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

Insulin-like growth factor system regulates oligodendroglial cell behavior: therapeutic potential in CNS

Amongst the many soluble extracellular factors stimulating intracellular signal transduction pathways and driving cellular processes such as proliferation, differentiation and survival, insulin-like growth factors (IGFs) stand out as indispensable factors for proper oligodendrocyte differentiation and accompanying myelin production. Owing to its potent myelinogenic capacity and its neuroprotective properties, IGFs hold therapeutic potential in demyelinating and neurodengenerative diseases. However, the IGF system is comprised of a complex molecular network involving regulatory binding proteins, proteases, cell surface and extracellular matrix components which orchestrate IGF-specific functions. Thus, the complexity by which these factors are tightly regulated makes a simplistic therapeutic approach towards treating demyelinating conditions unfeasible. In the present review, we address these issues and consider current therapeutic prospects of oligodendrocyte-targeted IGF-based therapies.

Insulin growth factor signaling mediates neuron-like differentiation of adipose-tissue-derived stem cells

Our previous study showed that adipose tissue-derived stem cells (ADSC) could be induced by isobutylmethylxanthine (IBMX) to differentiate into neuron-like cells. In the present study, ADSC were treated with IBMX in the presence or in the absence of each of eight specific inhibitors of different signaling pathways (JAK/STAT, PKA, PI3K, MEK, Wnt/Frizzled, ERK/MAPK, TGF-beta, and insulin growth factor [IGF]-I). PPP, a specific inhibitor of IGF-I signaling, was the only inhibitor that showed significant inhibition of IBMX-induced ADSC neuronal differentiation, as determined by changes in cell morphology in the initial screening. Further examination by immunofluorescence staining showed that the neuronal marker, beta-III-tubulin, was highly induced in IBMX-treated ADSC, and the induction was significantly suppressed by PPP. Western blotting, followed by densitometry showed that PPP suppressed IBMX-induced beta-III-tubulin expression by 43%, 88%, and 84% when used to treat the cells for 1, 3, and 24 hr, respectively. Treatment of ADSC with IBMX also led to the phosphorylation of IGF-I receptor at tyrosine 1136 (Y1136), as determined by immunofluorescence staining with an antibody that reacts specifically with Y1136. This effect was also abrogated by PPP. Thus, the IBMX-induced neuron-like differentiation of ADSC is mediated by IGF signaling through the phosphorylation of IGF-IR at Y1136.

Astrocytes--friends or foes in multiple sclerosis?

In multiple sclerosis (MS), the presence of demyelinating plaques has concentrated researchers' minds on the role of the oligodendrocyte in its pathophysiology. Recently, with the rediscovery of early and widespread loss of axons in the disease, new emphasis has been put on the role of axons and axon-oligodendrocyte interactions in MS. Despite the fact that, in 1904, Müller claimed that MS was a disease of astrocytes, more recently, astrocytes have taken a back seat, except as the cells that form the final glial scar after all hope of demyelination is over. However, perhaps it is time for the return of the astrocyte to popularity in the pathogenesis of MS, with recent reports on the dual role of astrocytes in aiding degeneration and demyelination, by promoting inflammation, damage of oligodendrocytes and axons, and glial scarring, but also in creating a permissive environment for remyelination by their action on oligodendrocyte precursor migration, oligodendrocyte proliferation, and differentiation. We review these findings to try to provide a cogent view of astrocytes in the pathology of MS.

Peripheral elevation of IGF-1 fails to alter Abeta clearance in multiple in vivo models

Increasing beta-amyloid (Abeta) clearance may alter the course of Alzheimer's disease progression and attenuate amyloid plaque pathology. Insulin-like growth factor I (IGF-1) augmentation has been suggested to increase Abeta clearance by facilitating transport of Abeta out of the brain. The availability of safe agents that increase IGF-1 levels therefore makes IGF-1 elevation an attractive target for disease modifying therapy in AD. The present series of studies sought to replicate published paradigms in which peripheral IGF-1 administration lowered brain Abeta acutely, with reduction in plaque pathology after chronic treatment. Thus Abeta levels were measured in several animal models following treatments that elevated IGF-1. Administration of IGF-1 to young or old rats for up to 3 days had no effect on Abeta levels in brain, CSF, or plasma. In adult beagles, 4 days of dosing with the growth hormone secretagogue, CP-424391, doubled baseline plasma IGF-1 levels, yet failed to alter CSF or plasma Abeta. 5-day treatment of young Tg2576 mice with IGF-1 produced robust elevations of IGF-1 levels in plasma, but no effects on Abeta were detected in brain, CSF, or plasma. Finally, 11-month-old Tg2576 mice were implanted with subcutaneous minipumps delivering IGF-1 for 1 month. No significant changes in Abeta (by ELISA or Western blot), plaque pathology, or phospho-tau epitopes were detected. These results do not demonstrate acute or chronic actions of peripherally administered IGF-1 on Abeta levels or the phosphorylation state of tau and therefore do not suggest any disease-modifying benefits of IGF-1 restorative therapy for AD through these mechanisms.

Expression of insulin-like growth factor system genes during the early postnatal neurogenesis in the mouse hippocampus

Insulin-like growth factor-1 (IGF-1) is essential to hippocampal neurogenesis and the neuronal response to hypoxia/ischemia injury. IGF (IGF-1 and -2) signaling is mediated primarily by the type 1 IGF receptor (IGF-1R) and modulated by six high-affinity binding proteins (IGFBP) and the type 2 IGF receptor (IGF-2R), collectively termed IGF system proteins. Defining the precise cells that express each is essential to understanding their roles. With the exception of IGFBP-1, we found that mouse hippocampus expresses mRNA for each of these proteins during the first 2 weeks of postnatal life. Compared to postnatal day 14 (P14), mRNA abundance at P5 was higher for IGF-1, IGFBP-2, -3, and -5 (by 71%, 108%, 100%, and 98%, respectively), lower for IGF-2, IGF-2R, and IGFBP-6 (by 65%, 78%, and 44%, respectively), and unchanged for IGF-1R and IGFBP-4. Using laser capture microdissection (LCM), we found that granule neurons and pyramidal neurons exhibited identical patterns of expression of IGF-1, IGF-1R, IGF-2R, IGFBP-2, and -4, but did not express other IGF system genes. We then compared IGF system expression in mature granule neurons and their progenitors. Progenitors exhibited higher mRNA levels of IGF-1 and IGF-1R (by 130% and 86%, respectively), lower levels of IGF-2R (by 72%), and similar levels of IGFBP-4. Our data support a role for IGF in hippocampal neurogenesis and provide evidence that IGF actions are regulated within a defined in vivo milieu.

Potential utility of rhIGF-1 in neuromuscular and/or degenerative disease

Neuromuscular/neurodegenerative disorders, such as the death of spinal cord motor neurons in amyotrophic lateral sclerosis (ALS) or the degeneration of spinal cord motor neuron axons in certain peripheral neuropathies, present a unique opportunity for therapeutic intervention with neurotrophic proteins. We have found that in mixed rat embryonic spinal cord cultures or in purified motor neuron preparations, recombinant human insulin-like growth factor 1 (rhIGF-1) enhances the survival of motor neurons at EC50 concentrations of 2 nM, consistent with an interaction at the tyrosine kinase-coupled rhIGF-1 receptor. In a model of programmed cell death in ovo, administration of rhIGF-1 produces a marked survival of motor neurons. In a variety of models of predominantly motor neuron or nerve injury in rodents, administration of rhIGF-1 prevents the death of motor neurons in neonatal facial nerve lesions, attenuates the loss of cholinergic phenotype in adult hypoglossal nerve axotomy and hastens recovery from sciatic nerve crush in mice. In a genetic model of motor neuron compromise, the wobbler mouse, rhIGF-1 (1 mg/kg s.c. daily) delayed the deterioration of grip strength and provided for a more normal distribution of fibre types. In addition, rhIGF-1 (0.3-1.0 mg/kg s.c. daily) prevents the motor and/or sensory neuropathy in rodents caused by vincristine, cisplatinum or Taxol. These combined data indicate that rhIGF-1 has marked effects on the survival of compromised motor neurons and the maintenance of their axons and functional connections. They also suggest the potential utility of rhIGF-1 for the treatment of diseases such as ALS and certain neuropathies.

Insulin-like Growth Factor Type-I Receptor Internalization and Recycling Mediate the Sustained Phosphorylation of Akt

Previously we demonstrated that insulin-like growth factor-I mediates the sustained phosphorylation of Akt, which is essential for long term survival and protection of glial progenitors from glutamate toxicity. These prosurvival effects correlated with prolonged activation and stability of the insulin-like growth factor type-I receptor. In the present study, we investigated the mechanisms whereby insulin-like growth factor-I signaling, through the insulin-like growth factor type-I receptor, mediates the sustained phosphorylation of Akt. We showed that insulin-like growth factor-I stimulation induced loss of receptors from the cell surface but that surface receptors recovered over time. Blocking receptor internalization inhibited Akt phosphorylation, whereas inhibition of receptor trafficking blocked receptor recovery at the cell surface and the sustained phosphorylation of Akt. Moreover the insulin-like growth factor type-I receptor localized with the transferrin receptor and Rab11-positive endosomes in a ligand-dependent manner, further supporting the conclusion that this receptor follows a recycling pathway. Our results provide evidence that ligand stimulation leads to internalization of the insulin-like growth factor type-I receptor, which mediates Akt phosphorylation, and that receptor recycling sustains Akt phosphorylation in glial progenitors. Mathematical modeling of receptor trafficking further supports these results and predicts an additional kinetic state of the receptor consistent with sustained Akt phosphorylation.

IGF2 knockout mice are resistant to kainic acid-induced seizures and neurodegeneration

Insulin-like growth factor 2 (Igf2), a member of the insulin gene family, is important for brain development and has known neurotrophic properties. Though Igf2, its receptors, and binding proteins, are expressed in the adult CNS, their role in the adult brain is less well-understood. Here we studied how Igf2 deficiency affects brains of adult Igf2 knockout (Igf2(-/-)) mice following neurotoxic insult produced by the glutamate analog kainic acid (KA). Igf2(-/-) mice exhibited attenuated epileptiform activity in response to KA and were less susceptible to hippocampal neurodegeneration compared with Igf2(+/+) mice. Other brain areas protected by the lack of Igf2 included the amygdala complex, septal nuclei, and thalamic region. Apoptosis, as determined by TUNEL and Hoechst 33342 staining, was accordingly less for Igf2(-/-) mice. Hippocampal slices from Igf2(-/-) mice also were protected against the effects epileptogenic effects of KA compared to Igf2(+/+) mice suggesting that neuroprotection afforded by a lack of Igf2 may be developmental in origin and experiments demonstrating enhanced synaptic inhibition in slices taken from Igf2(-/-) mice support this hypothesis. Taken together, these results suggest that Igf2 may be important for mechanisms and circuits that contribute to neurodegeneration and epilepsy.

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

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

IGF-I-induced oligodendrocyte progenitor proliferation requires PI3K/Akt, MEK/ERK, and Src-like tyrosine kinases

Insulin-like growth factor-I (IGF-I) is required for the growth of oligodendrocytes, although the underlying mechanisms are not fully understood. Our aim was to investigate the role of phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase kinase (MEK1), and Src family tyrosine kinases in IGF-I-stimulated proliferation of oligodendrocyte progenitors. IGF-I treatment increased the proliferation of cultured oligodendrocyte progenitors as determined by measuring incorporation of [(3)H]-thymidine and bromodeoxy-uridine (BrdU). IGF-I stimulated a transient phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK1) and extracellular signal-regulated kinases (ERK1/2) (targets of MEK1), as well as a rapid and sustained activation of Akt (a target of PI3K). Furthermore, inhibitors of PI3K (LY294002 and Wortmannin), MEK1 (PD98059 and U0126), and Src family tyrosine kinases (PP2) decreased IGF-I-induced proliferation, and blocked ERK1/2 activation. LY294002, Wortmannin and PP2 also blocked Akt activation. To further determine whether Akt is required for IGF-I stimulated oligodendrocyte progenitor proliferation, cultures were infected with adenovirus vectors expressing dominant-negative mutants of Akt or treated with pharmacological inhibitors of Akt. All treatments reduced IGF-I-induced oligodendrocyte progenitor proliferation. Our data indicate that stimulation of oligodendrocyte progenitor proliferation by IGF-I requires Src-like tyrosine kinases as well as the PI3K/Akt and MEK1/ERK signaling pathways.

Emerging roles of insulin-like growth factor-I in the adult brain

All tissues in the body are under the influence of insulin-like growth factor-I (IGF-I). Together with insulin, IGF-I is a key regulator of cell metabolism and growth. IGF-I also acts in the central nervous system, where it affects many different cell populations. In this brief review, we discuss the many roles of IGF-I in the adult brain, and present the idea that diseases affecting the brain will perturb IGF-I activity, although more refined studies at the molecular and cellular level are needed before we can firmly established this possibility. We also suggest that under normal physiological conditions IGF-I may play a significant role in higher brain functions underlying cognition, and may serve a homeostatic role during brain aging. Among newly emerging issues, the effects of IGF-I on non-neuronal cells within the nervous system and their impact in brain physiology and pathology are becoming very important in understanding the biology of this peptide in the brain.

Allometric growth ratios are independent of Igf2 gene dosage during development

In the mouse, allelic dosage of the paternally expressed gene coding for insulin-like growth factor II (Igf2), from null to bi-allelic, results in dose-dependent growth, an effect which appears to be fully established during a discrete period of embryogenesis that then persists throughout life. Here, we specifically quantify the influence of Igf2 allelic dosage on the proportionality of regional embryonic growth rather than overall growth. Remarkably, preservation of allometric growth ratios between head and body regions were observed throughout development, irrespective of the range of overall growth phenotype (60-130% of wild type). Evaluation of log-log plots suggests that each allele of Igf2 expressed corresponds to the equivalent of 2-4 days of relative growth. Igf2 is predominantly expressed in extra-embryonic mesoderm (E7.5-E8.25), 24 h before alterations in cell number are known to occur in embryos with disruption of the paternally expressed allele. We hypothesized that the preservation of proportionality may result from modification of extra-embryonic development and subsequent alteration of systemic nutritional supply. Morphological analyses of chorio-allantoic and placental development between E9 and E9.5 appeared Igf2 independent. This suggests either an intrinsic but systemic Igf2-dependent activity within the embryo or a more complex developmental mechanism accounts for the proportional phenotype. Allelic IGF2 expression is subject to stochastic variation in humans, with 10% of the population estimated to be functionally bi-allelic. Evaluation of allometric growth of normal and pathological human embryos, suggest intra-uterine growth phenotypes associated with altered IGF2 imprinting are also likely to be proportionate.

A non-transformed oligodendrocyte precursor cell line, OL-1, facilitates studies of insulin-like growth factor-I signaling during oligodendrocyte development

The process by which oligodendrocyte progenitors differentiate into mature oligodendrocytes is complex and incompletely understood in part because of the paucity of oligodendrocyte precursors cell lines that can be studied in culture. We have developed a non-immortalized rat oligodendrocyte precursor line, called OL-1, which behaves in a fashion consistent with developing oligodendrocytes in vivo. This OL-1 line provides a model for the study of oligodendrocyte development and offers an alternative to the CG-4 cell line. When OL-1 cells are propagated in conditioned growth media, they have morphology consistent with immature oligodendrocytes and exhibit A2B5 antigen positive and myelin basic protein-negative immunoreactivity. Withdrawal of conditioned growth media and culture in serum-free medium results in OL-1 cell maturation, manifested by a shift to myelin basic protein-positive immunoreactivity, A2B5 antigen-negative immunoreactivity, decreased NG2 mRNA expression, increased expression of proteolipid protein mRNA, and increased expression of CNP protein. In addition, the expression of proteolipid protein and its splicing variant DM-20 exhibit a pattern that is similar to brain proteolipid protein expression during development. When OL-1 cells are exposed to Insulin-like growth factor-I, there are significant increases in proteolipid protein mRNA expression (p<0.05), the number of cell processes (p<0.05), and cell number (p<0.05). Treatment with the caspase inhibitors Z-DEVD-FMK and Z-VAD-FMK (inhibitors of caspases 3, 6, 7, 8, 10 and 1, 3, 4, respectively), Insulin-like growth factor-I, or both, results in a similar increase in cell number. Because Insulin-like growth factor-I does not substantially increase the BrdU labeling of OL-1 cells, these data collectively indicate that Insulin-like growth factor-I increases OL-1 cell number predominately by promoting survival, rather than stimulating proliferation. This non-immortalized oligodendrocyte precursor cell line, therefore, exhibits behavior consistent with the in vivo development of oligodendrocytes and provides an excellent model for the study of developing oligodendrocytes.

The insulin-like growth factor system in multiple sclerosis

Multiple sclerosis (MS) is a chronic disorder of the central nervous system characterized by inflammation, demyelination, and axonal degeneration. Present therapeutic strategies for MS reduce inflammation and its destructive consequences, but are not effective in the progressive phase of the disease. There is a need for neuroprotective and restorative therapies in MS. Insulin-like growth factor-1 (IGF-1) is of considerable interest because it is not only a potent neuroprotective trophic factor but also a survival factor for cells of the oligodendrocyte lineage and possesses a potent myelinogenic capacity. However, the IGF system is complex and includes not only IGF-1 and IGF-2 and their receptors but also modulating IGF-binding proteins (IGFBPs), of which six have been identified. This chapter provides an overview of the role of the IGF system in the pathophysiology of MS, relevant findings in preclinical models, and discusses the possible use of IGF-1 as a therapeutic agent for MS.

IGF-1 stimulates de novo fatty acid biosynthesis by Schwann cells during myelination

Schwann cell (SC) differentiation to the myelinating phenotype is characterized by the elaboration of a lipid-rich membrane and the expression of myelin-specific proteins. Insulin-like growth factor-1 (IGF-1) has been identified as a growth factor that stimulates the early events of myelination in SCs that signals via the PI3K/Akt pathway. Given the role of IGF-1 in promoting myelination, we performed studies to determine if the fatty acid biosynthetic pathway was a target of IGF-1 signaling in the formation of myelin membrane in dorsal root ganglion neuron/Schwann cell (DRG/SC) cocultures. We report that the fatty acid profile of lipid extracts of cocultures treated with IGF-1 match that reported for native myelin membrane by electrospray mass spectroscopy analysis. We also demonstrate de novo fatty acid biosynthesis in response to IGF-1 treatment in DRG/SC cocultures metabolically labeled with (13)C-acetate as a carbon source for fatty acid synthesis. Consistent with this finding, Western blot analysis of lysates from both cocultures and purified SCs reveal that IGF-1 stimulates two key fatty acid synthesizing enzymes. Additionally, we show that stimulation of fatty acid synthesizing enzymes is mediated by the PI3K/Akt signaling pathway. We also show that the fatty acid synthesizing enzymes and associated signaling pathways are elevated during the period of myelin membrane formation in sciatic nerve. Collectively, these findings demonstrate that IGF-1 plays an important regulatory function during myelin membrane formation.

Genetic analysis of anterior posterior expression gradients in the developing mammalian forebrain

Intrinsic regulatory factors play critical roles in early cortical patterning, including the development of the anteroposterior (A-P) axis. To identify genes that are differentially expressed along the A-P axis of the developing cerebral cortex, we analyzed gene expression in presumptive frontal, parietal, and occipital cerebral walls of E12.5 mouse using complementary DNA microarrays. We identified 106 genes, including expressed sequence tags (ESTs), expressed in an A-P gradient in the embryonic brain and screened 88 by in situ hybridization for confirmation. Central nervous system (CNS) expression patterns of many of these genes were previously unknown. Others, such as Sfrp1, CoupTF1, and FABP7, were expressed in a manner consistent with previous studies, providing independent confirmation. Two related transcription factors, previously not implicated in CNS development, Fhl1 and Fhl2, were observed to be enriched in posterior and anterior telencephalon, respectively. We studied patterning gradients in Fhl1 knockout mice but observed no changes in gene expression related to A-P regionalization in the Fhl1 knockout mice. These data provide an important set of new candidates for studies of cortical patterning and maturation.

IGF-I specifically enhances axon outgrowth of corticospinal motor neurons

Corticospinal motor neurons (CSMN) are among the most complex CNS neurons; they control voluntary motor function and are prototypical projection neurons. In amyotrophic lateral sclerosis (ALS), both spinal motor neurons and CSMN degenerate; their damage contributes centrally to the loss of motor function in spinal cord injury. Direct investigation of CSMN is severely limited by inaccessibility in the heterogeneous cortex. Here, using new CSMN purification and culture approaches, and in vivo analyses, we report that insulin-like growth factor-1 (IGF-I) specifically enhances the extent and rate of murine CSMN axon outgrowth, mediated via the IGF-I receptor and downstream signaling pathways; this is distinct from IGF-I support of neuronal survival. In contrast, brain-derived neurotrophic factor (BDNF) enhances branching and arborization, but not axon outgrowth. These experiments define specific controls over directed differentiation of CSMN, indicate a distinct role of IGF-I in CSMN axon outgrowth during development, and might enable control over CSMN derived from neural precursors.

Transgenic mice overexpressing glycogen synthase kinase 3beta: a putative model of hyperactivity and mania

Lithium is used as treatment for bipolar disorder with particular efficacy in the treatment of mania. Lithium inhibits glycogen synthase kinase 3beta (GSK-3beta) directly or indirectly via stimulation of the kinase Akt-1. We therefore investigated the possibility that transgenic mice overexpressing GSK-3beta could be of relevance to model bipolar disorder. Transgenic mice showed hypophagia, an increased general locomotor activity, and decreased habituation as assessed in an open field, an increased acoustic startle response, and again decreased habituation. The forced swim test revealed a reduced immobility in transgenic mice, but this is probably related to the hyperactivity of the animals. There were no differences in baseline and stress-induced increases of plasma adrenocorticotrophic hormone and corticosterone levels. Molecular analysis suggests compensatory mechanisms in the striatum of these transgenic mice for the overload of active GSK-3beta by dimming the endogenous GSK-3beta signaling pathway via upregulation of Akt-1 expression. Brain-derived neurotrophic factor protein levels were increased in the hippocampus of the transgenic mice. This suggests some kind of compensatory mechanism to the observed reduction in brain weight, which has been related previously to a reduced size of the somatodendritic compartment. Together, in mice overexpressing GSK-3beta, specific intracellular signaling pathways are affected, which is accompanied by altered plasticity processes and increased activity and reactivity, whereas habituation processes seem to be decreased. The behavioral observations led to the suggestion that the model at hand recapitulates hyperactivity as observed in the manic phase of bipolar disorder.

Insulin-like growth factor-1 is an endogenous mediator of focal ischemia-induced neural progenitor proliferation

The adult mammalian brain contains resident neural progenitors in the subgranular zone of the dentate gyrus (DG) and the subventricular zone (SVZ) of the lateral ventricles. The proliferation of neural progenitors increases after focal cerebral ischemia in both of these regions, but the mechanisms that promote ischemia-induced neural progenitor proliferation are not yet understood. We hypothesize that diffusible factors from the ischemic area play a role in this process as the DG is remote from the area of infarction. In this study, we observed that the peak of neural progenitor proliferation in the ipsilateral DG was between day 2 and day 4 of reperfusion after transient middle cerebral artery occlusion in adult spontaneously hypertensive rats. GeneChip and real-time PCR analysis showed a three- to 102-fold increase in the expression of 15 diffusible, mitogenic factors in the ischemic cortex at 3 days of reperfusion. Of these, insulin-like growth factor-1 (IGF-1) showed increased protein expression in the activated astrocytes in the ischemic penumbra. In addition, the progenitors in both the SVZ and DG showed IGF-1 receptor expression. Inhibiting IGF-1 activity by introcerebroventricular infusion of IGF-1 antibody significantly prevented the ischemia-induced neural progenitor proliferation. These results indicate that IGF-1 formed in the ischemic penumbra might be one of the diffusible factors that mediate post-ischemic neural progenitor proliferation.

Systemic IGF-I treatment inhibits cell death in diabetic rat retina

Diabetic retinopathy can result in apoptotic cell death of retinal neurons, as well as significant visual loss. It is further known that insulin-like growth factor (IGF) levels are reduced in diabetes and that IGF-I can prevent cell death in many cell types. In this study, we tested the hypothesis that systemic treatment with IGF-I could inhibit death of neuroretinal cells in diabetic rats by examining the expression of proapoptotic markers. In diabetic rat retina, the number of TUNEL-immunoreactive cells increased approximately sixfold in the photoreceptor layer (P<.001) and eightfold in the inner nuclear layer (INL; P<.001); phospho-Akt (p-Akt; Thr 308) immunoreactivity increased eightfold in the ganglion cell layer (GCL; P<.001) and threefold in the INL (P<.01). Subcutaneous IGF-I treatment significantly reduced the number of TUNEL (P<.001) and p-Akt immunoreactive retinal cells (P<.05) in diabetic rats approximately to the level of the nondiabetic group. Qualitative results showed that caspase-3 and BAD immunoreactivities were also elevated in diabetes and reduced in IGF-I-treated animals. Elevated TUNEL and p-Akt immunoreactivities were localized to distinct cell layers in the retina of diabetic rats. Early intervention with systemic IGF-I reduced the presence of proapoptotic markers indicative of neuroretinal cell death, despite ongoing hyperglycemia and weight loss. The eye is a special sensory organ, and these data show that cell loss in the nervous system, even in uncontrolled diabetes, can be prevented by IGF-I administration.

RGMa inhibition promotes axonal growth and recovery after spinal cord injury

Repulsive guidance molecule (RGM) is a protein implicated in both axonal guidance and neural tube closure. We report RGMa as a potent inhibitor of axon regeneration in the adult central nervous system (CNS). RGMa inhibits mammalian CNS neurite outgrowth by a mechanism dependent on the activation of the RhoA–Rho kinase pathway. RGMa expression is observed in oligodendrocytes, myelinated fibers, and neurons of the adult rat spinal cord and is induced around the injury site after spinal cord injury. We developed an antibody to RGMa that efficiently blocks the effect of RGMa in vitro. Intrathecal administration of the antibody to rats with thoracic spinal cord hemisection results in significant axonal growth of the corticospinal tract and improves functional recovery. Thus, RGMa plays an important role in limiting axonal regeneration after CNS injury and the RGMa antibody offers a possible therapeutic agent in clinical conditions characterized by a failure of CNS regeneration.

The role of the phosphatidylinositide 3-kinase–protein kinase B pathway in schizophrenia

Neuroanatomical studies of brains from schizophrenic patients report evidence for neuronal dystrophy, while in genetic studies in schizophrenia there is evidence for mutations in growth factors and the downstream enzymes phosphatidylinositide 3-kinase (PI3K) and protein kinase B (PKB). Since the PI3K-PKB pathway is involved in cellular growth and proliferation, reduced activity of this cascade in schizophrenia could at least partly explain the neuronal dystrophy. Risk factors for schizophrenia, such as corticosteroids and cannabis, suppress the activity of the PI3K-PKB pathway. Conversely, estrogen and vitamin D, 2 factors with a moderate protective activity in schizophrenia, electroconvulsive shock therapy, and chronic treatment with antipsychotic compounds stimulate the pathway. Reduced activity of the PI3K-PKB pathway makes the brain more susceptible to virus infections, anoxia, and obstetric complications (recognized risk factors for schizophrenia), whereas a diminution of growth factor levels towards the end of puberty could contribute to an increase in schizophrenia symptoms observed around that time. On the other hand, constitutive (over)activation of the PI3K-PKB pathway increases cancer risk. Consequently, the presumed hypoactivity of the PI3K-PKB cascade might provide a partial explanation for the remarkable epidemiological finding of a reduced cancer rate in schizophrenic patients. Recognition of the role of a dysfunctional PI3K-PKB pathway in schizophrenia might help in the discovery of hitherto undetected causative gene mutations and could also lead to novel therapeutic approaches. However, a major challenge that remains to be solved is how the PI3K-PKB pathway can be activated without increasing the risk of cancer.

Insulin-like growth factor actions during development of neural stem cells and progenitors in the central nervous system

Insulin-like growth factor-I (IGF-I) plays a key role in normal development. Recent studies show that IGF-I exerts a wide variety actions in the central nervous system during development as well as in adulthood. This report reviews recent developments on IGF-I actions and its mechanisms in the central nervous system, with a focus on its actions during the development of neural stem cells and progenitors. Available data strongly indicate that IGF-I shortens the length of the cell cycle in neuron progenitors during embryonic life and has an influence on the growth of all neural cell types. The phosphatidylinositol-3 kinase/Akt and mitogen-activated protein kinase pathways seem to be the predominant mediators of IGF-I-stimulated neural cell proliferation and survival. IGF-I actions, however, likely depend on cell type, developmental stage, and microenvironmental milieu.

Study of effects of antiglaucoma eye drops on N-methyl-D-aspartate-induced retinal damage

PURPOSE

To study the effects of antiglaucoma eye drops on N-methyl-D-aspartate (NMDA)-induced retinal damage.

METHODS

Several antiglaucoma eye drops, beta-blockers, alpha/beta-blockers, an alpha1-blocker, an alpha2-agonist, and a prostaglandin derivative, were topically administrated to NMDA-treated rat eyes daily for 2 weeks, and the retinal thickness, the number of retrograde-labeled retinal ganglion cells (RGCs), and the results of a cDNA microarray analysis were studied.

RESULTS

Intravitreal administration of NMDA caused a significant decrease in the thickness of the retinal layers and induced upregulation of glial fibrillary acidic protein (GFAP). Topical administration of beta-blockers (timolol, betaxolol, and carteolol) and a prostaglandin derivative (latanoprost) showed almost no significant effects on retinal thickness, the number of RGCs, or expression of GFAP. In contrast, the alpha/beta-blockers (nipradilol and levobunolol), the alpha1-blocker (bunazosin HCl), and the alpha2-agonist (brimonidine) showed preservation effects on retinal thickness and the number of RGCs, and marked suppression of NMDA-induced upregulation of GFAP. Among 1101 genes related to cellular regulatory mechanisms, the expression of two genes, both for insulin-like growth factors, (IGF-1) and ErbB3, was altered upon administration of the alpha/beta-blockers, the alpha1-blocker, and the alpha2-agonist.

CONCLUSION

Our present study suggests that modulations of the alpha-adrenergic receptor, alpha1-blocking and alpha2-stimulation, by antiglaucoma eye drops may cause beneficial effects on NMDA-induced retinal damage in the rat.

Adult-onset deficiency in growth hormone and insulin-like growth factor-I decreases survival of dentate granule neurons: Insights into the regulation of adult hippocampal neurogenesis

Insulin-like growth factor-I (IGF-I), long thought to provide critical trophic support during development, also has emerged as a candidate for regulating ongoing neuronal production in adulthood. Whether and how IGF-I influences each phase of neurogenesis, however, remains unclear. In the current study, we used a selective model of growth hormone (GH) and plasma IGF-I deficiency to evaluate the role of GH and IGF-I in regulating cell proliferation, survival, and neuronal differentiation in the adult dentate gyrus. GH/IGF-I-deficient dwarf rats of the Lewis strain were made GH/IGF-I replete throughout development via twice daily injections of GH, and then GH/IGF-I deficiency was initiated in adulthood by removing animals from GH treatment. Bromodeoxyuridine (BrdU) labeling revealed no effect of GH/IGF-I deficiency on cell proliferation, but adult-onset depletion of GH and plasma IGF-I significantly reduced the survival of newly generated cells in the dentate gyrus. Colabeling for BrdU and markers of immature and mature neurons revealed a selective effect of GH/IGF-I deficiency on the survival of more mature new neurons. The number of BrdU-labeled cells expressing the immature neuronal marker TUC-4 did not differ between GH/IGF-I-deficient and -replete animals, but the number expressing only the marker of maturity NeuN was lower in depleted animals. Taken together, results from the present study suggest that, under conditions of short-term GH/IGF-I deficiency during adulthood, dentate granule cells continue to be produced, to commit to a neuronal fate, and to begin the process of neuronal maturation, whereas survival of the new neurons is impaired.

The Role of Growth Hormone in Neural Development

Growth hormone (GH) is integrally involved in the development of the central nervous system (CNS), as well as during its recovery from injury, two processes that share many similarities and may influence CNS functionality. This review discusses some of the most recent findings in the field and, in particular, the ontogeny, distribution, regulation and putative functions of GH and its receptor within the CNS, particularly during development. The relative roles of peripheral GH, acting in part through insulin-like growth factor-I, and of the autocrine/paracrine GH system within the brain are considered. The potential role of GH as a therapeutic agent to influence brain development and function is discussed.

Intracellular events mediating insulin-like growth factor I-induced oligodendrocyte development: modulation by cyclic AMP

Insulin-like growth factor I (IGF-I) is a potent inducer of oligodendrocyte development and myelination. Although IGF-I intracellular signaling has been well described in several cell types, intracellular mechanisms for IGF-I-induced oligodendrocyte development have not been defined. By using specific inhibitors of intracellular signaling pathways, we report here that the MAPK and phosphatidylinositol 3-kinase signaling pathways are required for the full effect of IGF-I on oligodendrocyte development in primary mixed rat cerebrocortical cell cultures. The MAPK activation, but not the phosphatidylinositol 3-kinase activation, leads to phosphorylation of the cAMP response element-binding protein, which is necessary for IGF-I to induce oligodendrocyte development. cAMP, although it does not show any effect on oligodendrocyte development, has an inhibitory effect on IGF-I-induced oligodendrocyte development that is mediated by the cAMP-dependent protein kinase. Furthermore, cAMP also has an inhibitory effect on IGF-I-dependent MAPK activation. This is a cAMP-dependent protein kinase-independent effect and probably contributes to the cAMP action on IGF-I-induced oligodendrocyte development.

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.

IGF-1 protects oligodendrocyte progenitor cells and improves neurological functions following cerebral hypoxia-ischemia in the neonatal rat

To investigate if insulin-like growth factor-1 (IGF-1) provides neuroprotection to oligodendrocyte progenitor cells (OPCs) following cerebral hypoxia-ischemia, a previously developed neonatal rat model of white matter damage was used in this study. Postnatal day 4 (P4) SD rat pups were subjected to bilateral common carotid artery ligation, followed by exposure to 8% oxygen for 10 min. IGF-1 (0.5 microg) or vehicle was injected into the left ventricle after artery ligation and before the hypoxic exposure. Cerebral hypoxia-ischemia caused death of O4+ late OPCs in the P5 rat brain and impaired myelination in the P9 and P21 rat brain. Caspase-3 activation was involved in the death of OPCs. Moreover, cerebral hypoxia-ischemia impaired neurobehavioral performance in juvenile rats. IGF-1 treatment attenuated damages to OPCs and improved neurological functions after cerebral hypoxia-ischemia. It reduced death of O4+ OPCs by 39% on P5 and enhanced myelination on P9 and P21. Bromodeoxyuridine uptake assay showed that cerebral hypoxia-ischemia inhibited proliferation of stem/progenitor cells in the subventricular zone and NG2+ early OPCs in the white matter area. IGF-1 treatment increased cell proliferation in the subventricular zone by 31% 1 day following hypoxic-ischemic insult. Proliferation of early and late OPCs in the IGF-1-treated group was 1.5- and 2.4-fold of that in the vehicle-treated group, respectively. In conclusion, IGF-1 provided potent neuroprotection to OPCs and improved neurological functions following cerebral hypoxia-ischemia in the neonatal rat. The neuroprotection of IGF-1 was associated with its antiapoptotic and mitogenic effects.

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.

Regulation of adult olfactory neurogenesis by insulin-like growth factor-I

Insulin-like growth factor-I (IGF-I) has multiple effects within the developing nervous system but its role in neurogenesis in the adult nervous system is less clear. The adult olfactory mucosa is a site of continuing neurogenesis that expresses IGF-I, its receptor and its binding proteins. The aim of the present study was to investigate the roles of IGF-I in regulating proliferation and differentiation in the olfactory mucosa. The action of IGF-I was assayed in serum-free culture combined with bromodeoxyuridine-labelling of proliferating cells and immunochemistry for specific cell types. IGF-I and its receptor were expressed by globose basal cells (the neuronal precursor) and by olfactory neurons. IGF-I reduced the numbers of proliferating neuronal precursors, induced their differentiation into neurons and promoted morphological differentiation of neurons. The evidence suggests that IGF-I is an autocrine and/or paracrine signal that induces neuronal precursors to differentiate into olfactory sensory neurons. These effects appear to be similar to the cellular effects of IGF-I in the developing nervous system.

IGF-1 and BDNF promote chick bulbospinal neurite outgrowth in vitro

Injured neurons in the CNS do not experience significant functional regeneration and so spinal cord insult often results in permanently compromised locomotor ability. The capability of a severed axon to re-grow is thought to depend on numerous factors, one of which is the decreased availability of neurotrophic factors. Application of trophic factors to axotomized neurons has been shown to enhance survival and neurite outgrowth. Although brainstem-spinal connections play a pivotal role in motor dysfunction after spinal cord injury, relatively little is known about the trophic sensitivity of these populations. This study explores the response of bulbospinal populations to various trophic factors. Several growth factors were initially examined for potential trophic effects on the projection neurons of the brainstem. Brain derived neurotrophic factor (BDNF) and insulin-like growth factor (IGF-1) significantly enhance mean process length in both the vestibulospinal neurons and spinal projection neurons from the raphe nuclei. Nerve growth factor (NGF), neurotrophin-4 (NT-4) and glial derived neurotrophic factor (GDNF) did not effect process outgrowth in vestibulospinal neurons. At the developmental stages used in this study, it was determined that receptors for BDNF and IGF-1 were present both on bulbospinal neurons and on surrounding cells with a non-neuronal morphology.

Targeted expression of IGF-1 in the central nervous system fails to protect mice from experimental autoimmune encephalomyelitis

Insulin-like growth factor 1 (IGF-1) has been identified as a critical molecule in the induction of myelination in the central nervous system (CNS). Systemic injection of IGF-1 has been shown to have a varied and transiently protective effect on the clinical course of experimental autoimmune encephalomyelitis (EAE). Since systemic IGF-1 can also modulate peripheral immune lymphocytes, we examined whether a sustained and local delivery of IGF-1 into the spinal cord would have any influence on the chronic course of EAE in C57/BL6 mice. The capability of adeno-associated virus (AAV) to be retrogradely transported efficiently from muscle to motor neurons of the spinal cord was used to overcome the difficulty routinely encountered when attempting chronic delivery of molecules into the CNS. We demonstrate that AAV-mediated delivery of IGF-1 in CNS did not have any beneficial effect on the clinical course of EAE. Injection of AAV-IGF1 after induction of the disease worsened the clinical symptoms. Furthermore, CNS expression of IGF-1 did not affect the pathogenic anti-MOG T cell response, as examined by proliferation and cytokine secretion. Thus, enhanced expression of IGF-1 in the CNS during inflammation does not have a significant effect on myelination. These data have important implications for the potential use of IGF-1 in the treatment of multiple sclerosis.

Early developmental changes in IGF-I, IGF-II, IGF binding protein-1, and IGF binding protein-3 concentration in the cerebrospinal fluid of children

IGF-I and IGF-II are ubiquitously expressed growth factors that have profound effects on the growth and differentiation of many cell types and tissues, including cells of the CNS. In biologic fluids, most IGFs are bound to one of six IGF binding proteins (IGFBPs 1-6). Increasing evidence strongly supports a role for IGF-I in CNS development, as it promotes neuronal proliferation and survival. However, little is known about IGF-I and its homolog IGF-II and their carrier proteins, IGFBPs, during the neonatal period in which brain size increases dramatically, myelination takes place, and neurons show limited capacity to proliferate. Herein, we have determined the concentrations of IGF-I, IGF-II, IGFBP-1, and IGFBP-3 in cerebral spinal fluid (CSF) samples that were collected from children who were 1 wk to 18 y of age. The concentrations of IGF-I, IGFBP-1, and IGFBP-3 in CSF from children <6 mo of age were significantly higher than in older children, whereas IGF-II was higher in the older group. This is in contrast to what is observed in the peripheral circulation, where IGF-I and IGFBP-3 are low at birth and rise rapidly during the first year, reaching peak levels during puberty. Higher concentrations of IGF-I, IGFBP-1, and IGFBP-3 in the CSF of very young children suggest that these proteins might participate in the active processes of myelination and synapse formation in the developing nervous system. We propose that IGF-I and certain IGFBPs are likely necessary for normal CNS development during critical stages of neonatal brain growth and development.

Essential role of protein kinase B gamma (PKB gamma/Akt3) in postnatal brain development but not in glucose homeostasis

Protein kinase B is implicated in many crucial cellular processes, such as metabolism, apoptosis and cell proliferation. In contrast to Pkb(alpha) and Pkb(beta)-deficient mice, Pkb(gamma)(-/-) mice are viable, show no growth retardation and display normal glucose metabolism. However, in adult Pkb(gamma)mutant mice, brain size and weight are dramatically reduced by about 25%. In vivo magnetic resonance imaging confirmed the reduction of Pkb(gamma)(-/-) brain volumes with a proportionally smaller ventricular system. Examination of the major brain structures revealed no anatomical malformations except for a pronounced thinning of white matter fibre connections in the corpus callosum. The reduction in brain weight of Pkb(gamma)(-/-) mice is caused, at least partially, by a significant reduction in both cell size and cell number. Our results provide novel insights into the physiological role of Pkb(gamma) and suggest a crucial role in postnatal brain development.

Integrin-growth factor interactions as regulators of oligodendroglial development and function

Central nervous system (CNS) development requires mechanisms for the regulation of cell number. Although growth factors are essential determinants of the proliferation and apoptosis that determine final numbers, the long-range nature of signals from diffusible growth factors makes them insufficient for the provision of the precise and localized signals required. Integration of integrin and growth factor receptor signaling in controlling cell behavior has been an important theme of research over the past several years. The focus of this review is on the mechanisms by which integrin-growth factor interactions regulate the development of oligodendrocytes and provide a mechanism for controlling, both in space and in time, oligodendrocyte numbers in the developing CNS.

Age-related effects of IGF-1 on the NMDA-, GH- and IGF-1-receptor mRNA transcripts in the rat hippocampus

Insulin-like growth factor-1 (IGF-1) and growth hormone (GH) have been suggested to promote memory and cognitive capabilities. In a recent publication we observed that GH increase the proportion of the NR2B subunit mRNA transcript of the NMDA receptor in rat hippocampus. NR2B has been suggested to be essential for spatial learning and long-term potentiation (LTP). This effect of GH might be IGF-1-mediated or a result of a co-ordination with IGF-1. To test this hypothesis further, we examined the effects of 10 daily s.c. injections of IGF-1 on NMDA receptor subunits (NR1, NR2A, and NR2B), GH receptor (GHR), GH binding protein (GHBP) and type 1 IGF receptor (IGF-1R) gene transcripts in the hippocampus. The NR2B subunit mRNA increased in young (11 weeks) but not in older (14-16 months) rats and the expression of the NR2A mRNA was decreased in both groups. The ratio of NR2B to NR2A is suggested to mirror the potential for synaptic plasticity. In both age groups, IGF-1 treatment resulted in a significant increase of this ratio at transcription level. The GHR mRNA increased in young rats, mimicking the effect of GH, while the IGF-1R mRNA was decreased in the older group of rats after IGF-1 treatment. These results suggest that IGF-1 in many aspects may mediate the actions earlier shown for GH.

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.

Role for Akt3/protein kinase Bgamma in attainment of normal brain size

Studies of Drosophila and mammals have revealed the importance of insulin signaling through phosphatidylinositol 3-kinase and the serine/threonine kinase Akt/protein kinase B for the regulation of cell, organ, and organismal growth. In mammals, three highly conserved proteins, Akt1, Akt2, and Akt3, comprise the Akt family, of which the first two are required for normal growth and metabolism, respectively. Here we address the function of Akt3. Like Akt1, Akt3 is not required for the maintenance of normal carbohydrate metabolism but is essential for the attainment of normal organ size. However, in contrast to Akt1-/- mice, which display a proportional decrease in the sizes of all organs, Akt3-/- mice present a selective 20% decrease in brain size. Moreover, although Akt1- and Akt3-deficient brains are reduced in size to approximately the same degree, the absence of Akt1 leads to a reduction in cell number, whereas the lack of Akt3 results in smaller and fewer cells. Finally, mammalian target of rapamycin signaling is attenuated in the brains of Akt3-/- but not Akt1-/- mice, suggesting that differential regulation of this pathway contributes to an isoform-specific regulation of cell growth.

Remyelinating and neuroprotective treatments in multiple sclerosis

Multiple sclerosis (MS) is the most common cause of neurological disability in young adults. The pathological hallmark is multifocal demyelination and inflammation in the CNS. In addition, there is also a variable extent of axonal damage. Remyelination has been seen in up to 70% of lesions but repair is generally incomplete. The demonstration of neuropathological heterogeneity of MS lesions suggests different pathophysiological subtypes and it is therefore unlikely that there is a uniform cause of incomplete remyelination in MS. In recent years, a great body of knowledge has accumulated in order to better understand the regulatory mechanisms of remyelination. This has led to a number of approaches to promote repair mechanisms, most of which have been successful in animal experiments. Unfortunately, the translation of these experimental data into clinical treatments has proven difficult. More information on the pathogenesis of MS, the reason why repair mechanisms fail in MS and a better understanding of the regulation of remyelination are required. This will ultimately lead to a specific treatment tailored for the individual patient and will probably involve a combination of immunomodulation, remyelination and neuroprotection.