Serum growth factors and neuroprotective surveillance: focus on IGF-1

IGF1, growth pathway polymorphisms and schizophrenia: a pooling study

It has been hypothesized that insulin-like growth factors (IGFs) and components of the growth-hormone (GH)-IGF axis may underlie reported associations of poor fetal and childhood growth with schizophrenia. We have investigated the association of schizophrenia with 16 SNPs spanning the IGF1 gene with an inter-marker distance of approximately 2-3 kb. We also examined associations with four common functional polymorphisms of genes involved in aspects of the GH-IGF system--the IGF1 receptor (IGF1R), insulin receptor substrate (IRS1), growth hormone (GH1), and IGF binding protein-3 (IGFBP3). The study was based on an analysis of pooled DNA samples from 648 UK and Irish cases of schizophrenia and 712 blood donor controls and of 297 Bulgarian parent offspring trios. In replicated pool analyses, none of the 16 SNPs in IGF1 nor the 4 key SNPs in the other growth pathway genes were associated with schizophrenia. SNP coverage of IGF1 was extensive, so our findings do not support a major role for IGF-I in the aetiology of schizophrenia.

Physical training decreases susceptibility to subsequent pilocarpine-induced seizures in the rat

Regular motor activity has many benefits for mental and physical condition but its implications for epilepsy are still controversial. In order to elucidate this problem, we have studied the effect of long-term physical activity on susceptibility to subsequent seizures. Male Wistar rats were subjected to repeated training sessions in a treadmill and swimming pool. Thereafter, seizures were induced by pilocarpine injections in trained and non-trained control groups. During the acute period of status epilepticus, we measured: (1) the latency of the first motor sign, (2) the intensity of seizures, (3) the time when it occurred within the 6-h observation period, and (4) the time when the acute period ended. All these behavioral parameters showed statistically significant changes suggesting that regular physical exercises decrease susceptibility to subsequently induced seizures and ameliorate the course of experimentally induced status epilepticus.

Choroid plexus megalin is involved in neuroprotection by serum insulin-like growth factor I

The involvement of circulating insulin-like growth factor I (IGF-I) in the beneficial effects of physical exercise on the brain makes this abundant serum growth factor a physiologically relevant neuroprotective signal. However, the mechanisms underlying neuroprotection by serum IGF-I remain primarily unknown. Among many other neuroprotective actions, IGF-I enhances clearance of brain amyloid beta (Abeta) by modulating transport/production of Abeta carriers at the blood-brain interface in the choroid plexus. We found that physical exercise increases the levels of the choroid plexus endocytic receptor megalin/low-density lipoprotein receptor-related protein-2 (LRP2), a multicargo transporter known to participate in brain uptake of Abeta carriers. By manipulating choroid plexus megalin levels through viral-directed overexpression and RNA interference, we observed that megalin mediates IGF-I-induced clearance of Abeta and is involved in IGF-I transport into the brain. Through this dual role, megalin participates in the neuroprotective actions of IGF-I including prevention of tau hyperphosphorylation and maintenance of cognitive function in a variety of animal models of cognitive loss. Because we found that in normal aged animals, choroid plexus megalin/LRP2 is decreased, an attenuated IGF-I/megalin input may contribute to increased risk of neurodegeneration, including late-onset Alzheimer's disease.

Effects of insulin-like growth factor 1 on voltage-gated ion channels in cultured rat hippocampal neurons

Insulin-like growth factor 1 (IGF-1) has important functions in the brain, including metabolic, neurotrophic, neuromodulatory, and neuroendocrine actions, and it is also prevents amyloid beta-induced death of hippocampal neurons. However, its functions on the voltage-gated ion channels in hippocampus remain uncertain. In the present study, we investigated the effects of IGF-1 on voltage-gated potassium, sodium, and calcium channels in the cultured rat hippocampal neurons using the whole-cell patch clamp recordings. Following incubation with different doses of IGF-1 for 24 h, a block of the peak transient A-type K+ currents amplitude (IC50: 4.425 ng/ml, Hill coefficient: 0.621) was observed. In addition, after the application of IGF-1, the amplitude of high-voltage activated Ca2+ currents significantly increased but activation kinetics did not significantly alter (V1/2: -33.45 +/- 1.32 mV, k = 6.16 +/- 1.05) compared to control conditions (V1/2: -33.19 +/- 2.28 mV, k = 7.26 +/- 1.71). However, the amplitude of Na+, K+, and low-voltage activated Ca2+ currents was not affected by the application of IGF-1. These data suggest that IGF-1 inhibits transient A-type K+ currents and enhances high-voltage-activated Ca2+ currents, but has no effects on Na+ and low-voltage-activated Ca2+ currents.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

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

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

Association of insulin-like growth factor I and insulin-like growth factor-binding protein-3 with intelligence quotient among 8- to 9-year-old children in the Avon Longitudinal Study of Parents and Children

BACKGROUND

Insulin-like growth factor I (IGF-I) is a hormone that mediates the effects of growth hormone and plays a critical role in somatic growth regulation and organ development. It is hypothesized that it also plays a key role in human brain development. Previous studies have investigated the association of low IGF-I levels attributable to growth hormone receptor deficiency with intelligence but produced mixed results. We are aware of no studies that investigated the association of IGF-I levels with IQ in population samples of normal children.

OBJECTIVES

To investigate the association of circulating levels of IGF-I and its principle binding protein, IGF-binding protein-3 (IGFBP-3), in childhood with subsequent measures of IQ.

METHODS

The cohort study was based on data for 547 white singleton boys and girls, members of the Avon Longitudinal Study of Parents and Children, with IGF-I and IGFBP-3 measurements (obtained at a mean age of 8.0 years) and IQ measured with the Wechsler Intelligence Scale for Children (at a mean age of 8.7 years). We also investigated associations with measures of speech and language based on the Wechsler Objective Reading Dimensions test (measured at an age of 7.5 years) and the Wechsler Objective Language Dimensions test (listening comprehension subtest only, measured at an age of 8.7 years). For some children (n = 407), IGF-I (but not IGFBP-3) levels had been measured at approximately 5 years of age in a previous study. Linear regression models were used to investigate associations of the IGF-I system with the measures of cognitive function.

RESULTS

Three hundred one boys and 246 girls were included in the sample. IGF-I levels (mean +/- SD) were 142.6 +/- 53.9 ng/mL for boys and 154.4 +/- 51.6 ng/mL for girls. IQ scores (mean +/- SD) were 106.05 +/- 16.6 and 105.27 +/- 15.6 for boys and girls, respectively. IGF-I levels were associated positively with intelligence. For every 100 ng/mL increase in IGF-I, IQ increased by 3.18 points (95% confidence interval [CI]: 0.52 to 5.84 points). These positive associations were seen in relation to the verbal component (coefficient: 4.27; 95% CI: 1.62 to 6.92), rather than the performance component (coefficient: 1.06; 95% CI: -1.67 to 3.78), of IQ. There was no evidence that associations with overall IQ differed between boys and girls. In a data set with complete information on confounders (n = 484), controlling for birth weight (adjusted for gestation), breastfeeding, and BMI slightly strengthened the associations of IGF-I levels with IQ. Additionally controlling for maternal education and IGFBP-3 levels attenuated the associations (change in IQ for every 100 ng/mL increase in IGF-I levels: 2.51 points; 95% CI: -0.42 to 5.44 points). The weakening of associations in models controlling for markers of parental socioeconomic position and education could reflect shared influences of parental IGF levels on parents' own educational attainment and their offspring's IGF-I levels. In unadjusted models examining associations of Wechsler Objective Reading Dimensions and Wechsler Objective Language Dimensions test scores with IGF-I levels, there was no strong evidence that performance on either of these tests was associated with circulating IGF-I levels, although positive associations were seen with both measures. Associations between IGF-I levels measured at age 5 and Wechsler Intelligence Scale for Children scores (n = 407) were similar to those for IGF-I levels measured at age 7 to 8. For every 100 ng/mL increase in IGF-I levels at 5 years of age, IQ increased by 2.3 points (95% CI: -0.21 to 4.89 points).

CONCLUSIONS

This study provides some preliminary evidence that IGF-I is associated with brain development in childhood. Additional longitudinal research is required to clarify the role of IGF-I in neurodevelopment. Because IGF-I levels are modifiable through diet and other environmental exposures, this may be one pathway through which the childhood environment may influence neurodevelopment.

Endurance exercise regimens induce differential effects on brain-derived neurotrophic factor, synapsin-I and insulin-like growth factor I after focal ischemia

The optimal amount of endurance exercise required to elevate proteins involved in neuroplasticity during stroke rehabilitation is not known. This study compared the effects of varying intensities and durations of endurance exercise using both motorized and voluntary running wheels after endothelin-I-induced focal ischemia in rats. Hippocampal levels of brain-derived neurotrophic factor, insulin-like growth factor I and synapsin-I were elevated in the ischemic hemisphere even in sedentary animals suggesting an intrinsic restorative response 2 weeks after ischemia. In the sensorimotor cortex and the hippocampus of the intact hemisphere, one episode of moderate walking exercise, but not more intense running, resulted in the greatest increases in levels of brain-derived neurotrophic factor and synapsin-I. Exercise did not increase brain-derived neurotrophic factor, insulin-like growth factor I or synapsin-I in the ischemic hemisphere. In voluntary running animals, both brain and serum insulin-like growth factor I appeared to be intensity dependent and were associated with decreasing serum levels of insulin-like growth factor I and increasing hippocampal levels of insulin-like growth factor I in the ischemic hemisphere. This supports the notion that exercise facilitates the movement of insulin-like growth factor I across the blood-brain barrier. Serum corticosterone levels were elevated by all exercise regimens and were highest in rats exposed to motorized running of greater speed or duration. The elevation of corticosterone did not seem to alter the expression of the proteins measured, however, graduated exercise protocols may be indicated early after stroke. These findings suggest that relatively modest exercise intervention can increase proteins involved in synaptic plasticity in areas of the brain that likely subserve motor relearning after stroke.

A role of insulin-like growth factor 1 in beta amyloid-induced disinhibition of hippocampal neurons

In the present study we investigated the effects of beta amyloid (Abeta) on inhibitory synaptic transmission in the cultured hippocampal neurons using whole-cell patch-clamp recordings and immunocytochemistry, and examined the role of insulin-like growth factor 1 (IGF-1). Incubation with 4 microM Abeta25-35 for 24 h significantly decreased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs), but had no effect on the mean amplitude. Pretreatment with 10 ng/ml IGF-1 for 24h prior to Abeta25-35 exposure blocked Abeta-induced disinhibition of hippocampal neurons. The frequency and mean amplitude of miniature IPSC (mIPSCs) were not significantly affected by Abeta. The rise and decay kinetics of sIPSCs and mIPSCs were similar for the control and Abeta25-35-treated hippocampal neurons. Immunocytochemistry showed no changes in the ratio of gamma-aminobutyric acid (GABA) positive cells subsequent to treatment with Abeta, or IGF-1. Together these data suggest that Abeta-induced the disinhibition in cultured hippocampal neurons, whereas IGF-1 could block this effect.

Local expression of GH and IGF-1 in the hippocampus of GH-deficient long-lived mice

Beneficial effects of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) on the development and function of the central nervous system are well documented. In spite of primary deficiency of GH and secondary IGF-1 deficiency, Ames dwarf mice live considerably longer than normal animals, exhibit apparently normal cognitive functions and maintain them into advanced age. In an attempt to reconcile these findings, we have examined local expression of GH and IGF-1 in the hippocampus of normal and Ames dwarf mice. We found that both hippocampal GH and IGF-1 protein levels are increased and the corresponding mRNAs are normal in Ames dwarf as compared with normal mice. Increased phosphorylation of Akt and cyclic AMP responsive element-binding protein (CREB) were detected in the hippocampus of Ames dwarf mice. Our results suggest that increase in hippocampal GH and IGF-1 protein expression and subsequent activation of PI3K/Akt-CREB signal transduction cascade might contribute to the maintenance of cognitive function and is likely to be responsible for the integrity of neuronal structure, and maintenance of youthful levels of cognitive function in these long-lived mice during aging.

SIRT1, neuronal cell survival and the insulin/IGF-1 aging paradox

Signaling through the insulin/IGF-1 pro-survival pathway is widely recognized to be neuroprotective as well as important for neuronal growth and physiology. In mammals, age-associated decline in circulating IGF-1 levels has been associated with neuronal aging and symptoms of neurodegeneration. Defects in IGF-1 receptor associated signaling has, however, been shown to significantly extend lifespan in models ranging from invertebrates to mouse. At least in C. elegans, restoring such defects in neurons alone reduces lifespan to wild-type levels. As we seek to delay brain aging and age-associated neuronal degeneration via nutritional and endocrinal supplements, an understanding of the mechanistic basis of this apparent paradox is important. Recent elucidation of the role of the protein deacetylase SIRT1 in cell survival and data associating IGF-1 with the regulation of SIRT1 expression may provide a direction towards resolving this issue.

Igf-I and postnatal growth of weaver mutant mice

IGF-I is an anabolic growth factor essential for growth and development, both as a mediator of growth hormone (GH) action and as a local stimulator of cell proliferation and differentiation. Although the importance of IGF-I in postnatal growth has been studied for several decades, its functions in pathological states are not fully understood. The weaver (wv) mutant mouse is a commonly used model for studying hereditary cerebellar ataxia and provides us with an opportunity to study the function of IGF-I in postnatal growth during neurodegeneration. In prepubertal wv mice, we found a parallel decrease in body weight and serum IGF-I. This parallel relationship was maintained in females, but not in males, as wv mice entered puberty. Interestingly, we found an increase in the levels of circulating IGF-I and hepatic mRNA preceded the catch-up of body weight of pubertal male wv mice. The increase in IGF-I levels coincided with a surge of circulating androgen at the onset of male puberty, suggesting that androgen might trigger the increase in IGF-I production in the pubertal and adult male wv mice. Overall, our results support the concept that IGF-I plays an important role in postnatal growth during and after neurodegeneration of wv mice. In addition, IGF-I's regulation of systemic growth during and after puberty is likely modulated by androgen in male wv mice.

Microspheres containing insulin-like growth factor I for treatment of chronic neurodegeneration

The therapeutic potential of peptide growth factors as insulin-like growth factor I (IGF-I) is currently under intense scrutiny in a wide variety of diseases, including neurodegenerative illnesses. A new poly(lactic-co-glycolide)-based microsphere IGF-I controlled release formulation for subcutaneous (SC) delivery has been developed by a triple emulsion method. The resulting microspheres displayed a mean diameter of 1.5microm, with an encapsulation efficiency of 74.3%. The protein retained integrity after the microencapsulation process as evaluated by circular dichroism and SDS-PAGE. The administration of IGF-I in microspheres caused at least a 30-fold increase in IGF-I mean residence time in rats and mice when compared with the conventional SC solution. Therefore, dosing can be changed from the conventional twice a day to once every 2 weeks. Therapeutic efficacy of this new formulation has been studied in mutant mice with inherited Purkinje cell degeneration (PCD). These mice show a chronic limb discoordination that is resolved after continuous systemic delivery of IGF-I. Normal motor coordination was maintained as long as IGF-I microsphere therapy is continued. Moreover, severely affected PCD mice, with marked ataxia, muscle wasting and shortened life-span showed a significant improvement after continuous IGF-I microsphere therapy as determined by enhanced motor coordination, marked weight gain and extended survival. This new formulation can be considered of great therapeutic promise for some chronic brain diseases.

Insulin-like growth factor I is required for vessel remodeling in the adult brain

Although vascular dysfunction is a major suspect in the etiology of several important neurodegenerative diseases, the signals involved in vessel homeostasis in the brain are still poorly understood. We have determined whether insulin-like growth factor I (IGF-I), a wide-spectrum growth factor with angiogenic actions, participates in vascular remodeling in the adult brain. IGF-I induces the growth of cultured brain endothelial cells through hypoxiainducible factor 1 alpha and vascular endothelial growth factor, a canonical angiogenic pathway. Furthermore, the systemic injection of IGF-I in adult mice increases brain vessel density. Physical exercise that stimulates widespread brain vessel growth in normal mice fails to do so in mice with low serum IGF-I. Brain injury that stimulates angiogenesis at the injury site also requires IGF-I to promote perilesion vessel growth, because blockade of IGF-I input by an anti-IGF-I abrogates vascular growth at the injury site. Thus, IGF-I participates in vessel remodeling in the adult brain. Low serum/brain IGF-I levels that are associated with old age and with several neurodegenerative diseases may be related to an increased risk of vascular dysfunction.

Role of serum insulin-like growth factor I in mammalian brain aging

Modern societies face new public health challenges associated with an increasingly aging population. Among these, pathological conditions linked to brain aging are paramount. Old age is a risk factor for important neurological impairments such as Alzheimer's disease or stroke. Even healthy elderly people usually present with milder forms of cognitive decline. This is possibly related to less-pronounced brain deficits seen in normal aging, including the shrinkage of neurons and the dense network of neurons and glia in the central nervous system known as the neuropil, a lower neurogenetic rate, impaired angiogenesis or brain accumulation of deleterious compounds. At least in mammals, age is also associated with a decline in insulin-like growth factor-I (IGF-I) levels, a well-known neuroprotective agent. Recently, a relationship between serum IGF-I and "house-keeping" mechanisms in the brain has been evidenced in laboratory rodents. Serum IGF-I increases adult neurogenesis, sustains neuronal health through a variety of fundamental homeostatic mechanisms, participates in brain angiogenesis, contributes to brain beta-amyloid clearance and affects learning and memory. Overall, diminished trophic input resulting from decreasing serum IGF-I levels during aging likely contributes to brain senescence in mammals.

Glutamate excitotoxicity attenuates insulin-like growth factor-I prosurvival signaling

Recent evidence suggests that impaired insulin/insulin-like growth factor I (IGF-I) input may be associated to neurodegeneration. Several major neurodegenerative diseases involve excitotoxic cell injury whereby excess glutamate signaling leads to neuronal death. Recently it was shown that glutamate inactivates Akt, a serine-kinase crucially involved in the prosurvival actions of IGF-I. We now report that excitotoxic doses of glutamate antagonize Akt activation by IGF-I and inhibit the neuroprotective effects of this growth factor on cultured neurons. Glutamate induces loss of sensitivity to IGF-I by phosphorylating the IGF-I receptor docking protein insulin-receptor-substrate (IRS)-1 in Ser(307) through a pathway involving activation of PKA and PKC in a hierarchical fashion. Administration of Ro320432, a selective PKC inhibitor, abrogates the inhibitory effects of glutamate on IGF-I-induced Akt activation in vitro and in vivo and is sufficient to block the neurotoxic action of glutamate on cultured neurons. Notably, administration of Ro320432 after ischemic insult, a major form of excitotoxic injury in vivo, results in a marked decrease ( approximately 50%) in infarct size. Therefore, uncoupling of IGF-I signaling by glutamate may constitute an additional route contributing to excitotoxic neuronal injury. Further work should determine the potential use of PKC inhibitors as a novel therapeutic strategy in ischemia and other excitotoxic insults.

Glucose transporter GLUT8 translocation in neurons is not insulin responsive

We examined the subcellular distribution of a novel glucose transporter isoform (GLUT8) in murine N2A neuroblastoma cells. Exogenous expression of GLUT8-green fluorescent protein (GFP) DNA constructs mimicked the endogenous GLUT8 localization to intracellular vesicles and minimally to the Giantin-positive Golgi. This distribution was unlike the distributions of endogenous GLUT1 and GLUT3 (predominant neuronal isoform), which were limited predominantly to the plasma membrane and minimal in the cytoplasm. Although GLUT4-GFP (insulin responsive isoform) was expressed transiently, no endogenous GLUT4 was detected in N2A cells. By employing stable transfectants that expressed GLUT8-GFP, the effect of insulin and insulin-like growth factor-I, potassium chloride (depolarized state), and 3% oxygen on translocation of GLUT8 to the plasma membrane of N2A cells was examined immunohistochemically and by subfractionation, followed by Western blot analysis. None of these agents translocated GLUT8 to the plasma membrane. However, when the internalization dileucine motif (L(12,13)) of GLUT8 was mutated to a dialanine motif (A(12,13)), GLUT8 colocalized with GLUT3 in the plasma membrane. We conclude that GLUT8 translocation to the N2A cellular plasma membrane is not observed secondary to the various stimuli investigated. Mutation of the N-terminal dileucine motif led to constitutive GLUT8 localization in the plasma membrane. The endogenous stimulus required for translocating neuronal GLUT8 is unknown. This stimulus, which is necessary for uncoupling the "cytoplasmic vesicular anchor" of GLUT8, would be crucial for its glucose-transporting function.

Cell death in the nervous system: lessons from insulin and insulin-like growth factors

Programmed cell death is an essential process for proper neural development. Cell death, with its similar regulatory and executory mechanisms, also contributes to the origin or progression of many or even all neurodegenerative diseases. An understanding of the mechanisms that regulate cell death during neural development may provide new targets and tools to prevent neurodegeneration. Many studies that have focused mainly on insulin-like growth factor-I (IGF-I), have shown that insulin-related growth factors are widely expressed in the developing and adult nervous system, and positively modulate a number of processes during neural development, as well as in adult neuronal and glial physiology. These factors also show neuroprotective effects following neural damage. Although some specific actions have been demonstrated to be anti-apoptotic, we propose that a broad neuroprotective role is the foundation for many of the observed functions of the insulin-related growth factors, whose therapeutical potential for nervous system disorders may be greater than currently accepted.

An antagonist of estrogen receptors blocks the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus of adult female rat

Interdependence between estradiol and insulin-like growth factor-I has been documented for different neural events, including neuronal differentiation, synaptic plasticity, neuroendocrine regulation and neuroprotection. In the present study we have assessed whether both factors interact in the regulation of neurogenesis in the adult rat dentate gyrus. Wistar albino female rats were bilaterally ovariectomized and treated with estradiol, insulin-like growth factor-I and/or the estrogen receptor antagonist ICI 182,780. Estradiol was administered in a subcutaneous silastic capsule. Insulin-like growth factor-I and ICI 182,780 were delivered in the lateral cerebral ventricle. Animals received six daily injections of 5-bromo-2-deoxyuridine and were killed 24 h after the last injection. The total number of 5-bromo-2-deoxyuridine-positive neurons was significantly increased in animals treated with insulin-like growth factor-I, compared with rats treated with vehicles, while rats treated with both insulin-like growth factor-I and estradiol showed a higher number of 5-bromo-2-deoxyuridine-positive neurons than rats treated with insulin-like growth factor-I or estradiol alone. The antiestrogen ICI 182,780 blocked the effect of insulin-like growth factor-I on the number of 5-bromo-2-deoxyuridine neurons with independence of whether the animals were treated or not with estradiol. These findings suggest that estrogen receptors are involved in the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus, and that estradiol and insulin-like growth factor-I have a cooperative interaction to promote neurogenesis. The interaction between insulin-like growth factor-I and estradiol may participate in changes in the rate of neurogenesis during different endocrine and physiological conditions, and may be related to the decline in neurogenesis with ageing.

Potential roles of insulin and IGF-1 in Alzheimer's disease

Aging is characterized by a significant decline of metabolic and hormonal functions, which often facilitates the onset of severe age-associated pathologies. One outstanding example of this is the reported association of deranged signaling by insulin and insulin-like-growth-factor 1 (IGF-1) with Alzheimer's disease (AD). Recent compelling biological data reveal effects of insulin and IGF-1 on molecular and cellular mechanisms underlying the pathology of AD. This review discusses available biological data that highlight the therapeutic potential of the insulin-IGF-1 signaling pathway in AD.

Insulin-like growth factor I modifies electrophysiological properties of rat brain stem neurons

On systemic injection, insulin-like growth factor I (IGF-I) elicits a prolonged increase in the excitability of dorsal column nuclei (DCN) cells in the brain stem as well as other target neurons within the brain. We have explored the cellular mechanisms involved in the stimulatory effects of IGF-I as well as its functional consequences. In a rat slice preparation, IGF-I induced a sustained depolarization of 2-5 mV in 81% of DCN neurons. Depolarization was accompanied with an increase in the input resistance (15%). Voltage-clamp recordings displayed that IGF-I decreased a K+-mediated A current (60%). Furthermore, IGF-I increased, in 78% of cells, the peak amplitude (25%), and rising slope (32%) of the excitatory postsynaptic potential evoked by dorsal column stimulation; in this case, a presynaptic facilitatory process appears to be involved. When anesthetized adult rats are injected in the carotid artery with IGF-I, extracellularly recorded propioceptive DCN neurons not only show increased spike activity but also an expansion of their cutaneous receptive field in 83% of DCN cells. Significantly, the increased excitability evoked by IGF-I in the DCN cells depends both in vivo and in vitro, on activation of p38 mitogen-activated protein kinase (MAPK), a Ser-kinase known to modulate K+ channel activity. We concluded that systemic IGF-I modulated the electrophysiological properties of target neurons within the brain. In turn, these changes probably contribute to functional reorganization processes such as expansion of neuronal receptive fields.

In vivo cellular and molecular mechanisms of neuronal apoptosis in the mammalian CNS

Apoptosis has been recognized to be an essential process during neural development. It is generally assumed that about half of the neurons produced during neurogenesis die before completion of the central nervous system (CNS) maturation, and this process affects nearly all classes of neurons. In this review, we discuss the experimental data in vivo on naturally occurring neuronal death in normal, transgenic and mutant animals, with special attention to the cerebellum as a study model. The emerging picture is that of a dual wave of apoptotic cell death affecting central neurons at different stages of their life. The first wave consists of an early neuronal death of proliferating precursors and young postmitotic neuroblasts, and appears to be closely linked to cell cycle regulation. The second wave affects postmitotic neurons at later stages, and is much better understood in functional terms, mainly on the basis of the neurotrophic concept in its broader definition. The molecular machinery of late apoptotic death of postmitotic neurons more commonly follows the mitochondrial pathway of intracellular signal transduction, but the death receptor pathway may also be involved.Undoubtedly, analysis of naturally occurring neuronal death (NOND) in vivo will offer a basis for parallel and future studies aiming to elucidate the mechanisms of pathologic neuronal loss occurring as the result of conditions such as neurodegenerative disorders, trauma or ischemia.

Brain repair and neuroprotection by serum insulin-like growth factor I

The existence of protective mechanisms in the adult brain is gradually being recognized as an important aspect of brain function. For many years, self-repair processes in the post-embryonic brain were considered of minor consequence or nonexistent. This notion dominated the study of neurotrophism. Thus, although the possibility that neurotrophic factors participate in brain function in adult life was prudently maintained, the majority of the studies on the role of trophic factors in the brain were focused on developmental aspects. With the recent recognition that the adult brain keeps a capacity for cell renewal, although limited, a new interest in the regenerative properties of brain tissue has emerged. New findings on the role of insulin-like growth factor I (IGF-I), a potent neurotrophic peptide present at high levels in serum, may illustrate this current trend. Circulating IGF-I is an important determinant of proper brain function in the adult. Its pleiotropic effects range from classical trophic actions on neurons such as housekeeping or anti-apoptotic/ pro-survival effects to modulation of brain-barrier permeability, neuronal excitability, or new neuron formation. More recent findings indicate that IGF-I participates in physiologically relevant neuroprotective mechanisms such as those triggered by physical exercise. The increasing number of neurotrophic features displayed by serum IGF-I reinforces the view of a physiological neuroprotective network formed by IGF-I, and possibly other still uncharacterized signals. Future studies with IGF-I, and hopefully other neurotrophic factors, will surely reveal and teach us how to potentiate the self-reparative properties of the adult brain.

Sedentary life impairs self-reparative processes in the brain: the role of serum insulin-like growth factor-I

Regular exercise has long being recognized as an important contributor to appropriate health status and is currently recommended to reduce the incidence of many diseases. More recent is the notion that sedentary life may also be a risk factor for neurodegenerative diseases even though for the last decade the beneficial effects of exercise on brain function have been widely documented. In the brain, exercise exerts both acute and long-term changes that can be interpreted as beneficial, such as increased levels of various neurotrophic factors or enhanced cognition. However, the signals involved in exercise-induced changes in the brain are not yet well known. It is generally thought that they arise from the periphery as a direct consequence of increased metabolic activity and aim to elicit adaptive changes in brain function. However, body-to-brain signaling induced by exercise also underlies a different aspect. Exercise induces changes in the brain that are essential for proper brain function. In this view, sedentarism, a relatively new cultural trait, negates the beneficial effects of exercise and paves the way to pathological derangement. A critical step in this process is exercise-induced uptake by the brain of insulin-like growth factor-I (IGF-I), a circulating hormone with potent neurotrophic activity. We summarize the evidence supporting the hypothesis that serum IGF-I is a neuroprotective hormone within a neuroprotective network modulated by physical activity.

Activation of hypoxia-inducible factor-1 in the rat cerebral cortex after transient global ischemia: potential role of insulin-like growth factor-1

Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that regulates the adaptive response to hypoxia in mammalian cells. It consists of a regulatory subunit HIF-1alpha, which accumulates under hypoxic conditions, and a constitutively expressed subunit HIF-1beta. In this study we analyzed HIF-1alpha expression in the rat cerebral cortex after transient global ischemia induced by cardiac arrest and resuscitation. Our results showed that HIF-1alpha accumulates as early as 1 hr of recovery and persists for at least 7 d. In addition, the expression of HIF-1 target genes, erythropoietin and Glut-1, were induced at 12 hr to 7d of recovery. A logical explanation for HIF-1alpha accumulation might be that the brain remained hypoxic for prolonged periods after resuscitation. By using the hypoxic marker 2-(2-nitroimidazole-1[H]-y1)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide (EF5), we showed that the brain is hypoxic during the first hours of recovery from cardiac arrest, but the tissue is no longer hypoxic at 2 d. Thus, the initial ischemic episode must have activated other nonhypoxic mechanisms that maintain prolonged HIF-1alpha accumulation. One such mechanism might be initiated by insulin-like growth factor-1 (IGF-1). Our results showed that IGF-1 expression was upregulated after cardiac arrest and resuscitation. In addition, we showed that IGF-1 was able to induce HIF-1alpha in pheochromocytoma cells and cultured neurons as well as in the brain of rats that received intracerebroventricular and systemic IGF-1 infusion. Moreover, infusion of a selective IGF-1 receptor antagonist abrogates HIF-1alpha accumulation after cardiac arrest and resuscitation. Our study suggest that activation of HIF-1 might be part of the mechanism by which IGF-1 promotes cell survival after cerebral ischemia.

IL-10 promotes survival of microglia without activating Akt

IL-10 is an anti-inflammatory cytokine that has recently been shown to promote survival of neurons and glia. Here we establish that IL-10 induces phosphorylation of Stat3 on Tyr(705) and serves as a survival factor for N13 microglial cells. Recombinant IL-10 (10 ng/ml) decreases growth factor withdrawal-induced apoptosis by 50%, as assessed by TUNEL. In contrast to IL-10, IGF-I increases enzymatic activity of PI 3-kinase and causes phosphorylation on serine(473) of Akt but does not prevent microglial apoptosis. These data establish that IL-10 activates Stat3 and inhibits the mitochondrial pathway of cell death without activating the Akt cell survival pathway.

Circulating insulin-like growth factor I mediates the protective effects of physical exercise against brain insults of different etiology and anatomy

Physical exercise ameliorates age-related neuronal loss and is currently recommended as a therapeutical aid in several neurodegenerative diseases. However, evidence is still lacking to firmly establish whether exercise constitutes a practical neuroprotective strategy. We now show that exercise provides a remarkable protection against brain insults of different etiology and anatomy. Laboratory rodents were submitted to treadmill running (1 km/d) either before or after neurotoxin insult of the hippocampus (domoic acid) or the brainstem (3-acetylpyridine) or along progression of inherited neurodegeneration affecting the cerebellum (Purkinje cell degeneration). In all cases, animals show recovery of behavioral performance compared with sedentary ones, i.e., intact spatial memory in hippocampal-injured mice, and normal or near to normal motor coordination in brainstem- and cerebellum-damaged animals. Furthermore, exercise blocked neuronal impairment or loss in all types of injuries. Because circulating insulin-like growth factor I (IGF-I), a potent neurotrophic hormone, mediates many of the effects of exercise on the brain, we determined whether neuroprotection by exercise is mediated by IGF-I. Indeed, subcutaneous administration of a blocking anti-IGF-I antibody to exercising animals to inhibit exercise-induced brain uptake of IGF-I abrogates the protective effects of exercise in all types of lesions; antibody-treated animals showed sedentary-like brain damage. These results indicate that exercise prevents and protects from brain damage through increased uptake of circulating IGF-I by the brain. The practice of physical exercise is thus strongly recommended as a preventive measure against neuronal demise. These findings also support the use of IGF-I as a therapeutical aid in brain diseases coursing with either acute or progressive neuronal death.