we found that INS isoforms are present in ChP from insulin-deficient autopsy donors.

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.

Carboxypeptidase B-like and trypsin-like activities in isolated rat pancreatic islets

Trypsin-like and carboxypeptidase B-like proteinases are believed to play important roles in the conversion of proinsulin into insulin as well as in the intracellular processing of a variety of other precursor forms. To facilitate the study of these enzymes we have developed sensitive methods for their detection in tissue preparations and incubation media. Studies with rat islet homogenates indicate the presence of both trypsin-like and carboxypeptidase B-like activities with slightly acidic pH optima. The trypsin-like activity was activated by thiols and inhibited by several thiol reagents but the carboxypeptidase was inhibited only by chelating agents. These properties suggest that these enzymes are related to the tissue cathepsins. Additional experimental approaches to the problems of positively identifying and localizing converting enzymes at the subcellular level are briefly discussed.

Molecular evolution of the polypeptide hormones

Any biological function is at least bimolecular and its evolution therefore is at least dual, with variations in two lines of molecules. The hormone specificity results from a particular fit between the three-dimensional structure of the agent and that of the receptor but, because receptors are not known at the structural level, a discussion on the evolution of the polypeptide hormones is mainly limited to the possible progressive changes of the latter. As for other proteins (enzymes, oxygen carriers etc.) two degrees of complexity can be distinguished according to whether the hormone comprises one or several polypeptide chains. Protein assembly can bring new biological properties, each subunit playing a particular role. In this case, the 'internal' evolution (chain-chain interactions) overlaps the 'external' evolution (hormone-receptor contacts). The 'monomeric' hormones present the following problems: evolution of the prohormone and of the converting enzyme (for insulin), duplication and differentiation of two lines of hormones either by amino acid substitutions (neurohypophysial hormones and neurophysins) or by substitutions and size modifications (corticotropin and lipotropin), duplication and fusion leading to internal homology in the single polypeptide chain (somatotropin, prolactin, placental lactogen). The 'dimeric' hormones lead to several problems: successive duplications giving different subunits, selective associations between subunits, unequal rates of evolution of the subunits, the function of each subunit (lutropin, follitropin, thyrotropin, choriogonadotropin). An attempt is made to integrate the evolution of polypeptide hormones in the frame of the evolution of proteins.

Polypeptide hormone-receptor interactions: the structure and receptor binding of insulin and glucagon

Insulin is a small globular protein with a well defined tertiary structure which is closely similar in all species with the exception of certain hystricomorphs such as the guinea pig. Insulin-like growth factor is homologous with insulin and probably has an insulin-like tertiary structure. In contrast glucagon is not a globular protein. It exists as an equilibrium population of conformers with low helix content at physiological concentrations but attains a largely helical conformation on association to trimers. The receptor binding of insulin depends critically on the correct three-dimensional juxtaposition of groups (A1, A21, B25, etc) and involves both hydrophobic and polar interactions. In insulin-like growth factor part of the insulin receptor region is thought to be buried in extra peptide, so explaining its weak binding to insulin receptors. In contrast the glucagon receptor complex probably involves largely hydrophobic contacts which are possible when a helical conformer is formed.

Molecular cloning of preproinsulin cDNAs from several osteoglossomorphs and a cyprinid

Several preproinsulin cDNAs were isolated and characterized from four members of the Osteoglossomorpha (an ancient teleost group); Osteoglossum bicirrhosum (arawana), Pantodon buchholzi (butterfly fish), Notopterus chitala (feather fin knife fish), Hiodon alosoides (goldeye) and Gnathonemus petersii (elephantnose). In addition, we isolated and characterized the preproinsulin cDNA from Catostomus commersoni (white sucker, as a representative of a generalized teleost). The comparative analysis of the sequences revealed conservation of the cystine residues known to be involved in the formation of the disulfide bridges, as well as residues involved in the hexamer formation, except for B-17 in the butterfly fish, the arawana and the goldeye. However, the N-terminus of the B-chain was very weakly conserved among the species studied. Residues known to be significant for maintaining receptor-binding conformation and those known to comprise the receptor-binding domain were all conserved, except for a conservative substitution at B13, aspartate substituted glutamate in the arawana, goldeye, butterfly fish and white sucker, and at B16, phenylalanine substituted tyrosine in the elephantnose. Phylogenetic analysis of the sequences revealed a monophyletic grouping of the osteoglossomorphs, and showed that they were not the most basal living teleost. Comparative sequence analysis of preproinsulins among the osteoglossomorphs was useful in assessment of intergroup relationship, relating elephantnose with the feather fin knife fish and the arawana, butterfly fish, and goldeye. This arrangement of species is consistent with relationships based on other more classical parameters, except for the goldeye which was assessed as being sister to all the osteoglossomorphs. The white sucker was grouped with the common carp and both are cyprinids.

Multiple forms of glucagon and somatostatin isolated from the intestine of the southern-hemisphere lamprey Geotria australis

Current views on Agnathan phylogeny favor the hypothesis that the genera of holarctic lampreys belong to a single family (Petromyzontidae) and form an interrelated progression in which Petromyzon is near to Ichthomyzon at the base of the phylogenetic tree and Lampetra is the most derived. A stock similar to that of contemporary Ichthomyzon is considered to have given rise to the southern hemisphere lamprey Geotria australis, the sole member of the Geotriidae. In the present study, two molecular forms of glucagon were isolated from an extract of G. australis intestine that differed in structure by six amino acid residues. One form shows two amino acid substitutions (Leu14 --> Met and Ala29 --> Ser) compared with the single molecular form of glucagon isolated from the sea lamprey Petromyzon marinus and the second form shows three substitutions (Asp15 --> Glu, Ser16 --> Ala, Ile24 --> Thr) compared with the single glucagon isolated from the river lamprey Lampetra fluviatilis. As Petromyzon and Lampetra glucagons differ by six amino acid residues, the data suggest that a duplication of the glucagon gene occurred prior to or early in lamprey evolution. Although both genes are strongly expressed in G. australis, the expression of one gene predominates in P. marinus while that of the other gene predominates in L. fluviatilis. Previous work has shown that, in the islet organ of G. australis, preprosomatostatin is processed almost exclusively to somatostatin-33. However, the present study demonstrates that somatostatin-14 is the major molecular form in G. australis intestine with somatostatin-33 present only as a minor component. This result demonstrates a tissue-dependent pathway of posttranslational processing of preprosomatostatin in the Geotria enteropancreatic system.

Insulin, glucagon and somatostatin localization in the pancreas of metamorphosed Xenopus laevis

The current study was designed to determine if insulin, glucagon and somatostatin-containing cells are present in the pancreas of adult Xenopus laevis. Localization methods utilized included cytochemical aldehyde fuchsin (AF) staining as well as the immunochemical peroxidase antiperoxidase (PAP) procedure for light microscopy. The results show numerous large clusters of AF-positive cells within a network of highly vascularized acinar tissue. PAP immunochemical localization with insulin antibody on adjacent sections demonstrates positive immunoreactivity to AF-positive cell groups and also the presence of immunoreactive insulin (IRI). Cells exhibiting this immunoreactivity are located in the central region of the islet-like structures. Serial sections not only show PAP immunoreactivity for IRI, but also for immunoreactive glucagon (IRG) and immunoreactive somatostatin (IRS) in the same islet-like structure. IRG and IRS-containing cells are situated around the periphery of the islet-like structures, surrounding the central core of IRI-containing cells. Antibody specificity was confirmed by homologous and heterologous antigen immuno-absorbance assays, as well as incubation of adjacent sections in preimmune sera. Based on this data we conclude that: the distribution of cells of the endocrine pancreas of metamorphosed Xenopus laevis is similar to that of many mammals and certain urodeles. Given the apparent specificity of the antigen-antibody reactions, it appears that Xenopus insulin, glucagon and somatostatin are structurally conserved.

Physiology of fish endocrine pancreas

From the very beginning of physiological studies on the endocine pancreas, fish have been used as experimental subjects. Fish insulin was one of the first vertebrate insulins isolated and one of the first insulins whose primary and then tertiary structures were reported. Before a second pancreatic hormone, glucagon, was characterized, a physiologically active 'impurity', similar to that in mammalian insulin preparations, was found in fish insulins.Fish have become the most widely used model for studies of biosynthesis and processing of the pancreatic hormones. It seems inconceivable, therefore, that until the recent past cod and tuna insulins have been the only purified piscine islet hormones available for physiological experiments. The situation has changed remarkably during the last decade.In this review the contemporary status of physiological studies on the fish pancreas is outlined with an emphasis on the following topics: 1) contents of pancreatic peptides in plasma and in islet tissue; 2) actions of piscine pancreatic hormones in fish; 3) specific metabolic consequences of an acute insufficiency of pancreatic peptides; 4) functional interrelations among pancreatic peptides which differ from those of mammals. The pitfalls, lacunae and the perspectives of contemporary physiological studies on fish endocrine pancreas are outlined.

Localization of insulin, glucagon and somatostatin in the pancreas of the adult newt, Notophthalmus viridescens

The current study is designed to demonstrate the presence of immunoreactive insulin (IRI), glucagon and somatostatin in the adult pancreas. Methods include aldehyde fuchsin (AF) staining and peroxidase anti-peroxidase (PAP) immunochemical localization for light microscopy as well as protein A gold (PAG) staining for scanning electron microscopy (SEM) in conjunction with backscattered electron imaging (BEI). Our results show the presence of large clusters of AF-positive cells within networks of highly vascularized pancreatic acinar tissue. PAP immunochemistry of pancreas serial sections exhibit positive immunoreactivity to the same AF-positive structure, thus demonstrating the presence of IRI. This immunoreactivity is found in a high percentage of cells in the islet-like structures. These cells tend to be centrally located within the cluster. Antibody specificity controls, including homologous antigen immunoabsorbance, as well as incubation of sections in normal guinea pig serum give negative immunoreactivity. Immunoreactive glucagon-containing cells and somatostatin-containing cells are distributed around the periphery of the central core of IRI-containing cells. SEM in conjunction with BEI confirm the presence of PAG within these cell clusters. We conclude that: (a) newt pancreatic IRI reacts in a specific manner with bovine antibody, suggesting a partial structural similarity to mammalian antigen; (b) IRI is localized within within pancreatic islet-like cell clusters and these IRI-containing cells form a central mass which is surrounded by glucagon and somatostatin-containing cells; this cellular distribution is similar to that found in many mammals. PAG conjugated insulin antibody is detectable by SEM in conjunction with BEI in islet cells of the newt pancreas.

Serum immunoreactive insulin levels in intact and regenerating postmetamorphic Xenopus laevis

In order to confirm the presence of immunoreactive insulin (IRI) in the serum of postmetamorphic Xenopus laevis, radioimmunoassay (RIA) methods were used. The concentration of hormone found in samples of blood serum taken from nonanaesthetized intact male and female animals by the guillotine method was 10.46 +/- 0.76 microU/ml. Significantly higher IRI concentrations were found in our intact animals anaesthetized in MS 222 at pH 3.5 (21.9 microU/ml) compared with intact controls anaesthetized in MS 222 adjusted to pH 7.0 (14.4 microU/ml). During the wound-healing stage subsequent to forelimb amputation in the experimental cases (0 hours to 3 days) anaesthetized in MS 222 pH 7.0, there were intervals of significantly elevated serum IRI followed by a period of decreased IRI concentration compared with the levels in anaesthetized (MS 222 pH 7.0) and nonanaesthetized intact controls. These fluctuations were due, presumably, to stress caused by amputational injury and/or anaesthetic. Serum IRI increased steadily from 3 to 14 days postamputation then remained stable for the balance of the regeneration period (28 days) compared with nonanesthetized intact controls. A positive correlation was found between immunoreactive insulin and glucose levels in the serum of our animals. However, no correlation exists between serum IRI levels and serum osmolality in the data.

The isolation, purification and amino-acid sequence of insulin from the teleost fish Cottus scorpius (daddy sculpin)

Insulin from the principal islets of the teleost fish, Cottus scorpius (daddy sculpin), has been isolated and sequenced. Purification involved acid/alcohol extraction, gel filtration, and reverse-phase high-performance liquid chromatography to yield nearly 1 mg pure insulin/g wet weight islet tissue. Biological potency was estimated as 40% compared to porcine insulin. The sculpin insulin crystallised in the absence of zinc ions although zinc is known to be present in the islets in significant amounts. Two other hormones, glucagon and pancreatic polypeptide, were copurified with the insulin, and an N-terminal sequence for pancreatic polypeptide was determined. The primary structure of sculpin insulin shows a number of sequence changes unique so far amongst teleost fish. These changes occur at A14 (Arg), A15 (Val), and B2 (Asp). The B chain contains 29 amino acids and there is no N-terminal extension as seen with several other fish. Presumably as a result of the amino acid substitutions, sculpin insulin does not readily form crystals containing zinc-insulin hexamers, despite the presence of the coordinating B10 His.

Characterization of cat insulin

Cat insulin was isolated and both chains were characterized by determination of the primary structures. The molecule was found to differ from human insulin at four positions, A8 (Ala), A10 (Val), A18 (His), and B30 (Ala). A comparison with other known insulin structures suggests that cat insulin has an uncommon property: it appears to be the only insulin found so far with His at position A18. The difference is compatible with a conserved overall conformation but this histidine occupies a position close to the suggested receptor interacting area and may influence some binding properties.

Nature of the B10 amino acid residue. Requirements for high biological activity of insulin

Human [10-asparagine-B] insulin ([ Asn10 -B] insulin), an analogue which differs from the parent molecule in that the histidine residue at position 10 of the B chain (B10) is replaced by asparagine, has been synthesized and isolated in purified form. In vitro biological assays indicated a potency of ca. 35% compared to insulin. We have previously shown that the replacement of histidine at position B10 by lysine resulted in an analogue displaying ca. 15% of the biological activity of natural hormone, while the substitution of leucine in this position produced a molecule exhibiting ca. 45% potency in in vivo assays. The data indicate that molecular size of the amino acid residue at position B10 may be important in the maintenance of a structure commensurate with high biological activity. Polarity at this position appears to be rather unimportant while a strongly basic group appears to be deleterious.

Role of zinc in insulin biosynthesis. Some possible zinc-insulin interactions in the pancreatic B-cell

The behaviour of proinsulin and insulin in the presence of zinc suggests it plays an important role in insulin's production in the B-cell for the vast majority of animal species. The postulate that proinsulin forms a zinc containing hexamer soon after its synthesis and that this organization of the molecule is maintained through all the subsequent processes is supported by our observation that the proinsulin hexamer is converted readily into the insulin hexamer. In addition the zinc ions enhance proinsulin's solubility and render insulin insoluble. Zinc ions also appear to play an important role in the microcrystalline character of the precipitated insulin granule. There may be advantages in condensing the stored material in this way; it will reduce contact with the surrounding membrane where the converting, and possibly other enzymes, are thought to be located, and it will tend to exclude incompletely converted hexamers.

Mapping of the residues responsible for the negative cooperativity of the receptor-binding region of insulin

Insulin binding to its receptor leads to negatively cooperative interactions among the receptor sites. Studies with 29 insulin analogues (animal insulins and proinsulin, insulin-like growth factor and chemically modified insulins) which vary 1,000-fold in their affinity for the receptor and in their biological potency, suggest that a discrete invariable region on the surface of the insulin monomer is responsible for including the negative cooperativity. This domain comprises some of the eight carboxy-terminal residues of the B-chain and the A21 asparagine. Burying of this 'cooperative site' in the dimerisation of insulin leads to a loss of negative cooperativity. A revised mapping of the insulin molecule is proposed, featuring distinct bioactive and cooperative sites.

Phylogeny of insulin. Some evolutionary aspects of insulin production with particular regard to the biosynthesis of insulin in Myxine glutinosa

Preceding phylogenetic studies on the occurrence of insulin have shown--e.g. by bioassays and immunocytochemical procedures--that insulin producing B-cells are present in all vertebrates and even in several invertebrates, both protostomian and deuterostomian. The most original B-cells are obviously endocrine cells of open type, situated in the mucosa of the alimentary tract. Moreover, the results of these studies show that insulin is not only a polypeptide hormone of considerable age but also that the insulin molecule seems to have been kept surprisingly stable during evolution. Best known of all non-mammalian insulins is that from the hagfish, Myxine glutinosa. It is probably the most original insulin of all in the vertebrate series. Both the amino-acid sequence and the three-dimensional structure of the dimer of hagfish insulin differ only little from those of pig insulin. The biosynthesis occurs via proinsulin and is also in most respects similar to mammalian insulin biosynthesis. There are, however, some differences. Although it readily crystallizes as tetragonal bipyramids, hagfish insulin does not form hexamers. In a test system, with isolated rat fat cells, its binding affinity is 23% and its potency 5% of that of pig insulin, a discrepancy indicating a "partial antagonism" on the receptors. Although the conversion of proinsulin to insulin seems to occur in the secretion granules, they contain no crystalline cores. Since a strictly tryptic-like enzyme was found to destroy hagfish insulin rapidly, the enzyme converting proinsulin to insulin must--in addition to a carboxy-peptidase-B-like activity--have a different specificity in Myxine.

Is the evolution of insulin Darwinian or due to selectively neutral mutation?

A model for the evolution of insulin mainly in terms of adaptive processes is discussed. The model depends critically on the relationship of sequence changes to the three-dimensional structure and the role of various parts of this structure in the conversion of the proinsulin molecule to the active form, the storage of insulin, its transport to the site of action and its interaction with a receptor.

The relation of polypeptide hormone structure and flexibility to receptor binding: the relevance of X-ray studies on insulins, glucagon and human placental lactogen

Thr relevance of the crystal structure of the polypeptide hormones, insulin, glucagon and human placental lactogen to conformation and flexibility in solution and to receptor binding is considered. X-ray studies for crystal forms of glucagon, human placental lactogen and three insulin derivatives (A1 acetyl insulin, A1-t-butoxy carbonyl insulin and A1 2,2-dimethyl-3-formyl-L-thiazolidine-4-carbonyl insulin) are reported. Neither glucagon nor human placental lactogen are as ordered as insulin in the crystal form. Glucagon crystals undergo distinct transformations on changing the pH of the mother liquor from pH 9.5 to pH 6, indicating that the glucagon molecule is flexible in the crystal, as it is in solution. On the other hand all insulin analogues have a similar three dimensional structure to that of native insulin. Three dimensional difference Fourier studies of two insulin derivatives at 3 A resolution indicate the position of the modifying groups and define the small conformational changes which have occurred. The in vitro biological activity and receptor binding decrease with the increasing size of the group added to A1. The correlation of the structure analysis with the biological data strongly implicate a region close to A1 in receptor binding. Insulin appears to bind to the receptor in a specific conformation similar to that observed in the crystal structure and in solution; amino acid residues which are separated in the primary structure but brought into close juxtaposition in the tertiary structure are important for full potency.