Igf1 gene disruption results in reduced brain size, CNS hypomyelination, and loss of hippocampal granule and striatal parvalbumin-containing neurons

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.

Low insulin-like growth factor (IGF-1) in the cerebrospinal fluid of children with progressive encephalopathy, hypsarrhythmia, and optic atrophy (PEHO) syndrome and cerebellar degeneration

PURPOSE

In patients with progressive encephalopathy, hypsarrhythmia, and optic atrophy (PEHO) syndrome, the pathophysiology underlying early progressive cerebellar and brainstem degeneration and severe epilepsy is unknown. Because insulin-like growth factor (IGF)-1 has been shown significantly to promote survival of cerebellar neurons, we wanted to see if the IGF system played a role in the pathogenesis of cerebellar atrophy.

METHODS

We used a sensitive enzyme immunoassay kit for measuring cerebrospinal fluid (CSF) IGF-1 and insulin-like growth-binding protein (IGFBP)-3 in four groups of patients: PEHO syndrome patients (eight), PEHO-like patients (seven), age-matched controls (31), and patients with other types of cerebellar atrophy (11).

RESULTS

Patients with PEHO syndrome and those with other progressive, degenerative cerebellar diseases had lower levels of CSF IGF-1 than the controls with other neurologic diseases. The CSF IGF-1 also allowed us to differentiate the "true" PEHO patients from the "PEHO-like" patients (those with similar clinical symptoms but without the typical neuroophthalmologic or neuroradiologic findings). The concentrations of IGFBP-3 did not significantly differ in any of the patient or control groups studied.

CONCLUSIONS

CSF IGF-1 levels might be used as a marker of the degeneration of neurons in specific areas.

Insulin-like growth factor I is essential for postnatal growth in response to growth hormone

Insulin-like growth factor I (IGF-I) is essential for cell growth and intrauterine development while both IGF-I and GH are required for postnatal growth. To explore the possibility of direct GH action on body growth, independent of IGF-I production, we have studied the effects of GH in an IGF-I-deficient mouse line created by the Cre/loxP system. The IGF-I null mice are born with 35% growth retardation and show delayed onset of peripubertal growth, grow significantly slower, and do not attain puberty. Their adult body weight was approximately one third and body length about two thirds that of their wild-type litter mates. Injection of recombinant human GH (rhGH, 3 mg/kg, twice daily, sc) between postnatal day 14 (P14) to P56 failed to stimulate their growth as measured as both body weight and length. In contrast, wild-type mice receiving the same doses of rhGH exhibited accelerated growth starting at P21 that continued until P56, when their body weight was increased by 30% and length by 12% compared with control mice treated with diluent. Despite the lack of response in growth, IGF-I null mice have normal levels of GH receptor expression in the liver and increased liver Jun B expression and liver size in response to rhGH treatment. Our results support an essential role for IGF-I in GH-induced postnatal body growth in mice.

In vitro and in vivo responses to short-term recombinant human insulin-like growth factor-1 (IGF-I) in a severely growth-retarded girl with ring chromosome 15 and deletion of a single allele for the type 1 IGF receptor gene

OBJECTIVES

Patients with single allele defects in the gene encoding the type 1 IGF receptor have been reported to have growth failure, but fibroblasts from affected patients have not exhibited insensitivity to the effects of IGF-I in vitro. The in vitro and in vivo responses to short-term recombinant human IGF-I (rhIGF-I) in a severely growth-retarded girl with ring chromosome 15 and deletion of a single allele for the type 1 IGF receptor gene have been investigated.

DESIGN AND PATIENT

The child exhibited prenatal and severe post-natal growth failure, and delayed psychomotor development. Southern blotting revealed a 50% reduction in IGF-I receptor DNA, and in an RNase protection assay (RPA), a quantitatively similar reduction in steady-state mRNA for type 1 IGF receptor. rhIGF-I was administered in graded doses of 40, 60 and 80 microg/kg twice daily by subcutaneous injection for periods of 2-2.5 days each.

RESULTS

During rhIGF-I treatment, mean urinary nitrogen excretion was unchanged and urinary calcium rose to 60% greater than in the pre-treatment period. rhIGF-I injections produced only a modest decrease in indices of GH secretion, assessed by frequent (every 20 min) sampling over periods of 12 h. There was no significant difference between the mean GH concentrations during rhIGF-I treatment (5.32 +/- 6.2 mU/l) compared with that before rhIGF-I treatment (8.46 +/- 10.2 mU/l). Mean IGFBP-3-values were increased (4.5 mg/l before vs. 5.4 mg/l during rhIGF-I). TSH values after injection of TRH were not significantly reduced by IGF-I (mean of all values, 18.6 mU/l vs. 15.5 mU/l during rhIGF-I treatment). In vitro binding of radiolabelled IGF-I to the patient's fibroblasts was less than that bound by control fibroblasts (patient, 0.69% binding by 248 000 cells, vs. 1.41% binding by 260 000 fibroblasts from an age-matched control). However, the patient's fibroblasts exhibited a growth response in vitro to the addition of IGF-I in a fashion similar to that of control fibroblasts.

CONCLUSIONS

These studies show evidence in each of the indices examined of in vivo resistance to IGF-I and suggest that the growth retardation observed in such patients may be the direct result of the absence of one of the alleles encoding the type 1 IGF receptor.

Evidence that IGF-I acts as an autocrine/paracrine growth factor in the magnocellular neurosecretory system: neuronal synthesis and induction of axonal sprouting

The ability of mature oxytocinergic (OT) and vasopressinergic (VP) neurons of the magnocellular neurosecretory system (MNS) to undergo axonal growth implies that one or more growth factors may be active in the adult MNS, yet little is known regarding their possible identity. One such potential factor is insulin-like growth factor I (IGF-I). We have examined the expression of IGF-I mRNA and IGF-I-immunoreactivity (IGF-I-ir) in the mature MNS and have also determined the in vivo response of OT and VP neurons to hypothalamic implants of IGF-I. In situ hybridization revealed moderate labeling of IGF-I mRNA in both the supraoptic (SON) and the paraventricular (PVN) nuclei of adult male rats. RT-PCR analysis confirmed the presence of authentic IGF-I mRNA in extracts of the basal hypothalamus. Faint IGF-I-ir was detected in scattered magnocellular neurons within both the PVN and the SON of normal rats, but IGF-I-ir was much more intense and the majority of MNS neurons including those in the accessory nuclei were immunoreactive in sections from rats given colchicine, as were some parvocellular neurons in the PVN. Confocal microscopy revealed that IGF-I-ir was present in both OT and VP neurons, but VP neurons contained the most intense IGF-I-ir. Finally, a dramatic growth response of OT but not of VP fibers was observed following implantation of polymer rods containing IGF-I into the hypothalamus. A dense OT fiber plexus grew along the cannula track and OT fibers invaded the leptomeninges ventral to the SON and encircled the rostral cerebral artery. To our knowledge this is the first demonstration of axonal sprouting by mature OT neurons in response to an identified growth factor and the first direct demonstration of sprouting in response to exogenous IGF-I in the adult CNS. These findings suggest that IGF-I is synthesized and transported by adult MNS neurons where it may act as an autocrine and/or paracrine growth factor.

The megencephaly mouse has disturbances in the insulin-like growth factor (IGF) system

Megencephaly, enlarged brain, is a major sign in several human neurological diseases. The mouse model for megencephaly, mceph/mceph, has an enlarged brain and a lowered body weight. In addition, it displays several neurological and motoric disturbances. Previous studies suggest that the brain enlargement results from hypertrophy of the brain cells, rather than hyperplasia. No structural abnormalities, edema or increased myelination have been found. In this study, a major imbalance in the mRNA expression of molecules in the insulin-like growth factor (IGF) system was found in brains of 9-10 weeks old mceph/mceph mice compared to +/+ wild-type mice. In mceph/mceph brains, we found upregulation of IGF binding proteins (BP)-2, -4, -5, and -6 mRNA, the regulating hormone transforming growth factor (TGF)beta1 mRNA and also a local downregulation of IGFBP-5 mRNA compared to wild-type brains by in situ hybridization. The altered expression of these mRNA species is colocalized in cerebral cortex, hippocampus, amygdala and piriform/entorhinal cortex. The mceph/mceph mice express less of the myelin component proteolipid protein (PLP) mRNA in corpus callosum. No expression difference of the housekeeping gene glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in brain or IGF system components in liver was found between mceph/mceph and wild-type mice. These data suggest that the IGF system has an important role in the excessive growth of the mceph/mceph brains.

Molecular, morphometric and functional analyses demonstrate that the growth hormone deficient little mouse is not hypomyelinated

To study the effects of naturally occurring growth hormone deficiency type I on CNS myelination, we compared the myelination of brains from little and wild-type littermate mice using molecular, histological, morphometric, and functional analyses. The little mouse produces only 6-8% of normal levels of growth hormone (GH) and approximately 20% of normal circulating levels of insulin-like growth factor 1 (IGF-1). Our data show that the expression of myelin basic protein (MBP) and proteolipid protein (PLP) of the little brain exhibit the same temporal pattern and amount as that of the wild-type brain. Furthermore, the density and size of myelinated axons and the myelin sheath thickness in the corpus callosum, anterior commissure and the optic nerve are comparable in the little and wild-type brains. These regions are reduced in size in the little mouse brain proportionate to the overall reduction in brain size implying a reduction in the total number of neurons. Therefore, it follows that the total myelin content is reduced, but when normalized to brain size, the myelin concentration is unchanged. Myelin staining patterns of whole brains were identical. Moreover, functional analysis of the visual pathway indicated no difference between the little and control mice. These results are inconsistent with previous reports of hypomyelination in the little mouse and suggest that this form of GH deficiency does not adversely affect the myelination process except possibly through neuronal proliferation. However, since axon size and density are maintained, the neuronal growth may conversely be inherently limited by other restricted brain growth.

Apoptotic cell death and caspase-3 activation induced by N-methyl-D-aspartate receptor antagonists and their prevention by insulin-like growth factor I

The effect of N-methyl-D-aspartate (NMDA) receptor antagonists on cell viability was studied in rat primary cortical cells. NMDA antagonists [MK-801 and 2-amino-5-phosphonovalerate (APV)] induced cell shrinkage, nuclear condensation or fragmentation, and internucleosomal DNA fragmentation. Treatment of cells with MK-801 (an NMDA antagonist) for 1-2 days induced apoptotic cell death in a dose-dependent manner (1 nM to 10 microM). NMDA (25 microM), however, inhibited the MK-801 (0.1 microM)-induced apoptotic cell death. MK-801 and APV decreased the concentration of intracellular calcium ion. Activation of caspase-3 was accompanied by MK-801-induced cell death in a dose-dependent manner, and an inhibitor of caspase-3 reduced the cell death. Further, cycloheximide (0.2 microg/ml) completely protected the cells from MK-801-induced apoptotic cell death and caspase-3 activation. Insulin-like growth factor I completely attenuated MK-801-induced apoptotic cell death and caspase-3 activation. These results demonstrated that the moderate NMDA receptor activation is probably involved in the survival signal of the neuron.

In vivo actions of fibroblast growth factor-2 and insulin-like growth factor-I on oligodendrocyte development and myelination in the central nervous system

The in vivo effects of fibroblast growth factor-2 (FGF-2) and insulin-like growth factor-I (IGF-I) on oligodendrocytes and CNS myelination were determined in the postnatal rat anterior medullary velum (AMV) following injection of both cytokines into the cerebrospinal fluid. Either FGF-2, IGF-I, or saline were administered via the lateral ventricle, twice daily commencing at postnatal day (P) 6. At P9, AMV were immunohistochemically labeled with the Rip antibody, to enable analysis of the numbers of myelin sheaths and of promyelinating and myelinating oligodendrocytes; promyelinating oligodendrocytes are a recognisable immature phenotype which express myelin-related proteins prior to forming myelin sheaths. In parallel experiments, AMV were treated for Western blot analysis to determine relative changes in expression of the myelin proteins 2', 3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and myelin oligodendrocyte glycoprotein (MOG), which, respectively, characterise early and late stages of myelin maturation. In FGF-2-treated AMV, the number of promyelinating oligodendrocytes increased by 87% compared to saline-injected controls. The numbers of myelinating oligodendrocytes and myelin sheaths were not decreased, but conspicuous unmyelinated gaps within fibre tracts were indications of retarded myelination following FGF-2 treatment. Western blot analysis demonstrated decreased expression of CNP and a near-total loss of MOG, confirming that FGF-2 decreased myelin maturation. In contrast, IGF-I had no effect on the number of promyelinating oligodendrocytes, but increased the numbers of myelinating oligodendrocytes and myelin sheaths by 100% and 93%, respectively. Western blot analysis showed that the amount of CNP was increased following IGF-I treatment, correlating with the greater number of oligodendrocytes, but that MOG expression was lower than in controls, suggesting that the increased number of myelin sheaths in IGF-I was not matched by increased myelin maturation. The results provide in vivo evidence that FGF-2 and IGF-I control the numbers of oligodendrocytes in the brain and, respectively, retard and promote myelination.

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.

Identification of a potent neurotrophic substance for ciliary ganglionic neurons in fetal calf serum as insulin-like growth factor II

When fetal calf serum (FCS) alone is used as a trophic support for cultured chicken parasympathetic ciliary ganglionic (cCG) neurons, it does not show any survival-promoting effects on these neurons. When FCS is applied to heparin-affinity chromatography, however, potent survival-promoting activity is obtained in the fraction eluted with 0.5 M NaCl. Using cCG neurons as a bioassay system, this neurotrophic activity was purified by a combination of heparin-affinity chromatography, gel filtration chromatography, and Sep-Pak C18 cartridge. The 40-50-kDa fractions from the gel filtration column with strong survival-promoting activity were shown to contain insulin-like growth factor II (IGF-II) by immunoblot analysis. By acidification, the survival-promoting activity and IGF-II were translocated together from the 40-50-kDa to the 7-10-kDa fractions, and the survival-promoting activity in the 7-10-kDa fractions was blocked by an anti-IGF-II neutralizing monoclonal antibody. These results indicate that the neurotrophic substance in 0.5 M NaCl-eluate from heparin-affinity chromatography is IGF-II and that mechanisms may exist in vivo for the activation of latent IGF-II, whose biological effects may be blocked by its specific binding proteins.

Alterations in the neural growth hormone axis following hypoxic-ischemic brain injury

Recently, there has been considerable interest in determining the role of the growth hormone receptor (GHR) in the central nervous system (CNS). The aim of this study was to investigate the role of circulating growth hormone (GH) and the neural GHR after hypoxic-ischemic (HI) brain injury in the 21-day old rat. We observed growth hormone receptor/binding protein (GHR/BP) immunoreactivity to be rapidly upregulated following a severe unilateral HI injury. There was a biphasic increase with an initial rise occurring in blood vessels within a few hours after injury followed by a secondary rise evident by 3 days post-hypoxia in microglia/macrophages, some of which are destined to express insulin-like growth factor-I (IGF-I). There was also an increased immunoreactivity in reactive astrocytes, some of which were in the process of dividing. Subsequently, we attempted to activate the endothelial GHR/BP which was found to be increased after injury by treating with 15 microgram g-1 day-1 s.c. bGH for 7 days. Circulating concentrations of IGF-I fell after injury and were restored with GH treatment (P=0.001), whereas treatment of normal animals had no effect on serum IGF-I. Peripheral GH treatment increased the cerebrospinal fluid (CSF) concentration of immunoreactive IGF-I in the injured rats (P=0.017). GH treatment also reversed the systemic catabolism caused by the injury but had no significant neuroprotective effects. These results indicate that GH therapy can be used to reverse the systemic catabolism that occurs after CNS injury. In addition, these data suggest a role for the neural GHR during the recovery from brain injury, both in terms of the induction of IGF-I and in terms of glial proliferation.

Transforming growth factor-beta, but not ciliary neurotrophic factor, inhibits DNA synthesis of adrenal medullary cells in vitro

Transforming growth factor-betas are members of a superfamily of multifunctional cytokines regulating cell growth and differentiation. Their functions in neural and endocrine cells are not well understood. We show here that transforming growth factor-betas are synthesized, stored and released by the neuroendocrine chromaffin cells, which also express the transforming growth factor-beta receptor type II. In contrast to the developmentally related sympathetic neurons, chromaffin cells continue to proliferate throughout postnatal life. Using 5-bromo-2'-deoxyuridine pulse labeling and tyrosine hydroxylase immunocytochemistry as a marker for young postnatal rat chromaffin cells, we show that treatment with fibroblast growth factor-2 (1 nM) and insulin-like growth factor-II (10 nM) increased the fraction of 5-bromo-2'-deoxyuridine-labeled nuclei from 1% to about 40% of the cells in the absence of serum. In the presence of fibroblast growth factor-2 and insulin-like growth factor-II, transforming growth factor-beta1 (0.08 nM) reduced 5-bromo-2'-deoxyuridine labeling by about 50%, without interfering with chromaffin cell survival or death. Doses lower and higher than 0.08 nM were less effective. Similar effects were seen with transforming growth factor-beta3. In contrast to transforming growth factor-beta, ciliary neurotrophic factor, which inhibits proliferation of sympathetic progenitor cells, was not effective on rat chromaffin cells from postnatal day 6. Glucocorticoids also suppress DNA synthesis in fibroblast growth factor-2/insulin-like growth factor-II-treated chromaffin cells. This effect was not mediated by chromaffin cell-derived transforming growth factor-beta, as shown by addition of neutralizing antibodies. We conclude that one function of adrenal medullary transforming growth factor-beta may be to act as a negative regulator of chromaffin cell division.

A role for insulin-like growth factor-I in the regulation of Schwann cell survival

During postnatal development in the peripheral nerve, differentiating Schwann cells are susceptible to apoptotic death. Schwann cell apoptosis is regulated by axons and serves as one mechanism through which axon and Schwann cell numbers are correctly matched. This regulation is mediated in part by the provision of limiting axon-derived trophic molecules, although neuregulin-1 (NRG-1) is the only trophic factor shown to date to support Schwann cell survival. In this report, we identify insulin-like growth factor-I (IGF-I) as an additional trophin that can promote Schwann cell survival in vitro. We find that IGF-I, like NRG-1, can prevent the apoptotic death of postnatal rat Schwann cells cultured under conditions of serum withdrawal. Moreover, we show that differentiating Schwann cells in the rat sciatic nerve express both the IGF-I receptor (IGF-I R) and IGF-I throughout postnatal development. These results indicate that IGF-I is likely to control Schwann cell viability in the developing peripheral nerve and, together with other findings, raise the interesting possibility that such survival regulation may switch during postnatal development from an axon-dependent mechanism to an autocrine and/or paracrine one.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

IGF-I deficient mice show reduced peripheral nerve conduction velocities and decreased axonal diameters and respond to exogenous IGF-I treatment

Although insulin-like growth factor-I (IGF-I) can act as a neurotrophic factor for peripheral neurons in vitro and in vivo following injury, the role IGF-I plays during normal development and functioning of the peripheral nervous system is unclear. Here, we report that transgenic mice with reduced levels (two genotypes: heterozygous Igf1+/- or homozygous insertional mutant Igf1m/m) or totally lacking IGF-I (homozygous Igf1-/-) show a decrease in motor and sensory nerve conduction velocities in vivo. In addition, A-fiber responses in isolated peroneal nerves from Igf1+/- and Igf1-/- mice are impaired. The nerve function impairment is most profound in Igf1-/- mice. Histopathology of the peroneal nerves in Igf1-/- mice demonstrates a shift to smaller axonal diameters but maintains the same total number of myelinated fibers as Igf1+/+ mice. Comparisons of myelin thickness with axonal diameter indicate that there is no significant reduction in peripheral nerve myelination in IGF-I-deficient mice. In addition, in Igf1m/m mice with very low serum levels of IGF-I, replacement therapy with exogenous recombinant hIGF-I restores both motor and sensory nerve conduction velocities. These findings demonstrate not only that IGF-I serves an important role in the growth and development of the peripheral nervous system, but also that systemic IGF-I treatment can enhance nerve function in IGF-I-deficient adult mice.

The effects of the N-terminal tripeptide of insulin-like growth factor-1, glycine-proline-glutamate in different regions following hypoxic-ischemic brain injury in adult rats

Insulin-like growth factor-1 has pleiotropic effects in the central nervous system and can act both as a survival and a differentiation factor. Insulin-like growth factor-1 can be proteolytically cleaved into des-N-(1-3)-insulin-like growth factor-1 and a N-terminal tripeptide fragment, glycine-proline-glutamate. Both insulin-like growth factor-1 and des-N-(1-3)-insulin-like growth factor-1 can improve neuronal survival after hypoxic-ischemic brain injury in vivo. The present study investigates the effects of glycine-proline-glutamate on different brain regions and neuronal populations after hypoxic-ischemic injury. Unilateral hypoxic-ischemic injury was induced in adult rats. Glycine-proline-glutamate (3 microg) was administered centrally 2 h after the injury and the extent of brain damage determined five days later. In a separate trial immunohistochemical techniques were used to determine the effects of glycine-proline-glutamate on specific populations of neurons in the striatum after the injury. Compared to the vehicle treatment, glycine-proline-glutamate (n=19) treatment reduced the extent of cortical damage and neuronal loss in the CA1-2 subregions of the hippocampus (P<0.05). In the striatum, there was a trend towards a reduction in neuronal loss after glycine-proline-glutamate treatment (P=0.053) compared to the vehicle (n=21)-treated animals. In a separate study, glycine-proline-glutamate (n=8) treatment prevented the loss of choline acetyltransferase (P<0.05), glutamate acid decarboxylase (P<0.05) and somatostatin (P<0.05) containing neurons in the ipsilateral striatum following hypoxic-ischemic brain injury and also increased the numbers of neuronal nitric oxide synthase (P<0.05) containing neurons in the contralateral side. These studies suggest that in addition to neuroprotective effects, glycine-proline-glutamate can influence neuronal activity after hypoxic-ischemic injury.

Insulin-like growth factor-I (IGF-I) ameliorates and IGF binding protein-1 (IGFBP-1) exacerbates the effects of undernutrition on brain growth during early postnatal life: studies in IGF-I and IGFBP-1 transgenic mice

Insulin-like growth factor-I (IGF-I) plays an important role in the stimulation of postnatal brain growth. In transgenic (Tg) mice, IGF-I overexpression stimulates postnatal brain growth, whereas decreased IGF-I availability caused by ectopic brain expression of IGF binding protein-1 [(IGFBP-1), an inhibitor of IGF-I action] retards postnatal brain growth. Because undernutrition during early postnatal development profoundly retards growth and maturation of the brain in rodents, we sought to determine the influence of IGF-I on undernutrition-induced brain growth retardation. Caloric restriction was imposed on IGF-I Tg mice, IGFBP-1 Tg mice, and their non-Tg littermates by separating half of each litter from their dams during the suckling period, postnatal d 1 to 21. Undernutrition reduced the brain growth of each group of mice, but the growth of undernourished IGF-I Tg mice was comparable to that of well-fed control mice (increased 4.13- and 4.22-fold, respectively) and greater than that of undernourished control mice (increased 3.45-fold), whereas undernourished IGFBP-1 Tg mice exhibited less growth (increased 3.15-fold) than undernourished control mice. When the effects of undernutrition were examined in specific brain regions of each group, the same pattern was observed, and IGF-I was found to be more effective in preserving the growth of the regions with the highest transgene expression (cerebral cortex, hippocampus, and diencephalon). Despite undernutrition, IGF-I transgene expression stimulated overgrowth of these regions as well as that of the posterior medial barrel subfield, a somatosensory area of the cerebral cortex in which IGF-I may be especially important in development. These data indicate that IGF-I can ameliorate the brain growth retardation caused by undernutrition imposed during development, although it is unclear whether IGF-I directly opposes the impact of undernutrition or acts independently of nutritional status. Nonetheless, these findings raise the possibility that the relatively high IGF-I expression during early postnatal life may be responsible for sparing the brain from the full impact of undernutrition during this time in development.

Overexpression of insulin-like growth factor-binding protein-2 in C6 glioma cells results in conditional alteration of cellular growth

To examine the relationship between the expression of insulin-like growth factor (IGF)-binding protein-2 (IGFBP-2) and cell growth in a cell type with a defined IGF/IGFBP system, an ovine IGFBP-2 complementary DNA was overexpressed in C6 glioma cells. C6 cells produce IGFBP-3, IGFBP-4, a negligible amount of IGFBP-2, and IGF-I. An ovine IGFBP-2 complementary DNA was transfected into C6 cells, and nine colonies that stably expressed variable levels of IGFBP-2 messenger RNA were selected. Synthesis of corresponding levels of IGFBP-2 was confirmed by ligand blot and immunoblot analyses of conditioned media. Three clones exhibited significantly reduced growth rates, and the remainder showed growth rates similar to those of the wild-type C6 cells. The clones, which overexpressed high levels of IGFBP-2 and IGF-I, had growth rates similar to the wild-type cells, whereas the three clones that overexpressed IGFBP-2 without a concomitant increase in IGF-I had reduced growth rates. In addition, a cell-associated IGFBP was identified in the slow growing clones, but not in the wild-type or the fast growing clones. This cell-associated IGFBP was deduced to be IGFBP-5 based on its molecular size, detection of IGFBP-5 messenger RNA only in slow growing clones, and competition of its binding by heparin. Growth of the slow growing clone, C6BP2-1, could not be overcome by the addition of exogenous IGF-I, suggesting that the cell-associated IGFBP-5 was the dominant regulator of IGF action. These observations suggested that 1) in C6 glioma cells cellular growth is altered by a disturbance in the equilibrium between IGF-I and IGFBPs and/or the functional properties of the IGFBPs; and 2) C6 cells may have a limited capacity to modulate IGF/IGFBP expression in response to changes in endogenous expression of IGFBPs. Endogenous regulation of the balance between IGFs and IGFBPs may be a model of regulation of cellular growth in tumor cells.

Phasic synaptic remodeling of the rat arcuate nucleus during the estrous cycle depends on insulin-like growth factor-I receptor activation

Insulin-like growth factor-I (IGF-I) has trophic and plastic effects on neurons and glial cells and modulates neuroendocrine events by acting at the level of the hypothalamus. IGF-I and estrogen signaling interact to regulate in vitro hypothalamic neuronal survival and differentiation. In vivo, IGF-I levels fluctuate in the rat hypothalamic arcuate nucleus during the estrous cycle in parallel with a phasic remodeling of synaptic contacts and glial cell processes. Both the fluctuation of IGF-I levels and the synaptic and glial changes are induced by estrogen. The possible role of IGF-I in the regulation of arcuate nucleus synaptic plasticity has been assessed in the present study by intracerebroventricular administration to cycling female rats of a specific IGF-I receptor antagonist. In agreement with previous findings, the number of synaptic inputs to arcuate neuronal somas in control rats showed a significant decrease between the morning of proestrus and the morning of estrus. This decline in synaptic inputs and the accompanying increase in glial ensheathing of neuronal somas were blocked by the IGF-I receptor antagonist. In contrast, the IGF-I receptor antagonist did not affect the basal number of synapses or the morphology of synaptic terminals or length of the synaptic contacts. These findings indicate that IGF-I receptor activation may be involved in the phasic remodeling of arcuate nucleus synapses during the estrous cycle. Res.

Alterations in insulin-like growth factor-1 gene and protein expression and type 1 insulin-like growth factor receptors in the brains of ageing rats

Ageing in mammals is characterized by a decline in plasma levels of insulin-like growth factor-1 that appears to contribute to both structural and functional changes in a number of tissues. Although insulin-like growth factor-1 has been shown to provide trophic support for neurons and administration of insulin-like growth factor-1 to ageing animals reverses some aspects of brain ageing, age-related changes in insulin-like growth factor-1 or type 1 insulin-like growth factor receptors in brain have not been well documented. In this series of studies, insulin-like growth factor-1 messenger RNA and protein concentrations, and type 1 insulin-like growth factor receptor levels were analysed in young (three to four- and 10-12-month-old), middle-aged (19-20-month-old) and old (29-32-month-old) Fisher 344 x Brown Norway rats. Localization of insulin-like growth factor-1 messenger RNA throughout the lifespan revealed that expression was greatest in arteries, arterioles, and arteriolar anastomoses with greater than 80% of these vessels producing insulin-like growth factor-1 messenger RNA. High levels of expression were also noted in the meninges. No age-related changes were detected by either in situ hybridization or quantitative dot blot analysis of cortical tissue. However, analysis of insulin-like growth factor-1 protein levels in cortex analysed after saline perfusion indicated a 36.5% decrease between 11 and 32 months-of-age (P<0.05). Similarly, analysis of type 1 insulin-like growth factor receptor messenger RNA revealed no changes with age but levels of type 1 insulin-like growth factor receptors indicated a substantial decrease with age (31% in hippocampus and 20.8 and 27.3% in cortical layers II/III and V/VI, respectively). Our results indicate that (i) vasculature and meninges are an important source of insulin-like growth factor-1 for the brain and that expression continues throughout life, (ii) there are no changes in insulin-like growth factor-1 gene expression with age but insulin-like growth factor-1 protein levels decrease suggesting that translational deficiencies or deficits in the transport of insulin-like growth factor-1 through the blood-brain barrier contribute to the decline in brain insulin-like growth factor-1 with age, and (iii) type 1 insulin-like growth factor receptor messenger RNA is unchanged with age but type 1 insulin-like growth factor receptors decrease in several brain regions. We conclude that significant perturbations occur in the insulin-like growth factor-1 axis with age. Since other studies suggest that i.c.v. administration of insulin-like growth factor-1 reverses functional and cognitive deficiencies with age, alterations within the insulin-like growth factor-1 axis may be an important contributing factor in brain ageing.

Inhibition of glycogen synthase kinase 3beta activity regulates proliferation of cultured cerebellar granule cells

Insulin-like growth factor I (IGF-I) is mitogenic for several types of neuronal progenitors including cerebellar granule neuron progenitors. The present study confirms that IGF-I can function as a mitogen in purified cultures of cerebellar granule cells and identifies intracellular signal transduction molecules that mediate this mitogenesis. In cultured granule cells, IGF-I inhibits GSK-3 activity and leads to phosphorylation of serine9 an inhibitory site on GSK-3beta. Phosphoinositide 3-kinase (PI3-K) activation by IGF-I can lead to phosphorylation and inactivation of GSK-3. A PI3-K inhibitor, LY294002, completely inhibited IGF-I-induced proliferation with half-maximal inhibition occurring at a concentration (1.5 micrograms) close to its reported IC50 value for inhibition of PI3-K. Lithium chloride (LiCl), a direct inhibitor of GSK-3beta, can alone stimulate granule cell proliferation and enhance proliferation induced by IGF-I. LiCl can reverse the inhibitory effect of LY294002 on granule cell proliferation suggesting that GSK-3 inhibition may be downstream of PI3-K activation in IGF-I's mitogenic pathway. Experiments further show that the expression of a dominant active form of GSK-3beta antagonizes IGF-I-induced mitogenesis. These studies support a role for inhibition of GSK-3beta activity in the signal transduction pathway by which IGF-I regulates granule neuron progenitor proliferation.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Methylmercury antagonizes the survival-promoting activity of insulin-like growth factor on developing cerebellar granule neurons

Methylmercury (MeHg), a widely distributed environmental toxicant, has a profound effect on the developing central nervous system. Human exposure to MeHg in utero has led to severe neurological abnormalities in children, including cognitive and motor dysfunction. The abnormalities appear to result from death of neurons and altered cytoarchitecture in the developing CNS. Death of cerebellar granule neurons occurs following both adult and in utero exposure to MeHg, indicating the vulnerability of these cells to the toxic action of MeHg. The studies reported here use purified cultures of developing mouse cerebellar granule neurons to evaluate whether MeHg directly acts on these developing neurons to inhibit their survival. These experiments show that, in purified cultures of cerebellar granule neurons maintained in medium containing insulin-like growth factor I (IGF-I) as the only added trophic factor, low micromolar concentrations of MeHg inhibit granule neuron survival. The reduction in survival produced by MeHg can be partially reversed by increasing the concentration of IGF-I, suggesting an antagonism between MeHg and IGF-I. Inhibition of phosphoinositide 3-kinase (PI3-K), an intracellular mediator of IGF-I's survival promoting action, can synergistically enhance MeHg's effect on survival. Further studies indicate that MeHg's inhibition of survival involves apoptotic death of granule neurons. This apoptosis appears to require activation of gene transcription and may involve an increase in expression of the immediate early transcription factor c-Jun. These studies suggest that MeHg can act on developing granule neurons to increase the expression of c-Jun and antagonize IGF-I's survival promoting activity.

Insulin-like growth factor-1 ameliorates age-related behavioral deficits

Insulin-like growth factor-1 has been found to be involved in the regulation of several aspects of brain metabolism, neural transmission, neural growth and differentiation. Because decreased insulin-like growth factor-1 and/or its receptors are likely to contribute to age-related abnormalities in behavior, the strategy of replacing this protein is one potential therapeutic alternative. The present study was designed to assess whether cognitive deficits with ageing may be partially overcome by increasing the availability of insulin-like growth factor-1 in the brain. Fischer-344 x Brown Norway hybrid (F1) male rats of two ages (four-months-old and 32-months-old) were preoperatively trained in behavioral tasks and subsequently implanted with osmotic minipumps to infuse the insulin-like growth factor-1 (23.5 microg/pump) or a vehicle, i.c.v. Animals were retested at two weeks and four weeks after surgery. Insulin-like growth factor-1 improved working memory in the repeated acquisition task and in the object recognition task. An improvement was also observed in the place discrimination task, which assesses reference memory. Insulin-like growth factor-1 had no effect on sensorimotor skills nor exploration, but mildly reversed some age-related deficits in emotionality. These data indicate a potentially important role for insulin-like growth factor-1 in the reversal of age-related behavioral impairments in rodents.

Basic FGF and FGF receptor 1 are expressed in microglia during experimental autoimmune encephalomyelitis: temporally distinct expression of midkine and pleiotrophin

Heparin-binding growth factors have been implicated in central nervous system development, regeneration and pathology. To assess the expression pattern and possible function in multiple sclerosis, the heparin-binding growth factors pleiotrophin (PTN), midkine (MK), basic fibroblast growth factor (FGF-2) and one of its receptors (FGFR1/flg) mRNA and protein levels were examined in an experimental autoimmune encephalomyelitis (EAE) model in the Lewis rat. We assessed the time course of expression of PTN, MK and FGF-2 during EAE and determined the cellular origin of FGF-2 and FGFR1 in normal spinal cord and during inflammatory demyelination. Basal expression of PTN and MK mRNAs in normal spinal cords was significantly upregulated after induction of EAE. MK expression was upregulated two to threefold correlating with disease progression, whereas PTN expression reached peak levels threefold above basal levels during the clinical recovery period. FGF-2 mRNA expression was low in normal spinal cord and dramatically increased in correlation with progressive demyelination. FGF-2 was confined to neurons in normal tissue and shifted dramatically to microglia, paralleling their activation during EAE. Double immunohistochemistry revealed colocalization of FGF-2 to activated microglia/macrophages with strongest expression in the macrophage-rich perivascular core area and microglial expression at the edges of white and gray matter perivascular regions. FGFR1, like its ligand, was induced in activated macrophages/microglia. Growth factor expression in demyelinating diseases could serve several functions, e.g., to modulate the activity of microglia/macrophage in an autocrine fashion, to induce the expression of other factors like insulin-like growth factor 1 or plasminogen activator, which can effect regeneration or degeneration, respectively, and finally to stimulate directly localized proliferation and/or regeneration of oligodendrocytes within the lesion area.

not really finished is crucial for development of the zebrafish outer retina and encodes a transcription factor highly homologous to human Nuclear Respiratory Factor-1 and avian Initiation Binding Repressor

Not really finished (nrf), a larval-lethal mutation in zebrafish generated by retroviral insertion, causes specific retinal defects. Analysis of mutant retinae reveals an extensive loss of photoreceptors and their precursors around the onset of visual function. These neurons undergo apoptosis during differentiation, affecting all classes of photoreceptors, suggesting an essential function of nrf for the development of all types of photoreceptors. In the mutant, some photoreceptors escape cell death, are functional and, as judged by opsin expression, belong to at least three classes of cones and one class of rods. The protein encoded by nrf is a close homologue of human Nuclear Respiratory Factor 1 and avian Initiation Binding Repressor, transcriptional regulators binding the upstream consensus sequence RCGCRYGCGY. At 24 hours of development, prior to neuronal differentiation, nrf is expressed ubiquitously throughout the developing retina and central nervous system. At 48 hours of development, expression of nrf is detected in the ganglion cell layer, in the neurons of the inner nuclear layer, and in the optic nerve and optic tracts, and, at 72 hours of development, is no longer detectable by in situ hybridization. Mutants contain no detectable nrf mRNA and die within 2 weeks postfertilization as larvae with reduced brain size. On the basis of its similarity with NRF-1 and IBR, nrf is likely involved in transcriptional regulation of multiple target genes, including those that encode mitochondrial proteins, growth factor receptors and other transcription factors. This demonstrates the power of insertional mutagenesis as a means for characterizing novel genes necessary for vertebrate retinal development.

Beta-adrenergic receptors mediate a stress-induced decrease in IGF-II mRNA in the rat cerebellum

1. Exposure to a combined forced swimming-confinement stress resulted in a decrease in insulin-like growth factor II (IGF-II) mRNA levels in the whole brain (without the cerebellum) and in the isolated brain areas of the cerebral cortex, the hippocampus, and the cerebellum. 2. In an effort to elucidate the neurotransmitter systems involved in this stress-induced decrease, animals were injected prior to exposure to the stress, with either propranolol, diazepam, or MK-801. 3. Administration of diazepam or MK-801 did not affect the stress-induced decrease in IGF-II mRNA in any of the three brain areas examined. 4. Administration of propranolol prior to the exposure to the stress inhibited the stress-induced decrease in IGF-II mRNA in the cerebellum. Propranolol had no such effect in the cerebral cortex or the hippocampus. 5. Our results suggest that in the cerebellum, the stress-induced decrease in IGF-II mRNA is mediated by beta 2-adrenergic receptors.

Mouse models of myelin diseases

Dys- and demyelination are the common endpoints of several inherited diseases of glial cells, which elaborate myelin and which maintain the myelin sheath very much like an "external" cellular organelle. Whereas some of the genes that are affected by mutations appear to be glial-specific, other genes are expressed in many cell types but their defect is restricted to oligodendrocytes or Schwann cells. Many of the disease genes and their encoded proteins have been studied with the help of mouse models, and a number of different molecular pathomechanisms have emerged which have been summarized in Figure 8. Some of the new concepts in the field, which have been addressed in this review, have only emerged because similar pathomechanisms were discovered for different myelin proteins. Mouse models have clearly helped to address both, the molecular pathology of myelin diseases and the normal function of myelin genes, but as discussed in this review, these questions turned out to be very different. Despite the progress in understanding the role of the abundant myelin proteins, there also remain a number of open questions that concern, among other things, the initial axon-glia recognition, the assembly process of the myelin sheath, and the long-term interaction of axons with their myelinating glia. Finally, animal models of human neurological diseases should not be restricted to the study of pathology, but they should also contribute to the development of experimental treatments. It is encouraging that a few attempts have been made.

Prosaposin prevents programmed cell death of rat cerebellar granule neurons in culture

Prosaposin, the precursor of sphingolipid activator proteins (saposin A-D), has been reported to be a neurotrophic factor in vitro and in vivo. Prosaposin mRNA is transiently expressed at a high level in developing cerebellum during the period of granule cell proliferation and maturation, suggesting its significance during development of cerebellum. Here we investigated the neuroprotective effect of prosaposin on cerebellar granule neurons, exposing primary cerebellar granule cells to low K+ which induced programmed cell death. Prosaposin rescued mature cerebellar granule neurons in a bimodal manner. A similar neuroprotective effect was obtained using TX14(A), a 14-mer neurotrophic peptide derivative of prosaposin. An additive neuroprotective effect was observed between BDNF and TX14(A), but not between IGF-1 and TX14(A). Prosaposin rescued 60% of nifedipine sensitive cerebellar granule neurons as well as IGF-1, while BDNF did not. Furthermore, the neuroprotective action of prosaposin was inhibited by LY294002, a specific inhibitor of PI 3-kinase. These findings indicated that prosaposin had a trophic effect upon newborn cerebellar granule cells and that the neuroprotective action was similar to that of IGF-1 rather than BDNF. Prosaposin may play a role in cerebellar development during programmed cell death of cerebellar neurons.

Induction of cell growth by insulin and insulin-like growth factor-I is associated with Jun expression in the otic vesicle

The present report investigates the cellular mechanisms involved in the regulation of cell proliferation by insulin and insulin-like growth factor-I (IGF-I) in the developing inner ear. The results show that insulin and IGF-I stimulate cell proliferation in the otic vesicle. This effect is associated with the induction of the expression of the nuclear proto-oncogene c-jun. The temporal profile of Jun expression coincided with the proliferative period of growth of the otic vesicle. IGF-I promoted the hydrolysis of a membrane glycosyl-phosphatidylinositol, which was characterised as the endogenous precursor for inositol phosphoglycan (IPG). Both purified IPG and a synthetic analogue, 6-O-(2-amino-2-deoxy-alpha-D-glucopyranosyl)-D-myoinositol-1,2-cyclic phosphate (C3), were able to mimic the effects of IGF-I on Jun expression. Anti-IPG antibodies blocked the effects of IGF-I, which were rescued by the addition of IPG or its analogue. These results suggest that the sequence involving the hydrolysis of membrane glycolipids and the expression of c-jun and c-fos proto-oncogenes is part of the mechanism that activates cell division in response to insulin and IGF-I during early organogenesis of the avian inner ear. The implications of these observations for otic development and regeneration are briefly discussed.

Biochemical and morphometric analyses show that myelination in the insulin-like growth factor 1 null brain is proportionate to its neuronal composition

To elucidate the role of insulin-like growth factor 1 (IGF1) in the normal development of brain myelination, we used behavioral, biochemical, and histological analyses to compare the myelination of brains from Igf1(-/-) and wild-type (WT) littermate mice. The studies were conducted at postnatal day 40, at which time the Igf1(-/-) mice weighed approximately 66% less than wild-type mice. However, the Igf1(-/-) brain weight was only reduced by approximately 34%. Formal neurological testing showed no sign of central or peripheral myelinopathy in Igf1(-/-) mice. Myelin composition was not significantly different, and myelin concentration, normalized to brain weight or protein, was equal in Igf1(-/-) and WT mice. Likewise, concentrations of myelin-specific proteins (MBP, myelin proteolipid protein, MAG, and 2',3'-cyclic nucleotide,3'-phosphodiesterase) were not significantly different in Igf1(-/-) and WT mice. The myelin-associated lipids galactocerebroside and sulfatide were modestly reduced in Igf1(-/-) brains. Regional oligodendrocyte populations and myelin staining patterns were comparable in Igf1(-/-) and WT brains, with the notable exception of the olfactory system. The Igf1(-/-) olfactory bulb was profoundly reduced in size and was depleted of mitral neurons and oligodendrocytes, and its efferent tracts were depleted of myelin. In summary, this study shows that myelination of the Igf1(-/-) brain is proportionate to its neuronal composition. Where projection neurons are preserved despite the deletion of IGF1, as in the cerebellar system, oligodendrocytes and myelination are indistinguishable from wild type. Where projection neurons are depleted, as in the olfactory bulb, oligodendrocytes are also depleted, and myelination is reduced in proportion to the reduced projection neuron mass. These data make a strong case for the primacy of axonal factors, not including IGF1, in determining oligodendrocyte survival and myelination.

Insulin-like growth factor-1 is a radial cell-associated neurotrophin that promotes neuronal recruitment from the adult songbird edpendyma/subependyma

In the adult songbird forebrain, neurons continue to be produced from precursor cells in the forebrain ependymal/subependymal zone (SZ), from which they migrate upon radial guide fibers. The new neurons and their radial cell partners may coderive from a common SZ progenitor, which may be the radial cell itself. On this basis, we asked whether radial cells might provide trophic support for the migration or survival of newly generated neurons. We focused upon the insulin-like growth factors (IGFs) IGF-1 and IGF-2, which have previously been shown to support the survival and differentiation of neural progenitor cells. We found that IGF-1 immunoreactivity was expressed heavily by adult zebra finch radial cells and their fibers, with little expression otherwise. IGF-2, in contrast, was expressed by parenchymal astrocytes and exhibited little radial cell expression. Despite their distinct distributions, IGF-1 and IGF-2 exerted similar trophic effects on finch SZ cells in vitro; both greatly increased the number of neurons migrating from explants of the adult finch SZ, relative to explants raised in low-insulin, IGF-1-deficient media. However, neither factor extended neuronal survival. These results suggest that in neurogenic regions of the adult avian forebrain, IGF-1 acts as a radial cell-associated neuronal differentiation and/or departure factor, which may serve to regulate neuronal recruitment into the adult brain.

Insulin-like growth factor-I analogue prevents apoptosis mediated through an interleukin-1 beta converting enzyme (caspase-1)-like protease of cerebellar external granular layer neurons: developmental stage-specific mechanisms of neuronal cell death

Using an organotypic slice culture system of neonatal rat cerebellum, we examined developmental stage-specific mechanisms of cell death of granule neurons. This culture system allows a serial process of granule neuron development including their proliferation during the early culture period and the proceeding migration from the external granular layer to the internal granular layer in the presence of a supraphysiological concentration (5 micrograms/ml) of insulin. Insulin deprivation induced apoptosis of granule neurons in external granular layer but not in internal granular layer. A truncated analogue of insulin-like growth factor-I (des (1-3) insulin-like growth factor-I) prevented this apoptosis at a concentration of 65-650 ng/ml. Some apoptotic granule neurons expressed proliferating cell nuclear antigen but not TAG-1, a marker protein of the postmitotic and premigratory granule neurons. Thus, this apoptosis occurred at a specific stage in granule neuron development: at the stage before TAG-1 expression and at least partly at the proliferative state. Ac-YVAD-CHO, an inhibitor of interleukin-1 beta converting enzyme (caspase-1)-like proteases, had a protective effect on this apoptosis. Interleukin-1 beta converting enzyme (caspase-1)-like protease activity increased under the apoptosis-induced condition. High concentration of K+, which is known to prevent granule neuron apoptosis in dissociated cultures, had a partial protective effect on this apoptosis. These findings suggest that (i) cerebellar granule neurons fall into apoptosis at the specific developmental stage unless stimulated by insulin-like growth factor-I (analogue), (ii) this apoptosis is mediated through an interleukin-1 beta converting enzyme-like protease, and (iii) this apoptosis consists of K(+)-sensitive and K(+)-insensitive components.

Regulation of experimental autoimmune encephalomyelitis with insulin-like growth factor (IGF-1) and IGF-1/IGF-binding protein-3 complex (IGF-1/IGFBP3)

Insulin-like growth factor (IGF)-1 is a cytokine that promotes oligodendrocyte development and myelin production. This study investigated whether treatment of chronic, relapsing murine experimental autoimmune encephalomyelitis (EAE) with IGF-1 or IGF-1 associated with its binding protein, IGFBP3, altered the course of disease. Administration of IGF-1/IGFBP3 (1-100 mg/kg per day) delayed the onset of disease in a dose-dependent manner and histologic examination showed a delay in inflammatory cells entering the central nervous system. However, once signs of EAE developed, disease was enhanced in the mice that had been given the highest dose of IGF-1/IGFBP3. Treatment with IGF-1/IGFBP3 after the onset of signs resulted in a severe relapse. Administration of free IGF-1 (10 mg/kg per day) provided mild protection when given before disease onset, but did not significantly alter the course of disease if given after disease onset. Possible mechanisms that could explain the altered disease in IGF-1/IGFBP3-treated mice included (a) IGF-1/IGFBP3 administration delayed the onset of EAE by downregulating ICAM-1 gene expression in the central nervous system, and (b) IGF-1/IGFBP3 treatment of EAE resulted in more severe disease due to enhanced expansion of encephalitogenic T cells. Although IGF-1 may enhance remyelination, these results indicate that administration of IGF-1 associated with IGFBP3 may also accentuate autoimmune demyelinating disease.

Insulin-like growth factor-I is a differentiation factor for postmitotic CNS stem cell-derived neuronal precursors: distinct actions from those of brain-derived neurotrophic factor

Insulin-like growth factor-I (IGF-I) has been reported previously to promote the proliferation, survival, and maturation of sympathetic neuroblasts, the genesis of retinal neurons, and the survival of CNS projection and motor neurons. Here we asked whether IGF-I could promote the in vitro differentiation of postmitotic mammalian CNS neuronal precursors derived from multipotent epidermal growth factor (EGF)-responsive stem cells. In the absence of IGF-I, virtually no neurons were present in cultured stem cell progeny, whereas IGF-I increased neuron number by eight- to 40-fold. Brief exposures (2 hr) to IGF-I were sufficient to allow for neuronal differentiation without affecting proliferation or survival. IGF-I actions could be mimicked by insulin and IGF-II at concentrations that correspond to the pharmacology of the IGF-I receptor, the latter for which the mRNA was detected in undifferentiated stem cell progeny. Although ineffectual alone at low concentrations (10 nM) that would activate its own receptor, insulin was able to potentiate the actions of IGF-I by acting on mitotically active neural precursors. When neuronal precursor differentiation by IGF-I was examined in relation to brain-derived neurotrophic factor (BDNF), two important observations were made: (1) BDNF could potentiate the differentiating actions of IGF-I plus insulin, and (2) BDNF could act on a separate population of precursors that did not require IGF-I plus insulin for differentiation. Taken together, these results suggest that IGF-I and BDNF may act together or sequentially to promote neuronal precursor differentiation.

Brain-derived neurotrophic factor stimulates neurite outgrowth in a calretinin-enriched neuronal culture system

A calretinin enriched cell culture system which comprised approximately 40% of the total neuronal population of the E14 rat embryo was established from the region of the thalamic eminence (TE), and the effects of several neurotrophins on the neurite growth of calretinin-immunoreactive (CR-IR) neurons was investigated. A 4-day treatment of BDNF significantly increased the ratio of CR-IR to microtubule-associated protein 2-immunoreactive neurons at concentrations between 50 and 250 ng/ml. IGF-I at 100 ng/ml and TGF-alpha at 250 ng/ml also increased this ratio. None of the neurotrophins examined increased the number of primary neurites. BDNF did, however, increase the number of secondary neurites. BDNF-treated primary and secondary neurites were also significantly longer than neurites from neurons in control cultures. IGF-I elicited an increase in primary neurite length, but did not affect either number or length of secondary neurites. TGF-alpha had no effect on either number or length of the primary and secondary neurites. These results indicate that the maturation and development of CR-IR neurites is specifically affected by BDNF. It is suggested that BDNF increases the CR concentration above the threshold of detection by immunohistochemistry in cells and stimulates the sprouting of secondary CR-IR neurites.

Insulin-like growth factor-I modulation of cerebellar cell populations is developmentally stage-dependent and mediated by specific intracellular pathways

Although development of transgenic animals overexpressing insulin-like growth factor-I has allowed the establishment of a role of this trophic factor in brain growth, detailed knowledge of the action of insulin-like growth factor-I on different brain areas is still lacking. We now provide evidence for a pleiotrophic role of this growth factor on cerebellar development. Insulin-like growth factor-I produced by cerebellar cultures is a survival factor for Purkinje cells and a mitogen/differentiation factor for cerebellar glioblasts. Trophic effects of insulin-like growth factor-I were observed only during specific developmental stages. In addition, insulin-like growth factor-I increased intracellular Ca2+ levels in Purkinje cells and c-Fos in dividing glioblasts. Survival-promoting effects of insulin-like growth factor-I on Purkinje cells required activation of protein kinase C, while glioblast division induced by insulin-like growth factor-I depended on phosphatidylinosytol 3-kinase activation. We conclude that insulin-like growth factor-I is a paracrine/autocrine pleiotrophic factor for both glia and neurons in the cerebellum. Its effects are mediated by distinct intracellular signals and appear to be specific to the developmental stage of the target cell. Since development of the different cell populations that compose a specific brain territory is not synchronized, the pleiotrophic action of growth factors such as insulin-like growth factor-I may be essential to ontogenetic processes underlying normal brain growth.

Insulin-like growth factor-I effects on gonadotropin-releasing hormone biosynthesis in GT1-7 cells

The immortalized GT1-7 cell line synthesizes and secretes GnRH, the key hormone of reproduction. However, GT1-7 cells lack the normal inputs from neurotransmitters, growth factors, and steroids, which are involved in the maturation and maintenance of GnRH neurons in the brain. We examined the effects of the neurotrophic factor insulin-like growth factor-I (IGF-I) on GnRH gene expression and the mechanism for these changes. Initially, effects of IGF-I on GnRH gene expression were determined by ribonuclease protection assay. In time-course experiments, IGF-I treatment caused significant increases in nuclear GnRH primary transcript levels, an index of GnRH gene transcription, 4 and 8 h after initiation of IGF-I treatment. GnRH messenger RNA (mRNA) levels in the cytoplasm were stimulated by IGF-I at 24 h of treatment. IGF-I also affected GT1-7 cell morphology, with an increase in process extension and cell-cell contacts. In contrast, GnRH peptide levels in the medium were initially stimulated and then suppressed by IGF-I, indicating an uncoupling of biosynthesis and secretion. The increase in GnRH mRNA levels induced by IGF-I is probably caused by a transcriptional mechanism, as evidenced by the increase in GnRH primary transcript levels before a change in GnRH mRNA levels, as well as our finding of a similar GnRH mRNA half-life for both control and IGF-I-treated cells. Interestingly, GT1-7 cells themselves were observed to express IGF-I immunoreactivity, suggesting the possibility of autoregulation by this neurotrophic factor. It is concluded that IGF-I is an important modulator of GnRH gene expression and release in the GT1-7 cell line. The reported stimulatory effects of IGF-I in vivo, and its hypothesized role in the development of GnRH neurons in the brain, suggest that IGF-I may make the GT1-7 cells line more like a mature GnRH neuron, as a model for future studies.

Macrophages in CNS remyelination: friend or foe?

Hematogenous macrophages and resident brain microglia are agents of demyelination in multiple sclerosis (MS) and paradoxically may also participate in remyelination. In vitro studies have shown that macrophage enrichment of aggregate brain cultures promotes myelination per se and enhances the capacity to remyelinate following a demyelinating episode. It has been hypothesized that remyelination in MS is implemented by surviving dedifferentiated oligodendrocytes or by newly recruited progenitors that migrate, proliferate and synthesize myelin in response to signalling molecules in the local environment. We postulate that macrophage-derived cytokines or growth factors may directly or indirectly promote oligodendroglial proliferation and differentiation, contributing to myelin repair in inflammatory demyelinating disease.

Regulation of insulin-like growth factor I (IGF-I) gene expression in brain of transgenic mice expressing an IGF-I-luciferase fusion gene

Insulin-like growth factor I (IGF-I) plays an important role in the development and function of the central nervous system (CNS). Little is known, however, about the factors and mechanisms involved in regulation of CNS IGF-I gene expression. To facilitate our goal to define mechanisms of IGF-I gene regulation in the CNS, we generated several lines of transgenic (Tg) mice that express firefly luciferase (LUC) under control of a 11.3-kb fragment from the 5' region of the rat IGF-I gene. Consistent with expression of the native IGF-I gene in murine brain, expression of the transgene predominated in neurons and astrocytes and used promoter 1, the major IGF-I promoter in the CNS and in most tissues. Transgene messenger RNA and protein expression rapidly increased after birth and peaked at postnatal (P) day 4 in all brain regions studied. LUC activities in all regions then gradually decreased to 0.5-4% of their peak values at P31, except for the olfactory bulb, which maintained about one third of its maximal activity. Compared with littermate controls, administration of dexamethasone decreased LUC activity and transgenic IGF-I messenger RNA abundance, whereas GH significantly increased the expression of the transgene. Addition of GH to cultured fetal brain cells from Tg mice for 12 h also increased LUC activity in a dose-dependent manner (77-388%). These results show that this IGF-I promoter transgene is expressed in a fashion similar to the endogenous IGF-I gene, and thus indicates that the transgene contains cis-elements essential for developmental, GH, and glucocorticoid regulation of IGF-I gene expression in the CNS. These Tg mice should serve as an useful model to study mechanisms of IGF-I gene regulation in the brain.

Neutralizing IGF-1 monoclonal antibody with cross-species reactivity

We report the generation of a murine IGF-1 monoclonal antibody designated 35I17, which exhibits unique cross-species reactivity. The antibody recognizes recombinant human and rat IGF-1 in ELISA, Western blots, and in an 125I-recombinant human IGF-1 Scintillation Proximity Assay. In addition, 35I17 blocks cell proliferation induced by recombinant human and rat IGF-1, and inhibits cell proliferation induced by sera from human, rat, calf, dog, goat, or mouse. The antibody inhibits rat IGF-1 binding to IGF-1 receptors, and prevents IGF-1-stimulated receptor and IRS-1 phosphorylation in LISN C4 cells, an IGF-1 receptor-transfected cell line. The cross-species and neutralizing properties of 35I17 may be useful in in vitro and in vivo animal studies for elucidating the role of IGF-1 in cancer, rheumatoid arthritis, and other diseases.

Analysis of rat vestibular hair cell development and regeneration using calretinin as an early marker

Despite increased interest in inner ear hair cell regeneration, it is still unclear what exact mechanisms underlie hair cell regeneration in mammals because of our limited understanding of hair cell development and the lack of specific hair cell markers. In this report, we studied hair cell development using immunohistochemistry on sections prepared from embryonic day (E) 13 to postnatal day 7 rat inner ear tissues. Of many epithelial, neuronal, and glial markers, we found that calcium-binding protein antibodies recognizing calretinin, calmodulin, or parvalbumin labeled immature hair cells in rat vestibular end organs. In particular, calretinin antiserum labeled the initial differentiating hair cells at E15, a stage immediately after the terminal mitosis of hair cell progenitors. The selective immunoreactivity of postmitotic presumptive hair cells, but not supporting cells or peripheral epithelial cells, was confirmed in utricular epithelial sheet cultures. Double labeling with calretinin and bromodeoxyuridine antibodies in long-term cultures showed that only a few mitotic utricular supporting cells became calretinin positive. Thus, although proliferation-mediated regeneration of new hair cells might directly contribute to hair cell regeneration in rat utricles after injury, it is very limited. In addition, double labeling with calretinin and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) revealed that differentiated hair cells underwent apoptosis during normal development at late embryonic and early postnatal stages in vivo and in vitro. Therefore, these experiments lay the groundwork for the time course of differentiation, regeneration, and apoptosis of mammalian vestibular hair cells. This work also suggests that calcium-binding proteins are useful markers for studies on inner ear hair cell differentiation and regeneration.

Insulin-like growth factor I gene deletion causing intrauterine growth retardation and severe short stature

The first human case of a homozygous molecular defect in the gene encoding insulin-like growth factor I (IGF-I) is described. The patient was a 15-year-old boy from a consanguineous pedigree who presented with severe intrauterine growth failure, sensorineural deafness and mild mental retardation. Endocrine evaluation of the growth hormone (GH)--IGF-I axis revealed elevated GH secretion, undetectable serum IGF-I and normal serum IGF-binding protein-3, acid-labile subunit, and GH-binding activity. Analysis of the IGF-I gene revealed a homozygous partial IGF-I gene deletion involving exons 4 and 5, which encodes a severely truncated mature IGF-I peptide. This patient demonstrates that complete disruption of the IGF-I gene in man is compatible with life, and indicates a major role for IGF-I in human fetal growth. In addition, his neurological abnormalities suggest that IGF-I may be involved in central nervous system development.

Insulin-like growth factor-I receptors in normal appearing white matter and chronic plaques in multiple sclerosis

Preclinical studies suggest that insulin-like growth factor-I (IGF-I) plays an important role in oligodendrocyte survival and myelination. We used human recombinant [125I]IGF-I to study IGF-I receptors in post-mortem brain tissue from patients with multiple sclerosis (MS). In normal appearing white matter, we found that IGF-I receptor densities and binding characteristics were not different between MS patients and controls. In chronic plaques, histologically characterized by astrogliosis, we found densities of IGF-I receptors which were in the same range as those measured in the normal appearing white matter. In vitro studies have shown that IGF-I also acts as a mitogenic factor for astrocytes. Since MS lesions are rapidly invaded by reactive astrocytes, IGF-I may not only protect oligodendrocytes and stimulate remyelination but also enhance the astrogliosis that limits repair.