Structural and functional characteristics of the Val44Met insulin-like growth factor I missense mutation: correlation with effects on growth and development

Identification of functional rare coding variants in IGF-1 gene in humans with exceptional longevity

Diminished signaling via insulin/insulin-like growth factor-1 (IGF-1) axis is associated with longevity in different model organisms. IGF-1 gene is highly conserved across species, with only few evolutionary changes identified in it. Despite its potential role in regulating lifespan, no coding variants in IGF-1 have been reported in human longevity cohorts to date. This study investigated the whole exome sequencing data from 2,487 individuals in a cohort of Ashkenazi Jewish centenarians, their offspring, and controls without familial longevity to identify functional IGF-1 coding variants. We identified two likely functional coding variants IGF-1:p.Ile91Leu and IGF-1:p.Ala118Thr in our longevity cohort. Notably, a centenarian specific novel variant IGF-1:p.Ile91Leu was located at the binding interface of IGF-1 - IGF-1R, whereas IGF-1:p.Ala118Thr was significantly associated with lower circulating levels of IGF-1. We performed extended all-atom molecular dynamics simulations to evaluate the impact of Ile91Leu on stability, binding dynamics and energetics of IGF-1 bound to IGF-1R. The IGF-1:p.Ile91Leu formed less stable interactions with IGF-1R's critical binding pocket residues and demonstrated lower binding affinity at the extracellular binding site compared to wild-type IGF-1. Our findings suggest that IGF-1:p.Ile91Leu and IGF-1:p.Ala118Thr variants attenuate IGF-1R activity by impairing IGF-1 binding and diminishing the circulatory levels of IGF-1, respectively. Consequently, diminished IGF-1 signaling resulting from these variants may contribute to exceptional longevity in humans.

Genetic causes of growth hormone insensitivity beyond GHR

Growth hormone insensitivity (GHI) syndrome, first described in 1966, is classically associated with monogenic defects in the GH receptor (GHR) gene which result in severe post-natal growth failure as consequences of insulin-like growth factor I (IGF-I) deficiency. Over the years, recognition of other monogenic defects downstream of GHR has greatly expanded understanding of primary causes of GHI and growth retardation, with either IGF-I deficiency or IGF-I insensitivity as clinical outcomes. Mutations in IGF1 and signaling component STAT5B disrupt IGF-I production, while defects in IGFALS and PAPPA2, disrupt transport and release of circulating IGF-I, respectively, affecting bioavailability of the growth-promoting IGF-I. Defects in IGF1R, cognate cell-surface receptor for IGF-I, disrupt not only IGF-I actions, but actions of the related IGF-II peptides. The importance of IGF-II for normal developmental growth is emphasized with recent identification of defects in the maternally imprinted IGF2 gene. Current application of next-generation genomic sequencing has expedited the pace of identifying new molecular defects in known genes or in new genes, thereby expanding the spectrum of GH and IGF insensitivity. This review discusses insights gained and future directions from patient-based molecular and functional studies.

Disorders of IGFs and IGF-1R signaling pathways

The insulin-like growth factor (IGF) system comprises two ligands, IGF-I and IGF-II, that regulate multiple physiological processes, including mammalian development, metabolism and growth, through the type 1 IGF receptor (IGF-1R). The growth hormone (GH)-IGF-I axis is the major regulator of longitudinal growth. IGF-II is expressed in many tissues, notably the placenta, to regulate human pre- and post-natal growth and development. This review provides a brief introduction to the IGF system and summarizes findings from reports arising from recent larger genomic sequencing studies of human genetic mutations in IGF1 and IGF2 and genes of proteins regulating IGF action, namely the IGF-1R, IGF-1R signaling pathway components and the IGF binding proteins (IGFBPs). A perspective on the effect of homozygous mutations on structure and function of the IGFs and IGF-1R is also given and this is related to the effects on growth.

Insights into pathological mutations in insulin-like growth factor I through in silico screening and molecular dynamics simulation

Insulin-like growth factor I (IGF-I) is an anabolic growth hormone indispensable for cell growth, proliferation, differentiation, and other metabolic processes. Three missense mutations in IGF-I have been identified to be disease-related, while more mutations are waiting for phenotype annotation. However, there is no previous work regarding effective and accurate identification of pathological mutations of IGF-I, neither regarding the effects of mutations on the protein structure and dynamics. In this study, we first predicted potential deleterious mutations present in IGF-I using 16 in silico tools. Then, these mutations were further evaluated through multiple bioinformatics methods including conservation analysis, physicochemical characterization, and molecular dynamics simulation. After rigorous screening, five mutations (T4M, V17M, V44M, R50W, and M59R) were finally selected, of which two have been previously reported to be deleterious. These mutations locate at conserved regions and change the residue size locally. In the conventional simulations, the mutations destabilized the overall IGF-I structure by destroying two important hydrogen bonds within the key region of "C-neck." This finding was further confirmed by the thermal unfolding simulations and the free-energy calculations, where the mutants were associated with faster and greater loss of helix and lower energy barriers in comparison with the wild-type protein. The rigorous phenotype prediction and comprehensive structural analysis of missense mutations will not only pave the way of screening for harmful mutations in IGF-I but also provide new prospects for the rational design of IGF-I analogues and tailored medicine.

Therapeutic Targeting of the IGF Axis

The insulin like growth factor (IGF) axis plays a fundamental role in normal growth and development, and when deregulated makes an important contribution to disease. Here, we review the functions mediated by ligand-induced IGF axis activation, and discuss the evidence for the involvement of IGF signaling in the pathogenesis of cancer, endocrine disorders including acromegaly, diabetes and thyroid eye disease, skin diseases such as acne and psoriasis, and the frailty that accompanies aging. We discuss the use of IGF axis inhibitors, focusing on the different approaches that have been taken to develop effective and tolerable ways to block this important signaling pathway. We outline the advantages and disadvantages of each approach, and discuss progress in evaluating these agents, including factors that contributed to the failure of many of these novel therapeutics in early phase cancer trials. Finally, we summarize grounds for cautious optimism for ongoing and future studies of IGF blockade in cancer and non-malignant disorders including thyroid eye disease and aging.

Insulin-Like Growth Factor-1 Signaling in Lung Development and Inflammatory Lung Diseases

Insulin-like growth factor-1 (IGF-1) was firstly identified as a hormone that mediates the biological effects of growth hormone. Accumulating data have indicated the role of IGF-1 signaling pathway in lung development and diseases such as congenital disorders, cancers, inflammation, and fibrosis. IGF-1 signaling modulates the development and differentiation of many types of lung cells, including airway basal cells, club cells, alveolar epithelial cells, and fibroblasts. IGF-1 signaling deficiency results in alveolar hyperplasia in humans and disrupted lung architecture in animal models. The components of IGF-1 signaling pathways are potentiated as biomarkers as they are dysregulated locally or systemically in lung diseases, whereas data may be inconsistent or even paradoxical among different studies. The usage of IGF-1-based therapeutic agents urges for more researches in developmental disorders and inflammatory lung diseases, as the majority of current data are collected from limited number of animal experiments and are generally less exuberant than those in lung cancer. Elucidation of these questions by further bench-to-bedside researches may provide us with rational clinical diagnostic approaches and agents concerning IGF-1 signaling in lung diseases.

How ligand binds to the type 1 insulin-like growth factor receptor

Human type 1 insulin-like growth factor receptor is a homodimeric receptor tyrosine kinase that signals into pathways directing normal cellular growth, differentiation and proliferation, with aberrant signalling implicated in cancer. Insulin-like growth factor binding is understood to relax conformational restraints within the homodimer, initiating transphosphorylation of the tyrosine kinase domains. However, no three-dimensional structures exist for the receptor ectodomain to inform atomic-level understanding of these events. Here, we present crystal structures of the ectodomain in apo form and in complex with insulin-like growth factor I, the latter obtained by crystal soaking. These structures not only provide a wealth of detail of the growth factor interaction with the receptor’s primary ligand-binding site but also indicate that ligand binding separates receptor domains by a mechanism of induced fit. Our findings are of importance to the design of agents targeting IGF-1R and its partner protein, the human insulin receptor.

The human type 1 insulin-like growth factor receptor (IGF-1R) is important for normal human growth and development. Here, the authors present the crystal structures of the IGF-1R ectodomain both in its apo form and in complex with its ligand insulin-like growth factor I and discuss the receptor activation mechanism.

IGF-1 in retinopathy of prematurity, a CNS neurovascular disease

The retina is part of the central nervous system and both the retina as well as the brain can suffer from severe damage after very preterm birth. Retinopathy of prematurity is one of the major causes of blindness in these children and brain neuronal impairments including cognitive defects, cerebral palsy and intraventricular hemorrhage (IVH) are also complications of very preterm birth. Insulin-like growth factor 1 (IGF-1) acts to promote proliferation, maturation, growth and survival of neural cells. Low levels of circulating IGF-1 are associated with ROP and defects in the IGF-1 gene are associated with CNS disorders including learning deficits and brain growth restriction. Treatment of preterm infants with recombinant IGF-1 may potentially prevent ROP and CNS disorders. This review compares the role of IGF-1 in ROP and CNS disorders. A recent phase 2 study showed a positive effect of IGF-1 on the severity of IVH but no effect on ROP. A phase 3 trial is planned.

Total Chemical Synthesis of an Intra-A-Chain Cystathionine Human Insulin Analogue with Enhanced Thermal Stability

Despite recent advances in the treatment of diabetes mellitus, storage of insulin formulations at 4 °C is still necessary to minimize chemical degradation. This is problematic in tropical regions where reliable refrigeration is not ubiquitous. Some degradation byproducts are caused by disulfide shuffling of cystine that leads to covalently bonded oligomers. Consequently we examined the utility of the non-reducible cystine isostere, cystathionine, within the A-chain. Reported herein is an efficient method for forming this mimic using simple monomeric building blocks. The intra-A-chain cystathionine insulin analogue was obtained in good overall yield, chemically characterized and demonstrated to possess native binding affinity for the insulin receptor isoform B. It was also shown to possess significantly enhanced thermal stability indicating potential application to next-generation insulin analogues.

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.

Association analysis of ten candidate genes in a large multinational cohort of small for gestational age children and children with idiopathic short stature (NESTEGG study)

BACKGROUND

Fetal growth failure has been associated with an increased risk of hypertension, cardiovascular disease and diabetes in adulthood. Exploring the mechanisms underlying this association should improve our understanding of these common adult diseases.

PATIENTS AND METHODS

We investigated 225 SNPs in 10 genes involved in growth and glucose metabolism (GH1, GHR, IGF1, IGF1R, STAT5A, STAT5B, MAPK1, MAPK3, PPARγ and INS) in 1,437 children from the multinational NESTEGG consortium: 345 patients born small for gestational age who remained short (SGA-S), 288 who showed catch-up growth (SGA-Cu), 410 idiopathic short stature (ISS) and 394 controls. We related genotype to pre- and/or postnatal growth parameters, response to growth hormone (if applicable) and blood pressure.

RESULTS

We found several clinical associations for GH1, GHR, IGF1, IGF1R, PPARγ and MAPK1. One SNP remained significant after Bonferroni's correction: IGF1R SNP rs4966035's minor allele A was significantly more prevalent among SGA and associated with smaller birth length (p = 0.000378) and birth weight (weaker association), independent of gestational age.

CONCLUSION

IGF1R SNP rs4966035 is significantly associated with birth length, independent of gestational age. This and other associations suggest that polymorphisms in these genes might partly explain the phenotype of short children born SGA and children with ISS.

Viral expression of insulin-like growth factor I E-peptides increases skeletal muscle mass but at the expense of strength

Insulin-like growth factor I (IGF-I) is a protein that regulates and promotes growth in skeletal muscle. The IGF-I precursor polypeptide contains a COOH-terminal extension called the E-peptide. Alternative splicing in the rodent produces two isoforms, IA and IB, where the mature IGF-I in both isoforms is identical yet the E-peptides, EA and EB, share less than 50% homology. Recent in vitro studies show that the E-peptides can enhance IGF-I signaling, leading to increased myoblast cell proliferation and migration. To determine the significance of these actions in vivo and to evaluate if they are physiologically beneficial, EA and EB were expressed in murine skeletal muscle via viral vectors. The viral constructs ensured production of E-peptides without the influence of additional IGF-I through an inactivating mutation in mature IGF-I. E-peptide expression altered ERK1/2 and Akt phosphorylation and increased satellite cell proliferation. EB expression resulted in significant muscle hypertrophy that was IGF-I receptor dependent. However, the increased mass was associated with a loss of muscle strength. EA and EB have similar effects in skeletal muscle signaling and on satellite cells, but EB is more potent at increasing muscle mass. Although sustained EB expression may drive hypertrophy, there are significant physiological consequences for muscle.

The pro-forms of insulin-like growth factor I (IGF-I) are predominant in skeletal muscle and alter IGF-I receptor activation

IGF-I is a key regulator of muscle development and growth. The pre-pro-peptide produced by the Igf1gene undergoes several posttranslational processing steps to result in a secreted mature protein, which is thought to be the obligate ligand for the IGF-I receptor (IGF-IR). The goals of this study were to determine what forms of IGF-I exist in skeletal muscle, and whether the mature IGF-I protein was the only form able to activate the IGF-IR. We measured the proportion of IGF-I species in murine skeletal muscle and found that the predominant forms were nonglycosylated pro-IGF-I and glycosylated pro-IGF-I, which retained the C-terminal E peptide extension, instead of mature IGF-I. These forms were validated using samples subjected to viral expression of IGF-I combined with furin and glycosidase digestion. To determine whether the larger molecular weight IGF-I forms were also ligands for the IGF-IR, we generated each specific form through transient transfection of 3T3 cells and used the enriched media to perform kinase receptor activation assays. Compared with mature IGF-I, nonglycosylated pro-IGF-I had similar ability to activate the IGF-IR, whereas glycosylation of pro-IGF-I significantly reduced receptor activation. Thus, it is important to understand not only the quantity, but also the proportion of IGF-I forms produced, to evaluate the true biological activity of this growth factor.

New Modulators for IGF-I Activity within IGF-I Processing Products

Insulin-like growth factor I (IGF-I) is a key regulator of muscle development and growth. The pre-pro-peptide produced by the Igf1 gene undergoes several post-translational processing steps to result in a secreted mature protein, which is thought to be the obligate ligand for the IGF-I receptor (IGF-IR). However, the significance of the additional forms and peptides produced from Igf1 is not clear. For instance, the C-terminal extensions called the E-peptides that are part of pro-IGF-I, have been implicated in playing roles in cell growth, including cell proliferation and migration and muscle hypertrophy in an IGF-IR independent manner. However, the activity of these peptides has been controversial. IGF-IR independent actions suggest the existence of an E-peptide receptor, yet such a protein has not been discovered. We propose a new concept: there is no E-peptide receptor, rather the E-peptides coordinate with IGF-I to modulate activity of the IGF-IR. Growing evidence reveals that the presence of an E-peptide alters IGF-I activity, whether as part of pro-IGF-I, or as a separate peptide. In this review, we will examine the past literature on IGF-I processing and E-peptide actions in skeletal muscle, address the previous attempts to separate IGF-I and E-peptide effects, propose a new model for IGF-I/E-peptide synergy, and suggest future experiments to test if the E-peptides truly modulate IGF-I activity.

The insulin-like growth factor mutation database (IGFmdb)

Insulin-like growth factors (IGF-I and IGF-II), and insulin are evolutionarily conserved hormonal regulators of eukaryotic growth and development. Through interactions with their cognate receptors, all three molecules can influence cellular growth, proliferation, differentiation, migration, and survival, as well as metabolic processes. As such, perturbations in signaling by IGFs and insulin are a well-documented cause of altered growth, development and survival during both embryonic and post-natal life. A key approach in understanding how IGFs and insulin elicit their biological effects has been through identifying structural features of the ligands that influence their receptor interactions. Over the years, the study of many hundreds of specifically engineered IGF and insulin analogues has provided a wealth of knowledge about how specific residues of these ligands contribute to ligand:receptor interactions. Some analogues have even provided the basis for designing therapeutic agents for the treatment of IGF and insulin-related diseases. As the list of IGF and insulin analogues continues to grow we find that, while many have been produced and studied, it would be of considerable value to have a central repository from which information about specific analogues and their receptor binding data were readily available in an easily searchable and comparable format. To address this, we have created the "Insulin-like growth factor mutation database" (IGFmdb). The IGFmdb is a web-based curated database of annotated ligand analogues and their receptor binding affinities that can be accessed via http://www.adelaide.edu.au/igfmutation. Currently the IGFmdb contains receptor-binding data for 67 IGF-II analogues that were publicly accessible prior to 2012, as well as 67 IGF-I analogues, including all of those produced and characterised in our laboratory. A small number of these are IGF species homologues. There are also 32 insulin analogues within IGFmdb that were reported within the included IGF analogue studies, representing only a small fraction of existing insulin mutants. Future developments of the IGFmdb will incorporate receptor-binding data for all publicly accessible IGF-I analogues and the data will be expanded to include IGF-binding protein (IGFBP) binding affinities.

Insulin-like growth factor-I E-peptide activity is dependent on the IGF-I receptor

Insulin-like growth factor-I (IGF-I) is an essential growth factor that regulates the processes necessary for cell proliferation, differentiation, and survival. The Igf1 gene encodes mature IGF-I and a carboxy-terminal extension called the E-peptide. In rodents, alternative splicing and post-translational processing produce two E-peptides (EA and EB). EB has been studied extensively and has been reported to promote cell proliferation and migration independently of IGF-I and its receptor (IGF-IR), but the mechanism by which EB causes these actions has not been identified. Further, the properties of EA have not been evaluated. Therefore, the goals of this study were to determine if EA and EB possessed similar activity and if these actions were IGF-IR independent. We utilized synthetic peptides for EA, EB, and a scrambled control to examine cellular responses. Both E-peptides increased MAPK signaling, which was blocked by pharmacologic IGF-IR inhibition. Although the E-peptides did not directly induce IGF-IR phosphorylation, the presence of either E-peptide increased IGF-IR activation by IGF-I, and this was achieved through enhanced cell surface bioavailability of the receptor. To determine if E-peptide biological actions required the IGF-IR, we took advantage of the murine C2C12 cell line as a platform to examine the key steps of skeletal muscle proliferation, migration and differentiation. EB increased myoblast proliferation and migration while EA delayed differentiation. The proliferation and migration effects were inhibited by MAPK or IGF-IR signaling blockade. Thus, in contrast to previous studies, we find that E-peptide signaling, mitogenic, and motogenic effects are dependent upon IGF-IR. We propose that the E-peptides have little independent activity, but instead affect growth via modulating IGF-I signaling, thereby increasing the complexity of IGF-I biological activity.

Neurodevelopmental effects of insulin-like growth factor signaling

Insulin-like growth factor (IGF) signaling greatly impacts the development and growth of the central nervous system (CNS). IGF-I and IGF-II, two ligands of the IGF system, exert a wide variety of actions both during development and in adulthood, promoting the survival and proliferation of neural cells. The IGFs also influence the growth and maturation of neural cells, augmenting dendritic growth and spine formation, axon outgrowth, synaptogenesis, and myelination. Specific IGF actions, however, likely depend on cell type, developmental stage, and local microenvironmental milieu within the brain. Emerging research also indicates that alterations in IGF signaling likely contribute to the pathogenesis of some neurological disorders. This review summarizes experimental studies and shed light on the critical roles of IGF signaling, as well as its mechanisms, during CNS development.

Genetic factors associated with small for gestational age birth and the use of human growth hormone in treating the disorder

The term small for gestational age (SGA) refers to infants whose birth weights and/or lengths are at least two standard deviation (SD) units less than the mean for gestational age. This condition affects approximately 3%–10% of newborns. Causes for SGA birth include environmental factors, placental factors such as abnormal uteroplacental blood flow, and inherited genetic mutations. In the past two decades, an enhanced understanding of genetics has identified several potential causes for SGA. These include mutations that affect the growth hormone (GH)/insulin-like growth factor (IGF)-1 axis, including mutations in the IGF-1 gene and acid-labile subunit (ALS) deficiency. In addition, select polymorphisms observed in patients with SGA include those involved in genes associated with obesity, type 2 diabetes, hypertension, ischemic heart disease and deletion of exon 3 growth hormone receptor (d3-GHR) polymorphism. Uniparental disomy (UPD) and imprinting effects may also underlie some of the phenotypes observed in SGA individuals. The variety of genetic mutations associated with SGA births helps explain the diversity of phenotype characteristics, such as impaired motor or mental development, present in individuals with this disorder. Predicting the effectiveness of recombinant human GH (hGH) therapy for each type of mutation remains challenging. Factors affecting response to hGH therapy include the dose and method of hGH administration as well as the age of initiation of hGH therapy. This article reviews the results of these studies and summarizes the success of hGH therapy in treating this difficult and genetically heterogenous disorder.

Insulin-like growth factor binding proteins: a structural perspective

Insulin-like growth factor binding proteins (IGFBP-1 to -6) bind insulin-like growth factors-I and -II (IGF-I and IGF-II) with high affinity. These binding proteins maintain IGFs in the circulation and direct them to target tissues, where they promote cell growth, proliferation, differentiation, and survival via the type 1 IGF receptor. IGFBPs also interact with many other molecules, which not only influence their modulation of IGF action but also mediate IGF-independent activities that regulate processes such as cell migration and apoptosis by modulating gene transcription. IGFBPs-1 to -6 are structurally similar proteins consisting of three distinct domains, N-terminal, linker, and C-terminal. There have been major advances in our understanding of IGFBP structure in the last decade and a half. While there is still no structure of an intact IGFBP, several structures of individual N- and C-domains have been solved. The structure of a complex of N-BP-4:IGF-I:C-BP-4 has also been solved, providing a detailed picture of the structural features of the IGF binding site and the mechanism of binding. Structural studies have also identified features important for interaction with extracellular matrix components and integrins. This review summarizes structural studies reported so far and highlights features important for binding not only IGF but also other partners. We also highlight future directions in which structural studies will add to our knowledge of the role played by the IGFBP family in normal growth and development, as well as in disease.

The severe short stature in two siblings with a heterozygous IGF1 mutation is not caused by a dominant negative effect of the putative truncated protein

OBJECTIVE

While in previous studies heterozygosity for an Insulin-Like Growth Factor 1 (IGF1) defect only modestly decreased height and head circumference, we recently reported on two siblings with severe short stature with a maternally transmitted heterozygous duplication of 4 nucleotides, resulting in a frame shift and a premature termination codon in the IGF1 gene. In this paper we describe the structural and functional characteristics of the putative truncated IGF-I protein.

DESIGN

Two children, their mother and maternal grandfather carried the mutation. In addition, two family members who were not affected were included in the study. Mutant (MT) IGF-I was synthesized in oxidized and reduced form using two methods. Neutral gel filtration studies were carried out with wild-type (WT) and synthetic MT IGF-I. Binding analysis of synthetic MT IGF-I to the IGF1R and insulin receptors were performed with EBNA-293 cells, stably transfected with the IGF-I receptor, and IM9 cells. L6 cells were used to examine the mitogenic potency and the potential antagonizing effect of synthetic MT IGF-I by [(3)H]-thymidine incorporation assays.

RESULTS

In the sera of both the carriers and non-carriers the proportion of (125)I-IGF-I that was associated with the 150 kDa complex was somewhat less (varying between ~37 and ~52%) than in normal pooled serum (~53-~63%) and, instead, slightly increased amounts of radioactivity were eluted in the 40-50 kDa fraction (consisting of binary IGF-IGFBP complexes) or remained unbound. Synthetic MT IGF-I did not bind to the IGF-I receptor, nor antagonize the growth-promoting effect of IGF-I. It did bind to IGFBPs, but was barely incorporated into 150 kDa complexes. Because in all cases WT IGF-I immunoreactivity was recovered in one peak, corresponding to the MW of WT IGF-I, i.e. ~7.6 kDa, an interaction of circulating truncated mutant peptide with WT IGF-I is very unlikely.

CONCLUSIONS

There is no evidence that the severe short stature associated with heterozygosity for this novel IGF1 mutation in children born from a mother with the same mutation is caused by a dominant negative effect of the truncated protein. We speculate that the growth failure is caused by a combination of partial IGF-I deficiency, placental IGF-I insufficiency, and other genetic factors.

All-atom structural models for complexes of insulin-like growth factors IGF1 and IGF2 with their cognate receptor

Type 1 insulin-like growth factor receptor (IGF1R) is a membrane-spanning glycoprotein of the insulin receptor family that has been implicated in a variety of cancers. The key questions related to molecular mechanisms governing ligand recognition by IGF1R remain unanswered, partly due to the lack of testable structural models of apo or ligand-bound receptor complexes. Using a homology model of the IGF1R ectodomain IGF1RDeltabeta, we present the first experimentally consistent all-atom structural models of IGF1/IGF1RDeltabeta and IGF2/IGF1RDeltabeta complexes. Our explicit-solvent molecular dynamics (MD) simulation of apo-IGF1RDeltabeta shows that it displays asymmetric flexibility mechanisms that result in one of two binding pockets accessible to growth factors IGF1 and IGF2, as demonstrated via an MD-assisted Monte Carlo docking procedure. Our MD-generated ensemble of structures of apo and IGF1-bound IGF1RDeltabeta agrees reasonably well with published small-angle X-ray scattering data. We observe simultaneous contacts of each growth factor with sites 1 and 2 of IGF1R, suggesting cross-linking of receptor subunits. Our models provide direct evidence in favor of suggested electrostatic complementarity between the C-domain (IGF1) and the cysteine-rich domain (IGF1R). Our IGF1/IGF1RDeltabeta model provides structural bases for the observation that a single IGF1 molecule binds to IGF1RDeltabeta at low concentrations in small-angle X-ray scattering studies. We also suggest new possible structural bases for differences in the affinities of insulin, IGF1, and IGF2 for their noncognate receptors.

The rationale and development of therapeutic insulin-like growth factor axis inhibition for lung and other cancers

The insulin-like growth factor (IGF) axis involves elements of endocrine, paracrine, and autocrine control. It is centrally involved in normal development and growth. Core signaling is driven through the IGF-1 receptor (IGF-1R) in either homo-multimeric complexes or hetero-multimeric complexes with the insulin receptor (IR). Signaling is affected by a large number of upstream and downstream factors, including the differential expression of various intracellular IR substrates, a range of stimulatory ligands (insulin, IGF-1, and IGF-2), the expression of specific clearance receptors (eg, IGF-2R), and different IGF-binding proteins. Considerable evidence exists to implicate aspects of the IGF axis in the development and maintenance of many different nonneoplastic and neoplastic diseases, including both small-cell lung cancer and non-small-cell lung cancer (NSCLC). A large number of different anticancer strategies directed against the IGF axis are being developed. Monoclonal antibodies directed against the IGF-1R are the furthest advanced clinically. Hyperglycemia appears to be a class effect. To date, the major difference among the antibodies used in clinical trials seems to be their plasma half-lives, leading to a number of different administration regimens being taken forward. Early signals of monotherapy activity have been notably reported in patients with Ewing sarcoma and in several other cancers. Encouraging increases in the NSCLC response rate have already been reported after the addition of an anti-IGF-1R antibody to first-line carboplatin and paclitaxel. Explorations of aspects of ligands, binding proteins, receptors, and receptor substrates are all ongoing to identify potential biomarkers predictive of benefit from IGF axis intervention.

Partial primary deficiency of insulin-like growth factor (IGF)-I activity associated with IGF1 mutation demonstrates its critical role in growth and brain development

CONTEXT

IGF-I is essential for fetal and postnatal development. Only three IGF1 defects leading to dramatic loss of binding to its type 1 receptor, IGF-1R, have been reported.

PATIENT

We describe a very lean boy who has intrauterine growth restriction and progressive postnatal growth failure associated with normal hearing, microcephaly, and mild intellectual impairment. He had markedly reduced concentrations of IGF-I, with IGFBP-3 and ALS serum levels in the upper normal range or above. IGF-I serum concentrations differed according to the immunoassay used. A higher than average GH dose was required for catch-up growth. Given the mismatch between IGF-I and IGFBP-3 levels, we sequenced his IGF1 gene.

RESULT

We identified a homozygous missense IGF1 mutation. This causes the replacement of a highly conserved amino acid (arginine 36) by a glutamine (R36Q) in the C domain of the predicted peptide. We showed that the abnormal IGF-I peptide has reduced mitogenic activity and partial loss of binding to its receptor IGF-1R. The patient's IGF-I level was undetectable in a highly specific monoclonal assay but elevated in a polyclonal assay.

CONCLUSION

This first report of mild deficiency of IGF-I activity demonstrates that the integrity of IGF-I signaling is important for normal growth and brain development. Molecular defects leading to partial loss of IGF-I activity may not be uncommon in patients born small for gestational age. The characterization of this complex phenotype and identification of such molecular defects have therapeutic implications, particularly now that, in addition to GH, recombinant IGF-I is available for clinical use.

Design of an insulin analog with enhanced receptor binding selectivity: rationale, structure, and therapeutic implications

Insulin binds with high affinity to the insulin receptor (IR) and with low affinity to the type 1 insulin-like growth factor (IGF) receptor (IGFR). Such cross-binding, which reflects homologies within the insulin-IGF signaling system, is of clinical interest in relation to the association between hyperinsulinemia and colorectal cancer. Here, we employ nonstandard mutagenesis to design an insulin analog with enhanced affinity for the IR but reduced affinity for the IGFR. Unnatural amino acids were introduced by chemical synthesis at the N- and C-capping positions of a recognition alpha-helix (residues A1 and A8). These sites adjoin the hormone-receptor interface as indicated by photocross-linking studies. Specificity is enhanced more than 3-fold on the following: (i) substitution of Gly(A1) by D-Ala or D-Leu, and (ii) substitution of Thr(A8) by diaminobutyric acid (Dab). The crystal structure of [D-Ala(A1),Dab(A8)]insulin, as determined within a T(6) zinc hexamer to a resolution of 1.35 A, is essentially identical to that of human insulin. The nonstandard side chains project into solvent at the edge of a conserved receptor-binding surface shared by insulin and IGF-I. Our results demonstrate that modifications at this edge discriminate between IR and IGFR. Because hyperinsulinemia is typically characterized by a 3-fold increase in integrated postprandial insulin concentrations, we envisage that such insulin analogs may facilitate studies of the initiation and progression of cancer in animal models. Future development of clinical analogs lacking significant IGFR cross-binding may enhance the safety of insulin replacement therapy in patients with type 2 diabetes mellitus at increased risk of colorectal cancer.

A novel approach to identify two distinct receptor binding surfaces of insulin-like growth factor II

Very little is known about the residues important for the interaction of insulin-like growth factor II (IGF-II) with the type 1 IGF receptor (IGF-1R) and the insulin receptor (IR). Insulin, to which IGF-II is homologous, is proposed to cross-link opposite halves of the IR dimer through two receptor binding surfaces, site 1 and site 2. In the present study we have analyzed the contribution of IGF-II residues equivalent to insulin's two binding surfaces toward the interaction of IGF-II with the IGF-1R and IR. Four "site 1" and six "site 2" analogues were produced and analyzed in terms of IGF-1R and IR binding and activation. The results show that Val(43), Phe(28), and Val(14) (equivalent to site 1) are critical to IGF-1R and IR binding, whereas mutation to alanine of Gln(18) affects only IGF-1R and not IR binding. Alanine substitutions at Glu(12), Asp(15), Phe(19), Leu(53), and Glu(57) analogues resulted in significant (>2-fold) decreases in affinity for both the IGF-1R and IR. Furthermore, taking a novel approach using a monomeric, single-chain minimized IGF-1R we have defined a distinct second binding surface formed by Glu(12), Phe(19), Leu(53), and Glu(57) that potentially engages the IGF-1R at one or more of the FnIII domains.

Abnormalities of IGF-I signaling in the pathogenesis of diseases of the bone, brain, and fetoplacental unit in humans

IGF-I action is essential for the regulation of tissue formation and remodeling, bone growth, prenatal growth, brain development, and muscle metabolism. Cellular effects of IGF-I are mediated through the IGF-I receptor, a transmembrane tyrosine kinase that phosphorylates intracellular substrates, resulting in the activation of multiple intracellular signaling cascades. Dysregulation of IGF-I actions due to impairment in the postreceptor signaling machinery may contribute to multiple diseases in humans. This article will review current information on IGF-I signaling and illustrate recent results demonstrating how impaired IGF-I signaling and action may contribute to the pathogenesis of human diseases, including osteoporosis, neurodegenerative disorders, and reduced fetal growth in utero.

Single gene mutations causing SGA

The growth hormone-insulin-like growth factor-I (GH-IGF-I) axis plays a key role in intra-uterine growth and development. This review will describe the consequences of genetic defects in various components of the GH-IGF-I axis on intra-uterine growth and development. Animal knockout experiments have provided evidence for the GH-independent secretion of IGF-I and its effect in utero. Reports of patients with a deletion or mutation of the IGF-I and IGF1R genes have provided insight into the role of intra-uterine IGF-I in the human. Homozygous defects of the IGF-I gene have dramatic effects on intra-uterine growth and development, whereas heterozygous defects of the IGF1R gene have a more variable clinical presentation. The phenotype in relation to the genotype of the different disorders will be reviewed in this chapter.

Alanine scanning of a putative receptor binding surface of insulin-like growth factor-I

Current evidence supports a binding model in which the insulin molecule contains two binding surfaces, site 1 and site 2, which contact the two halves of the insulin receptor. The interaction of these two surfaces with the insulin receptor results in a high affinity cross-linking of the two receptor alpha subunits and leads to receptor activation. Evidence suggests that insulin-like growth factor-I (IGF-I) may activate the IGF-I receptor in a similar mode. So far IGF-I residues structurally corresponding to the residues of the insulin site 1 together with residues in the C-domain of IGF-I have been found to be important for binding of IGF-I to the IGF-I receptor (e.g. Phe(23), Tyr(24), Tyr(31), Arg(36), Arg(37), Val(44), Tyr(60), and Ala(62)). However, an IGF-I second binding surface similar to site 2 of insulin has not been identified yet. In this study, we have analyzed whether IGF-I residues corresponding to the six residues of the insulin site 2 have a role in high affinity binding of IGF-I to the IGF-I receptor. Six single-substituted IGF-I analogues were produced, each containing an alanine substitution in one of the following positions (corresponding insulin residues in parentheses): Glu(9) (His(B10)), Asp(12) (Glu(B13)), Phe(16) (Leu(B17)), Asp(53) (Ser(A12)), Leu(54) (Leu(A13)), and Glu(58) (Glu(A17)). In addition, two analogues with 2 and 3 combined alanine substitutions were also produced (E9A,D12A IGF-I and E9A,D12A,E58A IGF-I). The results show that introducing alanine in positions Glu(9), Asp(12), Phe(16), Leu(54), and Glu(58) results in a significant reduction in IGF-I receptor binding affinity, whereas alanine substitution at position 53 had no effect on IGF-I receptor binding. The multiple substitutions resulted in a 33-100-fold reduction in IGF-I receptor binding affinity. These data suggest that IGF-I, in addition to the C-domain, uses surfaces similar to those of insulin in contacting its cognate receptor, although the relative contribution of the side chains of homologous residues varies.

IGF2: epigenetic regulation and role in development and disease

Insulin-like growth factor II (IGF2) is perhaps the most intricately regulated of all growth factors characterized to date. Its gene is imprinted--only one allele is active, depending on parental origin--and this pattern of expression is maintained epigenetically in almost all tissues. IGF2 activity is further controlled through differential expression of receptors and IGF-binding proteins (IGFBPs) that determine protein availability. This complex and multifaceted regulation emphasizes the importance of accurate IGF2 expression and activity. This review will examine the regulation of the IGF2 gene and what it has revealed about the phenomenon of imprinting, which is frequently disrupted in cancer. IGF2 protein function will be discussed, along with diseases that involve IGF2 overexpression. Roles for IGF2 in sonic hedgehog (Shh) signaling and angiogenesis will also be explored.

An investigation of the ligand binding properties and negative cooperativity of soluble insulin-like growth factor receptors

To investigate the interaction of the insulin-like growth factor (IGF) ligands with the insulin-like growth factor type 1 receptor (IGF-1R), we have generated two soluble variants of the IGF-1R. We have recombinantly expressed the ectodomain of IGF-1R or fused this domain to the constant domain from the Fc fragment of mouse immunoglobulin. The ligand binding properties of these soluble IGF-1Rs for IGF-I and IGF-II were investigated using conventional ligand competition assays and BIAcore biosensor technology. In ligand competition assays, the soluble IGF-1Rs both bound IGF-I with similar affinities and a 5-fold lower affinity than that seen for the wild type receptor. In addition, both soluble receptors bound IGF-II with similar affinities to the wild type receptor. BIAcore analyses showed that both soluble IGF-1Rs exhibited similar ligand-specific association and dissociation rates for IGF-I and for IGF-II. The soluble IGF-1R proteins both exhibited negative cooperativity for IGF-I, IGF-II, and the 24-60 antibody, which binds to the IGF-1R cysteine-rich domain. We conclude that the addition of the self-associating Fc domain to the IGF-1R ectodomain does not affect ligand binding affinity, which is in contrast to the soluble ectodomain of the IR. This study highlights some significant differences in ligand binding modes between the IGF-1R and the insulin receptor, which may ultimately contribute to the different biological activities conferred by the two receptors.

Structural basis for the lower affinity of the insulin-like growth factors for the insulin receptor

Insulin and the insulin-like growth factors (IGFs) bind with high affinity to their cognate receptor and with lower affinity to the noncognate receptor. The major structural difference between insulin and the IGFs is that the IGFs are single chain polypeptides containing A-, B-, C-, and D-domains, whereas the insulin molecule contains separate A- and B-chains. The C-domain of IGF-I is critical for high affinity binding to the insulin-like growth factor I receptor, and lack of a C-domain largely explains the low affinity of insulin for the insulin-like growth factor I receptor. It is less clear why the IGFs have lower affinity for the insulin receptor. In this study, 24 insulin analogues and four IGF analogues were expressed and analyzed to explore the role of amino acid differences in the A- and B-domains between insulin and the IGFs in binding affinity for the insulin receptor. Using the information obtained from single substituted analogues, four multiple substituted analogues were produced. A "quadruple insulin" analogue ([Phe(A8), Ser(A10), Thr(B5), Gln(B16)]Ins) showed affinity as IGF-I for the insulin receptor, and a "sextuple insulin" analogue ([Phe(A8), Ser(A10), Thr(A18), Thr(B5), Thr(B14), Gln(B16)]Ins) showed an affinity close to that of IGF-II for the insulin receptor, whereas a "quadruple IGF-I" analogue ([His(4), Tyr(15), Thr(49), Ile(51)]IGF-I) and a "sextuple IGF-II" analogue ([His(7), Ala(16), Tyr(18), Thr(48), Ile(50), Asn(58)]IGF-II) showed affinities similar to that of insulin for the insulin receptor. The mitogenic potency of these analogues correlated well with the binding properties. Thus, a small number of A- and B-domain substitutions that map to the IGF surface equivalent to the classical binding surface of insulin weaken two hotspots that bind to the insulin receptor site 1.

Cooperativity of the N- and C-terminal domains of insulin-like growth factor (IGF) binding protein 2 in IGF binding

A family of six insulin-like growth factor (IGF) binding proteins (IGFBP-1-6) binds IGF-I and IGF-II with high affinity and thus regulates their bioavailability and biological functions. IGFBPs consist of N- and C-terminal domains, which are highly conserved and cysteine-rich, joined by a variable linker domain. The role of the C-domain in IGF binding is not completely understood in that C-domain fragments have very low or even undetectable IGF binding affinity, but loss of the C-domain dramatically disrupts IGF binding by IGFBPs. We recently reported the solution structure and backbone dynamics of the C-domain of IGFBP-2 (C-BP-2) and identified a pH-dependent heparin binding site [Kuang, Z., Yao, S., Keizer, D. W., Wang, C. C., Bach, L. A., Forbes, B. E., Wallace, J. C., and Norton, R. S. (2006) Structure, dynamics and heparin binding of the C-terminal domain of insulin-like growth factor-binding protein-2 (IGFBP-2), J. Mol. Biol. 364, 690-704]. Here, we have analyzed the molecular interactions among the N-domain of IGFBP-2 (N-BP-2), C-BP-2, and IGFs using cross-linking and nuclear magnetic resonance (NMR) spectroscopy. The binding of C-BP-2 to the IGF-I.N-BP-2 binary complex was significantly stronger than the binding of C-BP-2 to IGF-I alone, switching from intermediate exchange to slow exchange on the NMR time scale. A conformational change or stabilization of the IGF-I Phe49-Leu54 region and the Phe49 aromatic ring upon binding to the N-domains, as well as an interdomain interaction between N-BP-2 and C-BP-2 (which is also detectable in the absence of ligand), may contribute to this cooperativity in IGF binding. Glycosaminoglycan binding by IGFBPs can affect their IGF binding although the effects appear to differ among different IGFBPs; here, we found that heparin bound to the IGF-I.N-BP-2.C-BP-2 ternary complex, but did not cause it to dissociate.

A Novel Binding Site for the Human Insulin-like Growth Factor-II (IGF-II)/Mannose 6-Phosphate Receptor on IGF-II

The mammalian insulin-like growth factor (IGF)-II/cation-independent mannose 6-phosphate receptor (IGF2R) binds IGF-II with high affinity. By targeting IGF-II to lysosomal degradation, it plays a role in the maintenance of correct IGF-II levels in the circulation and in target tissues. Loss of IGF2R function is associated with tumor progression; therefore, the IGF2R is often referred to as a tumor suppressor. The interaction between IGF2R and IGF-II involves domains 11 and 13 of the 15 extracellular domains of the receptor. Recently, a hydrophobic binding region was identified on domain 11 of the IGF2R. In contrast, relatively little is known about the residues of IGF-II that are involved in IGF2R binding and the determinants of IGF2R specificity for IGF-II over the structurally related IGF-I. Using a series of novel IGF-II analogues and surface plasmon resonance assays, this study revealed a novel binding surface on IGF-II critical for IGF2R binding. The hydrophobic residues Phe(19) and Leu(53) are critical for IGF2R binding, as are residues Thr(16) and Asp(52). Furthermore, Thr(16) was identified as playing a major role in determining why IGF-II, but not IGF-I, binds with high affinity to the IGF2R.

Insulin-like growth factor signaling regulates zebrafish embryonic growth and development by promoting cell survival and cell cycle progression

Although much is known about the global effects of insulin-like growth factor 1 receptor (IGF1R)-mediated signaling on fetal growth and the clinical manifestations resulting from IGF/IGF1R deficiencies, we have an incomplete understanding of the cellular actions of this essential pathway during vertebrate embryogenesis. In this study, we inhibited IGF1R signaling during zebrafish embryogenesis using antisense morpholino oligonucleotides or a dominant-negative IGF1R fusion protein. IGF1R inhibition resulted in reduced embryonic growth, arrested development and increased lethality. IGF1R-deficient embryos had significant defects in the retina, inner ear, motoneurons and heart. No patterning abnormalities, however, were found in the brain or other embryonic tissues. At the cellular level, IGF1R inhibition increased caspase 3 activity and induced neuronal apoptosis. Coinjection of antiapoptotic bcl2-like mRNA attenuated the elevated apoptosis and rescued the retinal and motoneuron defects, but not the developmental arrest. Subsequent cell cycle analysis indicated an increased percentage of cells in G1 and a decreased percentage in S phase in IGF1R-deficient embryos independent of apoptosis. These results provide novel insight into the cellular basis of IGF1R function and show that IGF1R signaling does not function as an anteriorizing signal but regulates embryonic growth and development by promoting cell survival and cell cycle progression.

The N-terminal subdomain of insulin-like growth factor (IGF) binding protein 6. Structure and interaction with IGFs

Insulin-like growth factor binding proteins (IGFBPs) modulate the activity and distribution of insulin-like growth factors (IGFs). IGFBP-6 differs from other IGFBPs in being a relatively specific inhibitor of IGF-II actions. Another distinctive feature of IGFBP-6 is its unique N-terminal disulfide linkages; the N-domains of IGFBPs 1-5 contain six disulfides and share a conserved GCGCC motif, but IGFBP-6 lacks the two adjacent cysteines in this motif, so its first three N-terminal disulfide linkages differ from those of the other IGFBPs. The contributions of the N- and C-domains of IGFBP-6 to its IGF binding properties and their structure-function relationships have been characterized in part, but the structure and function of the distinctive N-terminal subdomain of IGFBP-6 are unknown. Here we report the solution structure of a polypeptide corresponding to residues 1-45 of the N-terminal subdomain of IGFBP-6 (NN-BP-6). The extended structure of the N-terminal subdomain of IGFBP-6 is very different from that of the short two-stranded beta-sheet of the N-terminal subdomain of IGFBP-4 and, by implication, the other IGFBPs. NN-BP-6 contains a potential cation-binding motif; lanthanide ion binding was observed, but no significant interaction was found with physiologically relevant metal ions like calcium or magnesium. However, this subdomain of IGFBP-6 has a higher affinity for IGF-II than IGF-I, suggesting that it may contribute to the marked IGF-II binding preference of IGFBP-6. The extended structure and flexibility of this subdomain of IGFBP-6 could play a role in enhancing the rate of ligand association and thereby be significant in IGF recognition.

Clinical and biochemical characteristics of a male patient with a novel homozygous STAT5b mutation

CONTEXT

GH insensitivity can be caused by defects in the GH receptor (GHR) or in the postreceptor signaling pathway. Recently, two female patients with severe growth retardation and pulmonary and immunological problems were described with a defect in STAT5b, a critical intermediary of downstream GHR signaling.

OBJECTIVE

The objective was to determine the functional characteristics of a novel STAT5b mutation and describe the phenotype.

PATIENT

We describe an adult male patient with short stature [-5.9 sd score (SDS)], delayed puberty, and no history of pulmonary or immunological problems. GH-binding protein level as well as GH secretion characteristics were normal. Plasma prolactin level was elevated. Extremely low levels of IGF-I (-6.9 SDS), IGF-binding protein-3 (-12 SDS), and acid-labile subunit (-7.5 SDS) were found.

RESULTS

We found a homozygous frameshift mutation in the STAT5b gene (nucleotide 1102-3insC, Q368fsX376), resulting in an inactive truncated protein, lacking most of the DNA binding domain and the SH2-domain.

CONCLUSIONS

This report confirms the essential role of STAT5b in GH signaling in the human. We show for the first time that immunological or pulmonary problems or elevated GH secretion are not obligatory signs of STAT5b deficiency, whereas hyperprolactinemia appears to be part of the syndrome. Therefore, in patients with severe short stature, signs of GH insensitivity, and a normal GHR, analysis of the STAT5b gene is recommended.

A variable degree of intrauterine and postnatal growth retardation in a family with a missense mutation in the insulin-like growth factor I receptor

CONTEXT

The type 1 IGF-I receptor (IGF1R) mediates the biological functions of IGF-I. Binding of IGF-I to the IGF1R results in autophosphorylation of the intracellular beta-subunit and activation of intracellular signaling.

OBJECTIVE

The objective of this study was to evaluate the functional characteristics of a novel IGF1R mutation and describe the phenotypic features of two patients with this mutation.

DESIGN

The study was performed in a university hospital.

PATIENTS

We describe a 35-yr-old female with mild intrauterine growth failure, progressive postnatal growth retardation, severe failure to thrive, and microcephaly. Her daughter was born with severe intrauterine growth retardation and also showed postnatal failure to thrive and microcephaly.

RESULTS

We found a heterozygous G3148-->A nucleotide substitution in the IGF1R gene, changing a negatively charged glutamic acid at position 1050 into a positively charged lysine residue (E1050K). E1050 is a conserved residue in the intracellular kinase domain. Dermal fibroblasts of the mother showed normal binding of iodinated IGF-I, but autophosphorylation and activation of downstream signaling cascades upon challenging with IGF-I was markedly reduced. Consequently, the maximal [(3)H]thymidine incorporation upon challenge with a dose range of IGF-I was reduced compared with a panel of control cells (3.65 +/- 1.79-fold vs. 6.75 +/- 4.7-fold stimulation; P < 0.01). These data suggest that the mutation results in the inactivation of one copy of the IGF1R gene.

CONCLUSIONS

These two patients support the key role for IGF-I in intrauterine and postnatal growth. The different phenotypes of these and earlier described patients may be associated with variability in IGF-I signaling. The degree of intrauterine growth retardation may be partially determined by the presence or absence of maternal IGF-I resistance.

Gene duplication and functional divergence of the zebrafish insulin-like growth factor 1 receptors

Insulin-like growth factor (IGF) 1 receptor (IGF1R)-mediated signaling plays key roles in growth, development, and physiology. Recent studies have shown that there are two distinct ig f1r genes in zebrafish, termed ig f1ra and ig f1rb. In this study, we tested the hypothesis that zebrafish ig f1ra and ig f1rb resulted from a gene duplication event at the ig f1r locus and that this has led to their functional divergence. The genomic structures of zebrafish ig f1ra and ig f1rb were determined and their loci mapped. While zebrafish ig f1ra has 21 exons and is located on linkage group (LG) 18, zebrafish ig f1rb has 22 exons and mapped to LG 7. There is a strong syntenic relationship between the two zebrafish genes and the human IG F1R gene. Using a MO-based loss-of-function approach, we show that both Igf1ra and Igf1rb are required for zebrafish embryo viability and proper growth and development. Although Igf1ra and Igf1rb demonstrated a large degree of functional overlap with regard to cell differentiation in the developing eye, inner ear, heart, and muscle, they also exhibited functional distinction involving a greater requirement for Igf1rb in spontaneous muscle contractility. These findings suggest that the duplicated zebrafish ig f1r genes play largely overlapping but not identical functional roles in early development and provide novel insight into the functional evolution of the IGF1R/insulin receptor gene family.

Effect of IGF-I on DNA, RNA, and protein loss associated with brain atrophy and impaired learning in diabetic rats

Brain atrophy in diabetic dementia (DD) may be due to a catabolic state with DNA loss. Insulin-like growth factor (IGF) levels are reduced in diabetes, and IGF replacement may ameliorate brain protein loss. Subcutaneous minipumps released vehicle (Db + Veh) or IGF-I (Db + IGF-I) in diabetic rats. Brain wet, dry, and water weights as well as DNA, rRNA, poly(A)+ RNA, and protein contents per brain were significantly reduced in diabetic rats. In the remaining brain cells, there was a significant decline in ratios of (rRNA/DNA) and (protein/DNA). IGF-I administration partially prevented the loss of brain protein content independently of hyperglycemia (P < 0.03). This is the first demonstration of a severe disturbance in the brain protein regulatory pathway together with DNA loss in diabetes. Because Alzheimer's Disease (AD) is associated with a diabetes-like brain environment, a catabolic state may contribute to brain atrophy in sporadic AD as well as DD.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.

Molecular interactions of the IGF system

The insulin-like growth factor (IGF) system is a complex network of two soluble ligands; several cell surface transmembrane receptors and six soluble high-affinity binding-proteins. The IGF system is essential for normal embryonic and postnatal growth, and plays an important role in the function of a healthy immune system, lymphopoiesis, myogenesis and bone growth among other physiological functions. Deregulation of the IGF system leads to stimulation of cancer cell growth and survival. In order to manipulate the IGF system in the treatment of certain disorders, we must understand the protein-protein interactions at a molecular level. The complex molecular interactions of the ligands and receptors of the IGF system underlie all the biological actions mentioned above and will be the focus of this review.

Diabetes-Associated Mutations in Human Insulin: Crystal Structure and Photo-Cross-Linking Studies of A-Chain Variant InsulinWakayama†,‡

Naturally occurring mutations in insulin associated with diabetes mellitus identify critical determinants of its biological activity. Here, we describe the crystal structure of insulin Wakayama, a clinical variant in which a conserved valine in the A chain (residue A3) is substituted by leucine. The substitution occurs within a crevice adjoining the classical receptor-binding surface and impairs receptor binding by 500-fold, an unusually severe decrement among mutant insulins. To resolve whether such decreased activity is directly or indirectly mediated by the variant side chain, we have determined the crystal structure of Leu(A3)-insulin and investigated the photo-cross-linking properties of an A3 analogue containing p-azidophenylalanine. The structure, characterized in a novel crystal form as an R(6) zinc hexamer at 2.3 A resolution, is essentially identical to that of the wild-type R(6) hexamer. The variant side chain remains buried in a nativelike crevice with small adjustments in surrounding side chains. The corresponding photoactivatable analogue, although of low affinity, exhibits efficient cross-linking to the insulin receptor. The site of photo-cross-linking lies within a 14 kDa C-terminal domain of the alpha-subunit. This domain, unrelated in sequence to the major insulin-binding region in the N-terminal L1 beta-helix, is also contacted by photoactivatable probes at positions A8 and B25. Packing of Val(A3) at this interface may require a conformational change in the B chain to expose the A3-related crevice. The structure of insulin Wakayama thus evokes the reasoning of Sherlock Holmes in "the curious incident of the dog in the night": the apparent absence of structural perturbations (like the dog that did not bark) provides a critical clue to the function of a hidden receptor-binding surface.