Increase of extracellular insulin-like growth factor I (IGF-I) concentration following electrolytical lesion in rat hippocampus

The role for IGF-1-derived small neuropeptides as a therapeutic target for neurological disorders

INTRODUCTION

Exogenous IGF-1 protects the brain from ischemic injury and improves function. However, its clinical application to neurological disorders is limited by its large molecular size, poor central uptake and mitogenic potential.

AREAS COVERED

In this review, the authors have discussed the efficacy, pharmacokinetics and mechanisms of IGF-1 derivatives on protecting acute brain injury, preventing memory impairment and improving recovery from neurological degenerative conditions evaluated in various animal models. We have included natural metabolites of IGF-1, glycine-proline-glutamate (GPE), cleaved from N-terminal IGF-1 and cyclic glycine-proline (cGP) as well as the structural analogues of GPE and cGP, glycine-2-methyl-proline-glutamate and cyclo-l-glycyl-l-2-allylproline, respectively. In addition, the regulatory role for cGP in bioavailability of IGF-1 has also been discussed.

EXPERT OPINION

These small neuropeptides provide effective neuroprotection by offering an improved pharmacokinetic profile and more practical route of administration compared with IGF-1 administration. Developing modified neuropeptides to overcome the limitations of their endogenous counterparts represents a novel strategy of pharmaceutical discovery for neurological disorders. The mechanism of action may involve a regulation of IGF-1 bioavailability.

The IGF-derived tripeptide Gly-Pro-Glu is a weak NMDA receptor agonist

Glutamate acts as the universal agonist at ionotropic glutamate receptors in part because of its high degree of conformational flexibility. Other amino acids and small peptides, however, can activate N-methyl-d-aspartate (NMDA) receptors, albeit usually with lower affinity and efficacy. Here, we examined the action of glycine-proline-glutamate (GPE), a naturally occurring tripeptide formed in the brain following cleavage of IGF-I. GPE is thought to have biological activity in the brain, but its mechanism of action remains unclear. With its flanking glutamate and glycine residues, GPE could bind to either the agonist or coagonist sites on NMDA receptors, however, this has not been directly tested. Using whole cell patch-clamp recordings in combination with rapid solution exchange, we examined both steady-state currents induced by GPE as well as the effects of GPE on synaptically evoked currents. High concentrations of GPE evoked inward currents, which were blocked either by NMDA receptor competitive antagonists or the voltage-dependent channel blocker Mg(2+). GPE also produced a slight attenuation in the NMDA- and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated excitatory postsynaptic currents without altering the paired-pulse ratio. Our results suggest that GPE can activate NMDA receptors but at concentrations well above the expected concentration of GPE in the brain. Therefore, it is unlikely that endogenous GPE interacts with glutamate receptors under normal conditions.

Human conditions of insulin-like growth factor-I (IGF-I) deficiency

Insulin-like growth factor I (IGF-I) is a polypeptide hormone produced mainly by the liver in response to the endocrine GH stimulus, but it is also secreted by multiple tissues for autocrine/paracrine purposes. IGF-I is partly responsible for systemic GH activities although it possesses a wide number of own properties (anabolic, antioxidant, anti-inflammatory and cytoprotective actions). IGF-I is a closely regulated hormone. Consequently, its logical therapeutical applications seems to be limited to restore physiological circulating levels in order to recover the clinical consequences of IGF-I deficiency, conditions where, despite continuous discrepancies, IGF-I treatment has never been related to oncogenesis. Currently the best characterized conditions of IGF-I deficiency are Laron Syndrome, in children; liver cirrhosis, in adults; aging including age-related-cardiovascular and neurological diseases; and more recently, intrauterine growth restriction. The aim of this review is to summarize the increasing list of roles of IGF-I, both in physiological and pathological conditions, underlying that its potential therapeutical options seem to be limited to those proven states of local or systemic IGF-I deficiency as a replacement treatment, rather than increasing its level upper the normal range.

Insulin-like growth factor -1 (IGF-1) derived neuropeptides, a novel strategy for the development of pharmaceuticals for managing ischemic brain injury

Insulin-Like Growth Factor-1 (IGF-1) is neuroprotective and improves long-term function after brain injury. However, its clinical application to neurological disorders is limited by its large molecular size, poor central uptake, and mitogenic potential. Glycine-proline-glutamate (GPE) is naturally cleaved from the IGF-1 N-terminal and is also neuroprotective after ischemic injury, thus providing a potential novel strategy of drug discovery for management of neurological disorders. GPE is not enzymatically stable, thus intravenous infusion of GPE becomes necessary for stable and potent neuroprotection. The broad effective dose range and treatment window of 3-7 h after the lesion suggest its potential for treating acute brain injuries. The neuroprotective action of GPE is not age selective, is not dependent on cerebral reperfusion, plasma glucose concentrations, and core body temperature. G-2mPE, a GPE analogue designed to be more resistant to enzymatic activity, has a prolonged plasma half-life and is more potent in neuroprotection. Neuroprotection by GPE and its analogue may be involved in modulation of inflammation, promotion of astrocytosis, inhibition of apoptosis, and in vascular remodeling. Small neuropeptides have advantages over growth factors in the treatment of brain injury, and modified neuropeptides, designed to overcome the limitations of their endogenous counterparts, represent a novel strategy of pharmaceutical discovery for neurological disorders.

IGF-1 derived small neuropeptides and analogues: a novel strategy for the development of pharmaceuticals for neurological conditions

Insulin-like growth factor-1 (IGF-1) is neuroprotective and improves long-term function after brain injury. However, its clinical application to neurological disorders is limited by its large molecular size, poor central uptake and mitogenic potential. Glycine-proline-glutamate (GPE) is naturally cleaved from the IGF-1 N-terminal and it is also neuroprotective after ischemic injury, which provided a novel strategy of drug discovery for neurological disorders. GPE is not enzymatically stable, thus intravenous infusion of GPE becomes necessary for stable and potent neuroprotection. The broad effective dose range and treatment window of 3-7 h after the lesion suggest its potential for treating acute brain injuries. G-2meth-PE, a GPE analogue designed to be more enzymatic resistant, has a prolonged plasma half-life and is more potent in neuroprotection. Neuroprotection by GPE and its analogue may involve modulation of inflammation, promotion of astrocytosis, inhibition of apoptosis and vascular remodelling. Acute administration of GPE also prevents 6-OHDA-induced nigrostrial dopamine depletion. Delayed treatment with GPE does not prevent dopamine loss, but improves long-term function. Cyclo-glycyl-proline (cyclic Gly-Pro) is an endogenous DKP that may be derived from GPE. Cyclic Gly-Pro and its analogue cyclo-L-glycyl-L-2-allylproline (NNZ 2591) are both neuroprotective after ischaemic injury. NNZ2591 is highly enzymatic resistant and centrally accessible. Its peripheral administration improves somatosensory-motor function and long-term histological outcome after brain injury. Our research suggests that small neuropeptides have advantages over growth factors in the treatment of brain injury, and that modified neuropeptides designed to overcome the limitations of their endogenous counterparts represent a novel strategy of pharmaceutical discovery for neurological disorders.

Neuroprotective effects of the N-terminal tripeptide of insulin-like growth factor-1, glycine-proline-glutamate (GPE) following intravenous infusion in hypoxic-ischemic adult rats

The N-terminal tripeptide of insulin-like growth factor-1, GPE is neuroprotective when given intracerebroventricularly 2 h after hypoxic-ischemic (HI) brain injury in rats. We have now examined whether GPE can cross the blood-brain barrier and exert neuroprotective actions following intravenous administration. Following a single bolus intravenous injection, GPE was rapidly metabolized and cleared from the circulation. The short half-life (<2 min) in blood was subsequently associated with modest and inconsistent neuroprotection. In contrast, potent neuroprotection of GPE was consistently observed in all brain regions examined following 4 h intravenous infusion (12 mg/kg). The neuroprotective effects of GPE after infusion showed a broad effective dose range (1.2-120 mg/kg) and an extended window of treatment to 7-11 h after injury. The central penetration of GPE after intravenous infusion was injury-dependent. GPE also improved long-term somatofunction with a comparable neuronal outcome. GPE reduced both caspase-3-dependent and -independent apoptosis in the hippocampus. Treatment with GPE also inhibited microglial proliferation and prevented the injury-induced loss of astrocytes. In conclusion, the neuroprotective actions of GPE infusion were global, robust and displayed a broad effective dose range and treatment window. GPE's activity included the prevention of neuronal apoptosis, promotion of astrocyte survival and inhibition of microglial proliferation. With injury specific central penetration, GPE has considerable promise as a systemic neuroprotective treatment after acute encephalopathies.

Insulin-like growth factor-1 and post-ischemic brain injury

Insulin-like growth factor-1 (IGF-1) is a naturally occurring neurotrophic factor that plays an important role in promoting cell proliferation and differentiation during normal brain development and maturation. The present review examines recent evidence that endogenous IGF-1 also plays a significant role in recovery from insults such as hypoxia-ischemia and that giving additional exogenous IGF-1 can actively ameliorate damage. It is now well established that neurons and other cell types die many hours or even days after initial injury due to activation of programmed cell death pathways. IGF-1 and its binding proteins and receptors are intensely induced within damaged brain regions following brain injury, suggesting a possible a role for IGF-1 in brain recovery. Exogenous administration of IGF-1 within a few hours after brain injury is now known to be protective in both gray and white matter and leads to improved somatic function. In contrast, pre-treatment is ineffective, likely reflecting limited intracerebral penetration of IGF-1 into the uninjured brain. The neuroprotective effects of IGF-1 are mediated by IGF-1 receptors and its binding proteins and are specific to particular cellular phenotypes and brain regions. The window of opportunity for treatment with IGF-1 is limited to a few hours after normothermic brain injury, reflecting its specific actions on early, intracellular events in the apoptotic cascade. However, injury-associated mild post-hypoxic hypothermia, which delays the development of cell death, can shift and dramatically extend the window of opportunity for delayed treatment with IGF-1. Such a combined approach is likely to be essential for any clinical treatment.

Amygdala kindling decreases insulin-like growth factor-I receptor binding sites in the rat hippocampus

The neural excitability characteristic of kindling has been linked to structural alterations such as mossy fiber sprouting and synaptic reorganization within the hippocampus. Recent evidence suggests that growth factors may play a key role in kindling-related synaptic plasticity. Insulin-like growth factors-I and -II (IGF-I/-II) and insulin are structurally-related pleiotropic growth factors known to be involved in neural growth and differentiation. In the present study, we investigated the effect of kindling on [125I]IGF-I, [125I]IGF-II and [125I]insulin receptor binding in the hippocampus of adult rats. Our results indicate a progressive decrease in [125I]IGF-I (but not [125I]IGF-II or [125I]insulin) binding sites in the CA1, hilus and the granule cell layer of the kindled rats compared to sham-stimulated rats. These results, in keeping with the established neurotrophic effects of IGF-I, suggest a potential role for this growth factor in mediating the structural alterations associated with kindling.

Insulin-like growth factor-1 improves somatosensory function and reduces the extent of cortical infarction and ongoing neuronal loss after hypoxia-ischemia in rats

Treatment with insulin-like growth factor-1 has been demonstrated to reduce the extent of cortical infarction 5 days after hypoxic-ischemic brain injury. As neuronal death can be progressive and long lasting after initial injury, the present study examined the long-term effects of insulin-like growth factor-1 on late neuronal loss 20 days after hypoxic-ischemic injury, together with evaluating neurobehavioral outcome as assumed by somatosensory function. Unilateral brain injury was induced in adult rats by carotid artery ligation followed by 10 min of hypoxia (6% O2). A single dose of insulin-like growth factor-1 (50 microg) was administered intracerebroventricularly via a stereotaxically pre-fixed cannula 2 h after injury. A bilateral tactile stimulation test was used to examine the degree of somatosensory function at 3, 5, 10 and 20 days after the hypoxia in both insulin-like growth factor-1- (n=12) and its vehicle- (n=12) treated rats, along with sham-operated rats (n=9). Cortical infarction and percentage of selective neuronal loss in the cerebral cortex were examined 20 days after the hypoxic-ischemic injury in both treatment groups. Hypoxic-ischemic injury resulted in a significant delay in the time taken to contact the patch over the period examined (left/right ratio 5.1+/-0.79), particularly at 3 days (7.0+/-2.8) after the hypoxia, compared to sham-operated rats (1.1+/-0.9, P<0.05). The overall effect of insulin-like growth factor-1 in reducing the time taken to contact the patch was significant (P=0.03, 2.6+/-0.79) compared to the vehicle group. There was a trend towards a reduction of cortical infarction after insulin-like growth factor-1 treatment (P=0.058), however insulin-like growth factor-1 significantly reduced the percentage of selective neuronal loss (P=0.027) 20 days following the hypoxia. From these data we suggest that insulin-like growth factor-1 improves somatosensory function by reducing both the extent of cortical infarction and ongoing progressive neuronal death during brain recovery from hypoxic-ischemic injury.

Interactions of estrogens and insulin-like growth factor-I in the brain: implications for neuroprotection

Data from epidemiological studies suggest that the decline in estrogen following menopause could increase the risk of neurodegenerative diseases. Furthermore, experimental studies on different animal models have shown that estrogen is neuroprotective. The mechanisms involved in the neuroprotective effects of estrogen are still unclear. Anti-oxidant effects, activation of different membrane-associated intracellular signaling pathways, and activation of classical nuclear estrogen receptors (ERs) could contribute to neuroprotection. Interactions with neurotrophins and other growth factors may also be important for the neuroprotective effects of estradiol. In this review we focus on the interaction between insulin-like growth factor-I (IGF-I) and estrogen signaling in the brain and on the implications of this interaction for neuroprotection. During the development of the nervous system, IGF-I promotes the differentiation and survival of specific neuronal populations. In the adult brain, IGF-I is a neuromodulator, regulates synaptic plasticity, is involved in the response of neural tissue to injury and protects neurons against different neurodegenerative stimuli. As an endocrine signal, IGF-I represents a link between the growth and reproductive axes and the interaction between estradiol and IGF-I is of particular physiological relevance for the regulation of growth, sexual maturation and adult neuroendocrine function. There are several potential points of convergence between estradiol and IGF-I receptor (IGF-IR) signaling in the brain. Estrogen activates the mitogen-activated protein kinase (MAPK) pathway and has a synergistic effect with IGF-I on the activation of Akt, a kinase downstream of phosphoinositol-3 kinase. In addition, IGF-IR is necessary for the estradiol induced expression of the anti-apoptotic molecule Bcl-2 in hypothalamic neurons. The interaction of ERs and IGF-IR in the brain may depend on interactions between neural cells expressing ERs with neural cells expressing IGF-IR, or on direct interactions of the signaling pathways of alpha and beta ERs and IGF-IR in the same cell, since most neurons expressing IGF-IR also express at least one of the ER subtypes. In addition, studies on adult ovariectomized rats given intracerebroventricular (i.c.v.) infusions with antagonists for ERs or IGF-IR or with IGF-I have shown that there is a cross-regulation of the expression of ERs and IGF-IR in the brain. The interaction of estradiol and IGF-I and their receptors may be involved in different neural events. In the developing brain, ERs and IGF-IR are interdependent in the promotion of neuronal differentiation. In the adult, ERs and IGF-IR interact in the induction of synaptic plasticity. Furthermore, both in vitro and in vivo studies have shown that there is an interaction between ERs and IGF-IR in the promotion of neuronal survival and in the response of neural tissue to injury, suggesting that a parallel activation or co-activation of ERs and IGF-IR mediates neuroprotection.

Selective introduction of antisense oligonucleotides into single adult CNS progenitor cells using electroporation demonstrates the requirement of STAT3 activation for CNTF-induced gliogenesis

We have developed a novel method in which antisense DNA is selectively electroporated into individual adult neural progenitor cells. By electroporation of antisense oligonucleotides against signal transducer and activator of transcription 3 (STAT3) we demonstrate that ciliary neurotrophic factor (CNTF) is an instructive signal for astroglial type 2 cell fate specifically mediated via activation of STAT3. Activation of the mitogen-activated protein kinase (MAPK) signaling pathway induced only a transient increase in glial fibrillary acidic protein (GFAP) expression, and inhibition of this signaling pathway did not block the induction by CNTF of glial differentiation in progenitor cells. In addition we show that microelectroporation is a new powerful method for introducing antisense agents into single cells in complex cellular networks.

Impact of neonatal kainate treatment on hippocampal insulin-like growth factor receptors

The insulin-like growth factors-I and -II have neurotrophic properties and act through specific membrane receptors. High levels of binding sites for these growth factors are distributed discretely throughout the brain, being concentrated in the hippocampal formation. Functionally, the insulin-like growth factors, in addition to their growth-promoting actions, are considered to play important roles in normal cell functions, as well as in response to pharmacological or surgical manipulations. In adult rats, we have previously shown that systemic injection of kainate produces an overall decrease, in a time-dependent manner, in insulin-like growth factor-I and -II receptor binding sites in the hippocampus [Kar S. et al. (1997) Neuroscience 80, 1041-1055]. Given the evidence that insulin-like growth factors play a critical role during the early stages of brain development, the present study is a logical extension of this earlier report and established the effect of neonatal kainate injection on the developmental profile of insulin-like growth factor receptors. We have evaluated the time-course alteration of these receptors following systemic injection of kainate to newborn rats. After injection of a sublethal dose of kainate (5 mg/kg, i.p.) to postnatal one-day-old pups, [125I]insulin-like growth factor-I, [125I]insulin-like growth factor-II and [125I]insulin binding sites were studied at different postnatal days (7, 14, 21, 28 and 35) using receptor autoradiography. In the developing hippocampus, insulin-like growth factor-I and insulin binding sites are concentrated primarily in the dentate gyrus and the CA2/CA3 subfields, whereas insulin-like growth factor-II binding is discretely localized to the pyramidal layer and the granular layer of the dentate gyrus. Following kainate injection, we observed a slight increase in insulin-like growth factor-I binding sites in given hippocampal subfields starting at postnatal day 14, being significant at day 21. At later days, a progressive decrease was noted. This transient increase may represent an attempt for neuronal plasticity by up-regulating receptor levels. In contrast, insulin-like growth factor-II and insulin receptor binding sites are found to be decreased in various regions of the hippocampus in kainate-treated pups. Taken together, these results provide further evidence for the existence and differential alterations of insulin-like growth factor-I, insulin-like growth factor-II and insulin receptors in the developing rat hippocampus following kainate-induced lesion, suggesting possible involvement of these growth factors in brain plasticity.

The role of neuronal growth factors in neurodegenerative disorders of the human brain

Recent evidence suggests that neurotrophic factors that promote the survival or differentiation of developing neurons may also protect mature neurons from neuronal atrophy in the degenerating human brain. Furthermore, it has been proposed that the pathogenesis of human neurodegenerative disorders may be due to an alteration in neurotrophic factor and/or trk receptor levels. The use of neurotrophic factors as therapeutic agents is a novel approach aimed at restoring and maintaining neuronal function in the central nervous system (CNS). Research is currently being undertaken to determine potential mechanisms to deliver neurotrophic factors to selectively vulnerable regions of the CNS. However, while there is widespread interest in the use of neurotrophic factors to prevent and/or reduce the neuronal cell loss and atrophy observed in neurodegenerative disorders, little research has been performed examining the expression and functional role of these factors in the normal and diseased human brain. This review will discuss recent studies and examine the role members of the nerve growth factor family (NGF, BDNF and NT-3) and trk receptors as well as additional growth factors (GDNF, TGF-alpha and IGF-I) may play in neurodegenerative disorders of the human brain.

Induction of IGF-1 mRNA expression following traumatic injury to the postnatal brain

A variety of adult, non-neural tissues respond to injury by increasing expression of the gene which encodes for insulin-like growth factor-1 (IGF-1). This response is thought to be a key component in the regenerative capacity of these tissues. In contrast, the central nervous system (CNS) has relatively little regenerative capacity following injury. Interestingly, compared to many non-neuronal tissues, little IGF-1 mRNA can be detected in the adult CNS, raising the possibility that its lack of regenerative capacity is related its relative lack of IGF-1 expression. However, in the 2-week-old adolescent CNS IGF-1 mRNA can be detected in numerous brain regions. Therefore, the purpose of this study was to determine the responsiveness of the IGF-1 gene to injury in adolescent CNS tissue, a period in which expression of this gene is relatively abundant. Expression of IGF-1 mRNA was measured by means of a sensitive solution hybridization/RNase protection assay in the parieto-occipital lobes of 2-week-old and adult mice following penetrating injury. Levels of IGF-1 transcript in the injured brains were significantly increased above those of controls in both 2-week-old and adult brains 3-day post injury and remained elevated for 1 week after injury. These observations demonstrate that the adult CNS, like other tissues, can respond to injury by increasing expression of IGF-1 mRNA.

Insulin-like growth factor-I (IGF-I) immunoreactivity in the Alzheimer's disease temporal cortex and hippocampus

IGF-I has been shown to enhance neuronal survival and inhibit apoptosis. IGF-I immunoreactivity was examined in the Alzheimer's disease and normal post-mortem human hippocampus and temporal cortex to determine whether IGF-I protein levels are altered in response to neurodegeneration. IGF-I immunoreactivity was induced in a subpopulation of GFAP-immunopositive astroglia in the Alzheimer's disease temporal cortex. These observations raise the possibility that IGF-I has a neuroprotective role in the Alzheimer's disease brain.

Systemic administration of kainic acid induces selective time dependent decrease in [125I]insulin-like growth factor I, [125I]insulin-like growth factor II and [125I]insulin receptor binding sites in adult rat hippocampal formation

Administration of kainic acid evokes acute seizure in hippocampal pathways that results in a complex sequence of functional and structural alterations resembling human temporal lobe epilepsy. The structural alterations induced by kainic acid include selective loss of neurones in CA1-CA3 subfields and the hilar region of the dentate gyrus followed by sprouting and permanent reorganization of the synaptic connections of the mossy fibre pathways. Although the neuronal degeneration and process of reactive synaptogenesis have been extensively studied, at present little is known about means to prevent pathological conditions leading to kainate-induced cell death. In the present study, to address the role of insulin-like growth factors I and II, and insulin in neuronal survival as well as synaptic reorganization following kainate-induced seizure, the time course alterations of the corresponding receptors were evaluated. Additionally, using histological preparations, the temporal profile of neuronal degeneration and hypertrophy of resident astroglial cells were also studied. [125I]Insulin-like growth factor I binding was found to be decreased transiently in almost all regions of the hippocampal formation at 12 h following treatment with kainic acid. The dentate hilar region however, exhibited protracted decreases in [125I]insulin-like growth factor I receptor sites throughout (i.e. 30 days) the study. [125I]Insulin-like growth factor II receptor binding sites in the hippocampal formation were found to be differentially altered following systemic administration of kainic acid. A significant decrease in [125I]insulin-like growth factor II receptor sites was observed in CA1 subfield and the pyramidal cell layer of the Ammon's horn at all time points studied whereas the hilar region and the stratum radiatum did not exhibit alteration at any time. A kainate-induced decrease in [125I]insulin receptor binding was noted at all time points in the molecular layer of the dentate gyrus whereas binding in CA1-CA3 subfields and discrete layers of the Ammon's horn was found to be affected only after 12 h of treatment. These results, when analysed with reference to the observed histological changes and established neurotrophic/protective roles of insulin-like growth factors and insulin, suggest possible involvement of these growth factors in the cascade of neurotrophic events that is associated with the reorganization of the hippocampal formation observed following kainate-induced seizures.

Spatial and temporal changes in the insulin-like growth factor (IGF) axis indicate autocrine/paracrine actions of IGF-I within wounds of the rat brain

A precise role for insulin-like growth factors (IGFs), IGF-binding proteins (IGFBPs), and IGF-receptors (IGF-Rs) in damaged central nervous system (CNS) tissue has not been elucidated, although their expression in the ischemic brain has been demonstrated. However, little is known of IGF responses after CNS trauma. In this study, we have used ribonuclease protection assay, in situ hybridization, and immunohistochemistry to demonstrate that IGF-I, IGFBPs, and IGF-1R expression alters in response to a penetrating CNS injury. Within penetrant cerebral wounds in the acute phase of the response (1-7 days post lesion; dpl), increased levels of IGF-I, IGFBP-1, -2, -3, -6, and IGF-1R protein were localized to injury responsive astrocytes, neurons and cells of the monocyte lineage. IGF-I, IGFBP-2, and 3 showed a congruency in sites of messenger RNA (mRNA) and peptide expression, with IGF-I and IGFBP-2 mRNA expression predominating. IGF-I, IGFBP-1, and IGFBP-3 protein were also associated with the microvascular endothelium, which was accompanied by increased levels of IGFBP-3 mRNA. These early changes in IGFBP expression probably facilitate IGF-I action. Later in the wounding response (7-14 dpl), the expression of IGFBP-4 and IGFBP-5 peaked within astrocytes and neurons, with IGFBP-5 mRNA being specifically localized to the glia limitans within the wound, suggesting an inhibitory role for these proteins, down-regulating the effects of IGF-I chronically. Our evidence suggests that within penetrating CNS wounds, IGF-I acts in an autocrine/paracrine manner to regulate cellular responses, with its spatial and temporal availability being modulated by the differential presence of stimulatory vs. inhibitory IGFBPs.

Insulin-like growth factors (IGF) enhance three-dimensional (3D) growth of human glioblastomas

Human glioblastomas (gliomas) are characterized as rapidly growing brain tumors which are highly invasive but rarely metastatic. Human gliomas synthesize and secrete increased levels of insulin-like growth factors (IGFs) as well as expressing increased numbers of IGF receptors when compared to normal brain tissue. These observations suggest the existence of an IGF-mediated autocrine mechanism for glioma growth regulation. The purpose of this study was to examine the effect of human recombinant IGF (hrIGF) treatment on the in vitro growth of human glioma monolayer and three-dimensional (3D) multicellular spheroid cultures. The data demonstrate that hrIGF-I treatment of glioma cell lines slightly enhanced tumor monolayer proliferation as measured by [(3)H]thymidine incorporation. In contrast, treatment of glioma spheroids with hrIGF-I or hrDes(1-3)IGF-I, the truncated brain form of IGF-I, dramatically enhanced 3D tumor growth with a 1.5-2-fold reduction in spheroid doubling time (FRSDT). In addition, IGF-treated glioma spheroids were more densely packed than spheroids grown in media alone with no observed necrosis. These data suggest that IGFs will dramatically enhance glioma proliferation when 3D cell-cell contact occurs. This observed enhancement suggests that IGFs both synthesized in the brain and systemically support rapid proliferation of gliomas in vivo.

Insulin-like growth factor I modulates c-Fos induction and astrocytosis in response to neurotoxic insult

Insulin-like growth factor I participates in the cellular response to brain insult by increasing its messenger RNA expression and/or protein levels in the affected area. Although it has been suggested that insulin-like growth factor I is involved in a variety of cellular responses leading to homeostasis, mechanisms involved in its possible trophic effects are largely unknown. Since activation of c-Fos in postmitotic neurons takes place both in response to insulin-like growth factor I and after brain injury, we have investigated whether this early response gene may be involved in the actions of insulin-like growth factor I after brain insult. Partial deafferentation of the cerebellar cortex by 3-acetylpyridine injection elicited c-Fos protein expression on both Purkinje and granule cells of the cerebellar cortex. This neurotoxic insult also triggered gliosis, as determined by an increased number of glial fibrillary acidic protein-positive cells (reactive astrocytes) in the cerebellar cortex. When 3-acetylpyridine-injected animals received a continuous intracerebellar infusion of either a peptidic insulin-like growth factor I receptor antagonist or an insulin-like growth factor I antisense oligonucleotide for two weeks through an osmotic minipump, c-Fos expression was obliterated while reactive gliosis was greatly increased. On the contrary, continuous infusion of insulin-like growth factor I significantly decreased reactive gliosis without affecting the increase in c-Fos expression. These results indicate that insulin-like growth factor I is involved in both the neuronal (c-Fos) and the astrocytic (glial fibrillary acidic protein) activation in response to injury.

The role of the insulin-like growth factors in the central nervous system

Increasing evidence strongly supports a role for insulin-like growth factor-I (IGF-I) in central nervous system (CNS) development. IGF-I, IGF-II, the type IIGF receptor (the cell surface tyrosine kinase receptor that mediates IGF signals), and some IGF binding proteins (IGFBPs; secreted proteins that modulate IGF actions) are expressed in many regions of the CNS beginning in utero. The expression pattern of IGF system proteins during brain growth suggests highly regulated and developmentally timed IGF actions on specific neural cell populations. IGF-I expression is predominantly in neurons and, in many brain regions, peaks in a fashion temporally coincident with periods in development when neuron progenitor proliferation and/or neuritic outgrowth occurs. In contrast, IGF-II expression is confined mainly to cells of mesenchymal and neural crest origin. While expression of type I IGF receptors appears ubiquitous, that of IGFBPs is characterized by regional and developmental specificity, and often occurs coordinately with peaks of IGF expression. In vitro IGF-I has been shown to stimulate the proliferation of neuron progenitors and/or the survival of neurons and oligodendrocytes, and in some cultured neurons, to stimulate function. Transgenic (Tg) mice that overexpress IGF-I in the brain exhibit postnatal brain overgrowth without anatomic abnormality (20-85% increases in weight, depending on the magnitude of expression). In contrast, Tg mice that exhibit ectopic brain expression of IGFBP-1, an inhibitor of IGF action when present in molar excess, manifest postnatal brain growth retardation, and mice with ablated IGF-I gene expression, accomplished by homologous recombination, have brains that are 60% of normal size as adults. Taken together, these in vivo studies indicate that IGF-I can influence the development of most, if not all, brain regions, and suggest that the cerebral cortex and cerebellum are especially sensitive to IGF-I actions. IGF-I's growth-promoting in vivo actions result from its capacity to increase neuron number, at least in certain populations, and from its potent stimulation of myelination. These IGF-I actions, taken together with its neuroprotective effects following CNS and peripheral nerve injury, suggest that it may be of therapeutic benefit in a wide variety of disorders affecting the nervous system.

Basic fibroblast growth factor (bFGF) injection activates the glial reaction in the injured adult rat brain

Reactive gliosis is a reaction of glial cells to trauma which is characterized by a phenotypic modification of astrocytes, as well as by a proliferation and a migration of some of these cells to form a glial scar. This scar is currently considered as a physical impediment to neuronal regrowth but it may also be involved in wound healing since the astrocytes beside microglia play a phagocytic role in the clearance of post-traumatic debris. Growth factors are released in the area of the injury and at least some of them could be involved in gliosis. In order to test directly this possibility, we have injected one of them, the basic fibroblast growth factor (bFGF), into several brain areas (cortex, striatum, hippocampus or corpus callosum) of adult 2-month-old rats in the absence of lesion. A glial reaction was observed after 3 days and was maximum after 7 days. It was characterized by an increase in astrocyte proliferation and in glial fibrillary acidic protein (GFAP) expression, resulting in a higher number of GFAP-positive cells per surface unit, and by an increase in the size and branching of the astroglial processes. The GFAP mRNA levels were also strongly increased following the bFGF injection. These effects resemble the reactive gliosis observed after lesion and suggest that bFGF is actually involved in the triggering of glial reactions which follow brain injury. In further experiments, bFGF was injected in the site of electrolytic lesions made in the same various parts of the brain. These injections did not increase significantly the normal reactive gliosis induced by the lesion alone, but it accelerated some of the effects. It also resulted in a higher labeling index and GFAP mRNA levels were strongly enhanced after a 3-day-post-operative delay. This last observation strengthens the idea that one of the main factors driving the astrogliosis is the bFGF normally released in and around the site of the lesion.

Increased expression of type 1 insulin-like growth factor receptor messenger RNA in rat hippocampal formation is associated with aging and behavioral impairment

Insulin-like growth factor messenger RNAs are expressed in adult rat brain. However, little is known about the effects of aging on the expression of the insulin-like growth factors, their receptors, and their binding proteins in different regions of rat brain. The goal of the current study was to assess whether there is altered expression of the insulin-like growth factor system during normal aging in the hippocampal formation, a region particularly vulnerable to the aging process. A spatial learning task in the Morris water maze was used to assess the cognitive status of young (7-8-month-old) and aged (28-29-month-old) male Long-Evans rats. Sites of expression and abundance of insulin-like growth factor-I, type 1 insulin-like growth factor receptor, and insulin-like growth factor binding protein-4 messenger RNAs were then examined by in situ hybridization histochemistry and solution or northern blot hybridization assays. In situ hybridization histochemistry revealed no qualitative differences in the regional distribution of insulin-like growth factor-I, type 1 receptor, and insulin-like growth factor binding protein-4 messenger RNAs within the hippocampal formation of young and aged rats. However, quantitative analysis of messenger RNA abundance in hippocampal tissue homogenates showed a significant age-related increase in type 1 receptor messenger RNA (n = 25; t = -2.5; P < 0.02). Furthermore, linear regression analysis indicated that type 1 receptor messenger RNA abundance was significantly correlated with spatial learning impairment in the water maze (r = 0.44; P < 0.03) such that greater behavioral impairment was associated with higher type 1 receptor messenger RNA levels in the hippocampal formation. Neither insulin-like growth factor-I nor insulin-like growth factor binding protein-4 messenger RNA abundance was related to age or behavior. However, linear regression revealed a negative correlation between insulin-like growth factor-I messenger RNA abundance and type 1 receptor messenger RNA abundance in aged hippocampus (r = -0.72, P < 0.01). These data indicate that increased hippocampal expression of type 1 receptor messenger RNA is associated with aging and cognitive decline. The correlation between type 1 receptor and insulin-like growth factor-I messenger RNA abundance in the hippocampal formation of aged rats suggests that insulin-like growth factor availability may influence type 1 receptor expression. However, because no overall age difference was found in the amount of insulin-like growth factor-I messenger RNA in the hippocampal formation, decreased insulin-like growth factor from other sources such as the cerebrospinal fluid and the peripheral circulation may be involved in up-regulating type 1 receptor messenger RNA. Alternatively, type 1 receptor messenger RNA regulation may be part of a trophic response to the degenerative and regenerative events that occur within the hippocampal formation during aging.

Insulin-like growth factor-1 is a potent neuronal rescue agent after hypoxic-ischemic injury in fetal lambs

This study was designed to determine the potential of IGF-1 as a neuronal rescue agent after cerebral ischemia. Unanesthetized late gestation fetal sheep were subjected to 30-min cerebral ischemia by inflation of carotid artery occluder cuffs. 2 h later either 0.1 microgram rhIGF-1, 1 microgram rhIGF-1, 10 micrograms rhIGF-1, or vehicle was infused into a lateral cerebral ventricle over 1 h. Histologic outcome was assessed 5 d later. Overall neuronal loss was reduced with 0.1 microgram (P < 0.05) and 1 microgram (P < 0.002) rhIGF-1, but treatment with 10 micrograms was not effective. With 1 microgram rhIGF-1 neuronal loss scores were significantly lower in brain regions examined including cortex, hippocampus, and striatum, whereas with 0.1 microgram rhIGF-1 the parietal cortex and thalamus were not improved and the improvement seen in other regions was less than with 1 microgram rhIGF-1. Treatment with 1 microgram rhIGF-1 also delayed the onset of seizures and reduced their incidence. Moreover, the secondary phase of cytotoxic edema was reduced and delayed in onset. We conclude that low dose rhIGF-1 therapy promotes neuronal rescue after cerebral hypoxic-ischemic injury in utero, but the effect is dose dependent. Importantly, rhIGF-1 is effective and nontoxic when administered 2 h after the hypoxic ischemic insult. This distinguishes IGF-1 from most other neuroprotective therapies and suggests clinical application may be possible.

Parkinson's disease, trophic factors, and adrenal medullary chromaffin cell grafting: basic and clinical studies

Neural transplantation is one of the promising approaches for the treatment of Parkinson's disease. Although the strategy of using adrenal medulla as donor tissue, rather than fetal nigra tissue, started as an alternative method, recent experimental studies demonstrated the efficacy of adrenal medulla grafting as a neurotrophic source. Many methods to increase the survival of grafted chromaffin cells have been developed, some of which have already been applied clinically with encouraging results. This review summarizes the advancements of adrenal medulla grafting in basic and clinical studies. Special attention is focused on the relationship with neurotrophic factors and how we can enhance the survival of grafted chromaffin cells.

Insulin-like growth factor-1 mRNA is increased in deafferented hippocampus: spatiotemporal correspondence of a trophic event with axon sprouting

Deafferentation is known to induce axonal sprouting in adult brain, but the signals that direct this response are not understood. To evaluate the possible roles of insulin-like growth factor-1 (IGF-1) and basic fibroblast growth factor (bFGF) in central axonal sprouting, the present study used in situ hybridization to evaluate IGF-1 and bFGF mRNA expression in entorhinal deafferented rat hippocampus. Alternate tissue sections were processed for Fink-Heimer impregnation of axonal degeneration, Bandeiraea simplicifolia (BS-1) labeling of microglia, and glial fibrillary acidic protein immunocytochemistry. In control hippocampus, IGF-1 mRNA was localized to a few neurons, with no labeled cells in the dentate gyrus molecular layer; bFGF cRNA hybridization was diffuse in dendritic fields but was dense in CA2 stratum pyramidale. Both mRNA species were increased by deafferentation. The distribution of elevated IGF-1 mRNA corresponded precisely to fields of axonal degeneration and was greatest in the dentate gyrus outer molecular layer and stratum lacunosum moleculare. In these fields, IGF-1 mRNA was elevated by 2 days, reached maximal levels at 4 days, and declined by 10 days postlesion. Double labeling revealed that the majority of IGF-1 cRNA-labeled cells were microglia. In deafferented hippocampus, bFGF mRNA was broadly increased across fields both containing and lacking axonal degeneration. In the dentate, bFGF mRNA levels peaked at 5 days postlesion and remained elevated through 14 days. These results demonstrate that reactive microglia within deafferented hippocampal laminae express IGF-1 mRNA just prior to and during the period of reactive axonal growth and suggest that IGF-1 plays a role in directing the sprouting of spared afferents into these fields.

Measurement of somatostatin release in rat brain by microdialysis

We determined the most suitable conditions for measuring the somatostatin (SRIF) level by brain microdialysis and investigated its release from the hypothalamus. The relative recovery rate of SRIF was 8.4 +/- 0.5% (mean +/- SE) using a polycarbonate (PC) membrane with the push-pull method at a flow rate of 2 microliters/min. Using tubes with an internal diameter of 0.28 mm and lengths of 5, 25, 50 and 100 cm, the relative recovery rates using a PC membrane with the push method were 8.2 +/- 0.5%, 7.3 +/- 0.6%, 6.2 +/- 0.5% and 4.1 +/- 0.6%, respectively. When using tubes with an internal diameter of 0.1 mm and lengths of 5, 25, 50 and 100 cm, the relative recovery rates were 7.3 +/- 0.7%, 5.6 +/- 1.0%, 3.5 +/- 1.1% and 1.4 +/- 0.7%, respectively. The relative recovery rate was 5.2 +/- 0.5% with a polysulfone (PS-F, Fresenius) membrane, 4.5 +/- 0.4% with a PS-H (Hospal) membrane, 2.6 +/- 0.2% with an ethylenevinyl alcohol membrane (EVAL), 5.1 +/- 0.8% with a polyvinyl alcohol (PVA) membrane and 10.4 +/- 0.8% with a PS-K (Kaneka) membrane. With the push method, the extracellular SRIF level in rat pituitary was 42.8 +/- 1.8 pg/ml with a PC membrane, 23.1 +/- 2.9 pg/ml with an EVAL membrane at a flow rate of 2 microliters/min. With the push-pull method, it was 52.7 +/- 5.2 pg/ml using a PC membrane, 33.5 +/- 2.8 pg/ml using a PVA membrane and 54.4 +/- 3.2 pg/ml using a PS-K membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of insulin-like growth factor binding protein-4 and -5 mRNAs in adult rat forebrain

Accumulating evidence indicates that the insulin-like growth factors (IGFs) can act as neurotrophic factors. A family of at least six IGF binding proteins (IGFBPs) has been characterized. The IGFBPs prolong the half-life of IGFs in plasma and may modulate IGF action in a cell- or tissue-specific fashion. Two recently characterized IGFBPs, IGFBP-4 and -5, have been shown by northern blot hybridization to be expressed in rat brain, but their cellular sites of synthesis are poorly characterized. Because IGFBP-4 and IGFBP-5 could potentially modulate IGF actions in the brain, we used in situ hybridization histochemistry and 35S-labeled IGFBP-4 and IGFBP-5 riboprobes to localize sites of IGFBP-4 and -5 mRNA expression in adult rat brain. The two IGFBP mRNAs are abundantly expressed within discrete regions of brain. The expression patterns of the two genes are largely nonoverlapping. Notably, IGFBP-4 mRNA is highly expressed within hippocampal and cortical areas, whereas IGFBP-5 mRNA is not detected above background in these areas. Within the hippocampus, abundant IGFBP-4 mRNA expression is detected in pyramidal neurons of the subfields of Ammon's horn and the subiculum and in the granule cell layer of the anterior hippocampal continuation. In the cortex, IGFBP-4 mRNA is widely expressed in most areas and layers. In contrast, IGFBP-5, but not IGFBP-4, mRNA is detected within thalamic nuclei, leptomeninges, and perivascular sheaths. The distinct expression patterns of IGFBP-4 and -5 mRNAs within the brain suggest that these IGFBPs may modulate paracrine/autocrine actions of the IGFs in discrete brain regions or compartmentalization of the IGFs within the brain.

The effects of IGF-1 treatment after hypoxic-ischemic brain injury in adult rats

Intraventricular injection of insulin-like growth factor 1 (IGF-1) 2 h after hypoxic-ischemic injury reduces neuronal loss. To clarify the mode of action, we compared histological outcome between treatment groups in the following three studies: 0, 0.5, 5, and 50 micrograms IGF-1 given 2 h after injury; 0 and 20 micrograms IGF-1 given 1 h before; and 20 micrograms IGF-1 and insulin or vehicle alone given 2 h after. Unilateral hypoxic-ischemic injury was induced in adult rats by ligation of the right carotid and exposure to 6% O2 for 10 min. Histological outcome was evaluated in the cortex, striatum, and hippocampus 5 days later. Five to 50 micrograms IGF-1 reduced the incidence of infarction and neuronal loss in a dose-dependent manner in all regions (p < 0.05), and 50 micrograms reduced the infarction rate from 87 to 26% (p < 0.01). Pretreatment did not alter outcome. IGF-1 improved outcome compared with equimolar doses of insulin (p < 0.05) and did not affect systemic glucose concentrations or cortical temperature. The results indicate that the neuronal protective effects of IGF-1 are specific and are not mediated via insulin receptors, hypothermia, or hypoglycemic mechanisms. Centrally administered IGF-1 appears to provide worthwhile trophic support to cells within most cerebral structures after transient hypoxic-ischemic injury.

Effect of embryonic hippocampal transplantation in amygdaloid kindled rat

Embryonic neural tissue was transplanted into previously kindled rats. A thirteen- to fourteen-day embryonic hippocampal cell suspension was grafted in the stratum oriens near the CA2 area of the hippocampus. Almost 80% of the animals had a good recovery and became seizure-free. Injection of neocortical cells or saline did not show any positive effect on the kindling susceptibility. Although 20 day embryonic cell transplantation was also effective, the effect did not last as long as the 13- to 14-day embryonic transplantation. These observations open the possibility that the neural grafts may be used for therapy of medically intractable epilepsies.

Expression of insulin-like growth factor I by astrocytes in response to injury

Astrocytes are known to express several growth factors in response to injury and neurological disease. Insulin-like growth factor I (IGF-I) induces astrocytes to divide in vitro and is expressed by developing, but not adult astrocytes both in vivo and in vitro. We tested whether IGF-I is re-expressed by reactive astrocytes in response to injury. We found that astrocytes surrounding the lesioned parenchyma after introduction of a cannula through the cerebral cortex, hippocampus and midbrain contain high levels of immunoreactive IGF-I, as determined by immunocytochemistry using a highly sensitive and specific anti-IGF-I monoclonal antibody. Interestingly, the contralateral hippocampus also contained IGF-I positive astrocytes although in substantial lower numbers. Intact animals showed no detectable IGF-I immunoreactivity in astrocytes. IGF-I was detected at the first time point tested after the lesion was made, 1 week, and for at least 1 month thereafter. Reactive astrocytes expressing high levels of glial fibrillary acidic protein were found in a much wider distribution all along the lesioned area and beyond. We conclude that mechanical injury of the brain induces a specific pattern of expression of IGF-I by a subpopulation of astrocytes. These findings suggest that IGF-I is participating in the response of astrocytes to injury.