Use of transgenic mice for understanding the physiology of insulin-like growth factors

IGF1R Deficiency Modulates Brain Signaling Pathways and Disturbs Mitochondria and Redox Homeostasis

Insulin-like growth factor 1 receptor (IGF1R)-mediated signaling pathways modulate important neurophysiological aspects in the central nervous system, including neurogenesis, synaptic plasticity and complex cognitive functions. In the present study, we intended to characterize the impact of IGF1R deficiency in the brain, focusing on PI3K/Akt and MAPK/ERK1/2 signaling pathways and mitochondria-related parameters. For this purpose, we used 13-week-old UBC-CreERT2; Igf1r^fl/fl male mice in which Igf1r was conditionally deleted. IGF1R deficiency caused a decrease in brain weight as well as the activation of the IR/PI3K/Akt and inhibition of the MAPK/ERK1/2/CREB signaling pathways. Despite no alterations in the activity of caspases 3 and 9, a significant alteration in phosphorylated GSK3β and an increase in phosphorylated Tau protein levels were observed. In addition, significant disturbances in mitochondrial dynamics and content and altered activity of the mitochondrial respiratory chain complexes were noticed. An increase in oxidative stress, characterized by decreased nuclear factor E2-related factor 2 (NRF2) protein levels and aconitase activity and increased H₂O₂ levels were also found in the brain of IGF1R-deficient mice. Overall, our observations confirm the complexity of IGF1R in mediating brain signaling responses and suggest that its deficiency negatively impacts brain cells homeostasis and survival by affecting mitochondria and redox homeostasis.

Ethanol-Induced White Matter Atrophy Is Associated with Impaired Expression of Aspartyl-Asparaginyl-β-Hydroxylase (ASPH) and Notch Signaling in an Experimental Rat Model

Alcohol-induced white matter (WM) degeneration is linked to cognitive-motor deficits and impairs insulin/insulin-like growth factor (IGF) and Notch networks regulating oligodendrocyte function. Ethanol downregulates Aspartyl-Asparaginyl-β-Hydroxylase (ASPH) which drives Notch. These experiments determined if alcohol-related WM degeneration was linked to inhibition of ASPH and Notch. Adult Long Evans rats were fed for 3, 6 or 8 weeks with liquid diets containing 26% ethanol (caloric) and in the last two weeks prior to each endpoint they were binged with 2 g/kg ethanol, 3×/week. Controls were studied in parallel. Histological sections of the frontal lobe and cerebellar vermis were used for image analysis. Frontal WM proteins were used for Western blotting and duplex ELISAs. The ethanol exposures caused progressive reductions in frontal and cerebellar WM. Ethanol-mediated frontal WM atrophy was associated with reduced expression of ASPH, Jagged 1, HES-1, and HIF-1α. These findings link ethanol-induced WM atrophy to inhibition of ASPH expression and signaling through Notch networks, including HIF-1α.

IGF-1-induced enhancement of PRNP expression depends on the negative regulation of transcription factor FOXO3a

The conformational conversion of the cellular prion protein (PrP^C) into its β-sheet-rich scrapie isoform (PrP^Sc) causes fatal prion diseases, which are also called transmissible spongiform encephalopathies (TSEs). Recent studies suggest that the expression of PrP^C by the PRNP gene is crucial for the development of TSEs. Therefore, the identification of the exogenous and endogenous stimulating factors that regulate PRNP expression would help to understand the pathogenesis of TSEs. Here, we demonstrate that forkhead box O3a (FOXO3a) negatively regulates PRNP expression by binding to the PRNP promoter, which is negatively regulated by insulin-like growth factor 1 (IGF-1). Our results show that the IGF-1-induced enhancement of PRNP mRNA and protein levels is due to the activation of the PI3K-Akt signaling pathway. The activation of Akt then induces the phosphorylation of FOXO3a, leading to its translocation from the nucleus to the cytoplasm and preventing its binding to the PRNP promoter. Treatment with the PI3K-Akt inhibitor LY294002 induces the nuclear retention of FOXO3a, which leads to a decrease in PRNP expression. We present a new IGF-1-PI3K-Akt-FOXO3a pathway, which influences PRNP expression. The results of this work are vital for understanding the function of PrP^C and for future therapeutic approaches to human TSEs.

Neonatal repetitive maternal separation causes long-lasting alterations in various neurotrophic factor expression in the cerebral cortex of rats

AIMS

This study was carried out to examine the effects of early postnatal maternal separation stress on the development of the cerebral cortex with respect to time-dependent fluctuations of neurotrophic factor ligand and receptor expression.

MAIN METHODS

Wistar rats were separated from their mothers for 3h per day during postnatal days (PND) 10 to 15. The cerebral cortex was analyzed by real-time RT-PCR for the evaluation of the expression of mRNA for brain-derived neurotrophic factor (BDNF), TrkB, insulin-like growth factor-1 (IGF-1), and type 1 IGF receptor (IGF-1R) on PND16, 20, 30, and 60.

KEY FINDINGS

The expression of these neurotrophic factor ligands and receptors in the cerebral cortex was enhanced on PND16 and PND20, and then it returned to baseline levels on PND30. By PND60, however, the expression levels were attenuated.

SIGNIFICANCE

The important implication of this study is the persistent abnormal fluctuation of neurotrophic factor expression for a prolonged period, triggered even after the brain growth spurt. Given that neurotrophic factors play important roles in brain development, it can be speculated that the altered expression of these factors induced by maternal separation may interrupt normal brain development and ultimately lead to functional disruption. However, the possibility of such changes leading to various functional disruptions and the underlying mechanisms involved require further study.

Alternatively spliced transcripts of Sparus aurata insulin-like growth factor 1 are differentially expressed in adult tissues and during early development

Spliced variants of insulin-like growth factor 1 (IGF-1), a small peptide with a critical role in metabolism and growth, have been identified in various vertebrate species. However, despite recent functional data in mammalian systems suggesting specific roles (e.g. in muscle formation) for their pro-peptides and/or E domains, their function remains unclear. In this study, three alternatively spliced variants of Sparus aurata proIGF-1 (1a, 1b, and 1c) were identified and their expression analyzed. In adult fish, IGF-1 gene expression was observed in various soft tissues (highest levels in liver) and calcified tissues, with IGF-1c being always the most expressed isoform. In developing larvae, each isoform presented a specific pattern of expression, characterized by different onset and extent and consistent with a possible role of IGF-1a and 1b during early post-hatching events (e.g. bone or muscle formation), while IGF-1c would be rather involved in early larvae formation but probably acts in concerted action with other isoforms at later stages. We also propose that, in adults, IGF-1a and 1b isoforms may have a local action, while isoform 1c would assume a systemic action, as its mammalian counterpart. This hypothesis was further supported by in silico analysis of isoform distribution, revealing that only IGF-1c/Ea isoform has been conserved throughout evolution and that other fish isoforms (i.e. 1a and 1b) may be associated with mechanisms of osmoregulation. We finally propose that IGF-1 variants may exhibit different modes of action (systemic or local) and may be involved in different developmental and adaptive mechanisms.

The insulin-like growth factor system and the fetal brain: effects of poor maternal nutrition

The insulin-like growth factor (IGF) signaling system plays indispensable roles in pre- and post-natal brain growth and development. A large body of studies using both in vivo null mutant and transgenic mice and in vitro neuronal culture techniques indicate that IGF-I acts directly on the brain while IGF-II effects are mediated to a large extent by IGF-II control of placental growth. It appears that all of the mechanisms, except migration, that are involved in normal brain development, e.g., proliferation, apoptosis, maturation and differentiation, are influenced by IGF-I. While IGF system members are produced in the brain, recent reports in post-natal animals indicate that normal brain health and function are dependent upon transfer of circulating IGF-I from the liver and its transfer across the blood brain barrier. Data showing that this phenomenon applies to pre-natal brain growth and development would make an important contribution to fetal physiology. A number of kinase pathways are able to participate in IGF signaling in brain with respect to nutrient restriction; among the most important are the PI3K/AKT, Ras-Raf-MEK-ERK and mTOR-nutrient sensing pathways. Both maternal and fetal IGF-I peripheral plasma concentrations are greatly reduced in nutrient restriction while IGF-II does not appear to be affected. Nutrient restriction also affects IGF binding protein concentrations while effects on the IGF-I receptor appear to vary with the paradigm. Studies on the effects of nutrient restriction on the fetal primate brain in relation to activity of the IGF system are needed to determine the applicability of rodent studies to humans.

Insulin-like growth factor I treatment for cerebellar ataxia: Addressing a common pathway in the pathological cascade?

In the present work we review evidence supporting the use of insulin-like growth factor I (IGF-I) for treatment of cerebellar ataxia, a heterogeneous group of neurodegenerative diseases of low incidence but high societal impact. Most types of ataxia display not only motor discoordination, but also additional neurological problems including peripheral nerve dysfunctions. Therefore, a feasible therapy should combine different strategies aimed to correct the various disturbances specific for each type of ataxia. For cerebellar deficits, and most probably also for other types of brain deficits, the use of a wide-spectrum neuroprotective factor such as IGF-I may prove beneficial. Intriguingly, both ataxic animals as well as human patients show altered serum IGF-I levels. While the pathogenic significance of IGF-I, if any, in this varied group of diseases is difficult to envisage, disrupted IGF-I neuroprotective signaling may constitute a common stage in the pathological cascade associated to neuronal death. Indeed, treatment with IGF-I has proven effective in animal models of ataxia. Based on this pre-clinical evidence we propose that IGF-I should be tested in clinical trials of cerebellar ataxia in those cases where either serum IGF-I deficiency (as in primary cerebellar atrophy) or loss of sensitivity to IGF-I (as in ataxia telangiectasia) has been reported. Taking advantage of the widely protective and anabolic actions of IGF-I on peripheral tissues, this neurotrophic factor may provide additional therapeutic advantages for many of the disturbances commonly associated to ataxia such as cardiopathy, muscle wasting, or immune dysfunction.

STAT3 activation in response to growth factors or cytokines participates in retina precursor proliferation

Growth factors and cytokines play an important role in the development of central nervous systems including neurons of the retina. However, the molecular pathways that trigger cell growth remain unclear in neuronal precursors. In the present studies, we used a retinal explant culture system to investigate the response of signal transducer and activator of transcription factors (STATs) to extrinsic factors during mouse retinal development. Retinas from embryonic and neonatal stages showed that STAT3 but not STAT1 was activated in response to ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), fibroblast growth factor-1 (FGF1), fibroblast growth factor-2 (FGF2), epidermal growth factor (EGF), interferon-alpha (IFN-alpha) and interferon-gamma (IFN-gamma) in distinct patterns. STAT3 activation was detected in the outermost retina layer in response to CNTF, LIF, FGF1, and IFN-alpha 24 hr after stimulation in postnatal day 1 (PN1) explants, but not FGF2, EGF, IFN-gamma, and retinoic acid (RA). Cytokine stimulation increased the number of cells incorporating BrdU and the labelled cells co-localized with phosphorylated STAT3, indicating that STAT3 may play an essential role in coupling extrinsic factors to retina precursor cell (RPC) proliferation. Furthermore, persistent expression of two neural precursor markers, Hes1 and Otx2 was detected in outer retinal layers and correlated with STAT3 activation by CNTF, suggesting that STAT3 activation may play a critical role in stimulating mitotic precursors. These results strongly support a model that STAT3-mediated signalling regulates precursor populations during mouse retina development.

The impact of the GH-IGF-I axis on gonadotropin secretion: inferences from animal models

Given the tight, temporal coupling between growth and reproductive development, the idea that a common signal may regulate both adolescent growth and the initiation of puberty has been the focus of much research. Since the rate-limiting step for the onset of puberty is the appropriate hypothalamic secretion of gonadotropin-releasing hormone (GnRH), any factor important for the initiation of puberty must affect GnRH pulsatility. This review examines the hypothesis that GH and/or IGF-I are growth-related signals that regulate the release of GnRH, initiating puberty. By extension, this review also addresses the hypothesis that the GH axis also impacts GnRH and gonadotropin secretion in post-pubertal individuals and, thus, affects the maintenance of fertility in adults. The review examines data from a range of animal models employing a number of different strategies which directly manipulate the activity of either GH or IGF-I. The success of these strategies for producing the desired effects on the GH-IGF-I axis is somewhat variable. Although IGF-I may only play a permissive role in the maintenance of adult fertility, acting at the level of the gonad to increase sensitivity to gonadotropin stimulation, the data indicate that IGF-I is essential for reproductive maturation. However, in addition to its well-documented effects on the gonad, the specific mode of action of IGF-I on the neuroendocrine hypothalamus and GnRH pulsatility remains to be determined. Available evidence suggests that such action by IGF-I may be mediated through neurotransmitter effects on GnRH neurons, changing the availability of metabolic substrates for neuronal activity, or remodeling of synaptic input into GnRH neurons.

Tyrosine kinase and phosphatidylinositol 3-kinase activation are required for cyclic adenosine 3',5'-monophosphate-dependent potentiation of deoxyribonucleic acid synthesis induced by insulin-like growth factor-I in FRTL-5 cells

In previous studies, we showed that pretreatment of rat FRTL-5 thyroid cells with TSH, or other agents that increased intracellular cAMP, markedly potentiated DNA synthesis in response to insulin-like growth factor-I (IGF-I). In addition, we found that TSH pretreatment caused an increase in tyrosine phosphorylation of intracellular proteins including an unidentified 125-kDa protein that was well correlated with the TSH-potentiating effect on DNA synthesis induced by IGF-I. These results suggested that cAMP amplified IGF-I-dependent signals for cell growth through changes of cAMP-dependent tyrosine phosphorylation. The present studies were undertaken to determine how tyrosine kinase activation followed by an increase in tyrosine phosphorylation is required for cAMP-dependent potentiation of DNA synthesis induced by IGF-I in this cell line. First of all, we measured tyrosine kinase or protein-tyrosine phosphatase activities in the cell lysates by the in vitro assay. Chronic treatment with TSH or (Bu)2-cAMP stimulated tyrosine kinase activity in the particulate fraction and protein-tyrosine phosphatase activity in the soluble fraction, suggesting that tyrosine kinase plays more important roles for a cAMP-dependent increase in tyrosine phosphorylation of intracellular proteins. The increased tyrosine kinase activity was sensitive to genistein, a potent tyrosine kinase inhibitor. Genistein abolished both the cAMP-dependent increase in tyrosine phosphorylation of the 125-kDa protein and the enhanced DNA synthesis induced by IGF-I in a similar concentration-dependent manner. The only tyrosine-phosphorylated protein associated with the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase in response to cAMP was 125 kDa. In addition, we found that PI 3-kinase activity bound to p85 subunit significantly increased after (Bu)2cAMP treatment. These results suggested that cAMP stimulates PI 3-kinase through tyrosine phosphorylation of the 125-kDa protein. We then measured DNA synthesis in cells pretreated for 24 h with TSH or (Bu)2cAMP in the absence or presence of LY294002, a PI 3-kinase inhibitor, followed by treatment with IGF-I for 24 h. Presence of LY294002 during TSH or (Bu)2cAMP pretreatment completely abolished cAMP-dependent potentiation of DNA synthesis induced by IGF-I. These results suggest that in FRTL-5 cells cAMP activates genistein-sensitive tyrosine kinases that in turn activate PI 3-kinase activity. These mechanisms appear to be necessary for cAMP-dependent potentiation of the DNA synthesis induced by IGF-I.

Relationship of insulin-like growth factor-I, insulin-like growth factor binding protein-3, insulin, growth hormone in cord blood and maternal factors with birth height and birthweight

BACKGROUND

To determine whether the following factors are related to birthweight or birth height, we measured insulin-like growth factor (IGF)-I, insulin-like growth factor binding protein (IGFBP)-3, insulin and growth hormone (GH) levels in cord blood and also observed the relationship between birthweight, birth height and maternal factors.

METHODS

One hundred and ninety-four cord bloods were collected, 106 from males and 88 from females. Three newborns were small for gestational age (SGA), 168 were appropriate (AGA) and 23 were large (LGA); 21 newborns were preterm and 172 were term.

RESULTS

Levels of IGF-I and IGFBP-3, measured by enzyme-linked immunosorbent assay, were significantly lower in preterm babies (35.3 +/- 15.1 and 1025.6 +/- 562.8 ng/mL, respectively) than in term babies (61.6 +/- 39.5 and 1252.6 +/- 403.2 ng/mL, respectively; P < 0.01), but neither insulin nor GH levels, measured by radioimmunoassay, showed any significant difference between the two groups (P > 0.05). Among term babies, IGF-I and IGFBP-3 levels were significantly higher in the LGA group (96.1 +/- 34.1 and 1544.7 +/- 418.1 ng/mL, respectively) than in the AGA group (56.4 +/- 37.6 and 1212.8 +/- 383.4 ng/mL, respectively; P < 0.01). Levels of IGF-I and IGFBP-3 showed significant correlation with birthweight and length, respectively (P < 0.01), although GH and insulin levels did not (P > 0.05). There was a significant correlation between IGF-I and IGFBP-3 levels (P < 0.01, r = 0.64), but IGF-I and IGFBP-3 levels showed no relationship with GH or insulin levels. Birthweight correlated significantly with prepartum maternal weight, maternal weight gain and maternal height (P < 0.05), but birth length correlated significantly only with maternal height (P < 0.05).

CONCLUSIONS

Our results suggest that fetal growth depends on fetal levels of IGF-I and IGFBP-3 and maternal factors, not on insulin or GH. Levels of IGF-I and IGFBP-3 may not be regulated by insulin alone, but by the complex interactions between several factors, such as insulin, GH and maternal factors.

Insulin-Like growth factor I: implications in aging

According to the somatomedin model, growth hormone (GH)-dependent hepatic synthesis is responsible for maintaining circulating insulin-like growth factor (IGF)-I levels. On the other hand, the local autocrine/paracrine IGF-I expression in peripheral tissue is generally GH-independent and reflects the effects of various and tissue-specific trophic hormones. Circulating IGF-I levels undergo important age-related variations increasing at puberty and decreasing, thereafter, to low levels in the elderly. Low IGF-I levels in the elderly mainly reflect impaired somatotroph secretion but the decline in gonadal sex steroid levels, some protein and micronutrients malnutrition as well as age-dependent variations in IGF-binding proteins may also play a role in the age-related decrease in IGF-I activity. This, in turn, partially accounts for age-related changes in bones, muscles, cardiovascular system, central nervous system and the immune system. However, it is currently unclear whether treatment with exogenous IGF-I can retard or reverse age-related changes in body structure and function.

Synergistic roles of neuregulin-1 and insulin-like growth factor-I in activation of the phosphatidylinositol 3-kinase pathway and cardiac chamber morphogenesis

Cardiac chamber morphogenesis requires the coordinated growth of both cardiac muscle and endocardial cell lineages. Paracrine growth factors may modulate the coordinated cellular specification and differentiation during cardiac chamber morphogenesis, as suggested by the essential role of endothelial-derived growth factors, neuregulin-1, and insulin-like growth factor-I. Using the whole mouse embryo culture system for delivery of diffusible factors into the cardiac chamber, neuregulin-1 was shown to promote trabeculation of the ventricular wall. Another factor, insulin-like growth factor-I, had no apparent effect by itself. Combined treatment with neuregulin-1 and insulin-like growth factor-I strongly induced DNA synthesis of cardiomyocytes and expansion of both the ventricular compact zone and the atrioventricular cushions leading to chamber growth and maturation. In cultured cardiomyocytes, combined neuregulin-1 and insulin-like growth factor-I also had a synergistic effect to promote DNA synthesis and cellular growth, which were prevented by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. Adenoviral delivery of dominant negative Rac1, which acts downstream of phosphatidylinositol 3-kinase, blocked the effect of combined neuregulin-1/insulin-like growth factor-I treatment. These studies support the concept that the interaction of neuregulin-1 and insulin-like growth factor-I pathways plays an important role in coordinating cardiac chamber morphogenesis and may occur through convergent activation of phosphatidylinositol 3-kinase.

Postnatal growth responses to insulin-like growth factor I in insulin receptor substrate-1-deficient mice

Organ weight was compared in adult mice with deletion of one (IRS-1-/+) or both (IRS-1-/-) copies of the insulin receptor substrate-1 (IRS-1) gene and IRS-1+/+ littermates. IRS-1-/+ mice showed modest reductions in weight of most organs in proportion to a decrease in body weight. IRS-1-/- mice showed major reductions in weight of heart, liver, and spleen that were directly proportional to a decrease in body weight. In IRS-1-/- mice, kidney and particularly small intestine and brain exhibited proportionately smaller weight reductions, and gastrocnemius muscle showed a proportionately greater weight reduction than the decrease in body weight. Growth deficits in IRS-1-/- mice could reflect impaired actions of multiple hormones or cytokines that activate IRS-1. To assess the requirement for IRS-1 in insulin-like growth factor I (IGF-I)-dependent postnatal growth, IRS-1-/+ mice were cross-bred with mice that widely overexpress a human IGF-I transgene (IGF+) to generate IGF+ and wild-type mice on an IRS-1+/+, IRS-1-/+, and IRS-1-/- background. IGF-I overexpression increased body weight and weight of brain, small intestine, kidney, spleen, heart, and gastrocnemius muscle in IRS-1+/+ mice. IGF-I overexpression could not completely reverse the body growth retardation in IRS-1-/- mice. Absolute or partial IRS-1 deficiency impaired IGF-I-induced body overgrowth more in females than in males. In males and females, IGF-I stimulated similar overgrowth of brain regardless of IRS-1 status, and intestine and spleen showed dose dependence on IRS-1 for IGF-I-induced growth. IGF-I-induced growth of gastrocnemius muscle had an absolute requirement for IRS-1. IGF-I-induced growth of kidney and heart was impaired by IRS-1 deficiency only in females. In vivo, therefore, most organs do not require IRS-1 for IGF-I-induced growth and can use alternate signaling molecules to mediate IGF-I action. Other organs, such as gastrocnemius muscle, require IRS-1 for IGF-I-induced growth in vivo.

Cellular aspects of trophic actions in the nervous system

During the past three decades the number of molecules exhibiting trophic actions in the brain has increased drastically. These molecules promote and/or control proliferation, differentiation, migration, and survival (sometimes even the death) of their target cells. In this review a comprehensive overview of small diffusible factors showing trophic actions in the central nervous system (CNS) is given. The factors discussed are neurotrophins, epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, insulin-like growth factors, ciliary neurotrophic factor and related molecules, glial-derived growth factor and related molecules, transforming growth factor-beta and related molecules, neurotransmitters, and hormones. All factors are discussed with respect to their trophic actions, their expression patterns in the brain, and molecular aspects of their receptors and intracellular signaling pathways. It becomes evident that there does not exist "the" trophic factor in the CNS but rather a multitude of them interacting with each other in a complicated network of trophic actions forming and maintaining the adult nervous system.