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

The neurobiology of insulin-like growth factor I: From neuroprotection to modulation of brain states

After decades of research in the neurobiology of IGF-I, its role as a prototypical neurotrophic factor is undisputed. However, many of its actions in the adult brain indicate that this growth factor is not only involved in brain development or in the response to injury. Following a three-layer assessment of its role in the central nervous system, we consider that at the cellular level, IGF-I is indeed a bona fide neurotrophic factor, modulating along ontogeny the generation and function of all the major types of brain cells, contributing to sculpt brain architecture and adaptive responses to damage. At the circuit level, IGF-I modulates neuronal excitability and synaptic plasticity at multiple sites, whereas at the system level, IGF-I intervenes in energy allocation, proteostasis, circadian cycles, mood, and cognition. Local and peripheral sources of brain IGF-I input contribute to a spatially restricted, compartmentalized, and timed modulation of brain activity. To better define these variety of actions, we consider IGF-I a modulator of brain states. This definition aims to reconcile all aspects of IGF-I neurobiology, and may provide a new conceptual framework in the design of future research on the actions of this multitasking neuromodulator in the brain.

Impaired SNF2L Chromatin Remodeling Prolongs Accessibility at Promoters Enriched for Fos/Jun Binding Sites and Delays Granule Neuron Differentiation

Chromatin remodeling proteins utilize the energy from ATP hydrolysis to mobilize nucleosomes often creating accessibility for transcription factors within gene regulatory elements. Aberrant chromatin remodeling has diverse effects on neuroprogenitor homeostasis altering progenitor competence, proliferation, survival, or cell fate. Previous work has shown that inactivation of the ISWI genes, Smarca5 (encoding Snf2h) and Smarca1 (encoding Snf2l) have dramatic effects on brain development. Smarca5 conditional knockout mice have reduced progenitor expansion and severe forebrain hypoplasia, with a similar effect on the postnatal growth of the cerebellum. In contrast, Smarca1 mutants exhibited enlarged forebrains with delayed progenitor differentiation and increased neuronal output. Here, we utilized cerebellar granule neuron precursor (GNP) cultures from Smarca1 mutant mice (Ex6DEL) to explore the requirement for Snf2l on progenitor homeostasis. The Ex6DEL GNPs showed delayed differentiation upon plating that was not attributed to changes in the Sonic Hedgehog pathway but was associated with overexpression of numerous positive effectors of proliferation, including targets of Wnt activation. Transcriptome analysis identified increased expression of Fosb and Fosl2 while ATACseq experiments identified a large increase in chromatin accessibility at promoters many enriched for Fos/Jun binding sites. Nonetheless, the elevated proliferation index was transient and the Ex6DEL cultures initiated differentiation with a high concordance in gene expression changes to the wild type cultures. Genes specific to Ex6DEL differentiation were associated with an increased activation of the ERK signaling pathway. Taken together, this data provides the first indication of how Smarca1 mutations alter progenitor cell homeostasis and contribute to changes in brain size.

Insulin-like Growth Factor (IGF)-1 treatment stabilizes the microvascular cytoskeleton under ischemic conditions

Our previous studies showed that Insulin-like Growth Factor (IGF)-1 reduced blood brain barrier permeability and decreased infarct volume caused by middle cerebral artery occlusion (MCAo) in middle aged female rats. Similarly, cultures of primary brain microvessel endothelial cells from middle-aged female rats and exposed to stroke-like conditions (oxygen glucose deprivation; OGD) confirmed that IGF-1 reduced dye transfer across this cell monolayer. Surprisingly, IGF-1 did not attenuate endothelial cell death caused by OGD. To reconcile these findings, the present study tested the hypothesis that, at the earliest phase of ischemia, IGF-1 promotes barrier function by increasing anchorage and stabilizing cell geometry of surviving endothelial cells. Cultures of human brain microvessel endothelial cells were subject to oxygen-glucose deprivation (OGD) in the presence of IGF-1, IGF-1 + JB-1 (IGFR inhibitor) or vehicle. OGD disrupted the cell monolayer and reduced cell-cell interactions, which was preserved in IGF-1-treated cultures and reversed by concurrent treatment with JB-1. IGF-1-mediated preservation of the endothelial monolayer was reversed with LY294002 treatment, but not by Rapamycin, indicating that IGF-1 s actions on cell-cell contacts are likely mediated via the PI3K pathway. In vivo, microvessel morphology was evaluated in middle-aged female rats that were subjected to ischemia by MCAo, and treated ICV with IGFI, IGF-1 + JB-1, or artificial CSF (aCSF; vehicle) after reperfusion. Compared to vehicle controls, IGF-1 treated animals displayed larger microvessel diameters in the peri-infarct area and increased staining density for vinculin, an anchorage protein. Both these measures were reversed by concurrent IGF-1 + JB-1 treatment. Moreover these effects were restricted to 24 h after ischemia-reperfusion and no treatment effects were seen at 5d post stroke. Collectively, these data suggest that in the earliest hours during ischemia, IGF-1 promotes receptor-mediated anchorage of endothelial cells, and its actions may be accurately characterized as vasculoprotective.

Insulin-Like Growth Factors in the Pathogenesis of Neurological Diseases in Children

Insulin-like growth factors play a key role for neuronal growth, differentiation, the survival of neurons and synaptic formation. The action of IGF-1 is most pronounced in the developing brain. In this paper we will try to give an answer to the following questions: Why are studies in children important? What clinical studies in neonatal asphyxia, infantile spasms, progressive encephalopathy-hypsarrhythmia-optical atrophy (PEHO) syndrome, infantile ceroid lipofuscinosis (INCL), autistic spectrum disorders (ASD) and subacute sclerosing encephalopathy (SSPE) have been carried out? What are IGF-based therapeutic strategies? What are the therapeutic approaches? We conclude that there are now great hopes for the therapeutic use of IGF-1 for some neurological disorders (particularly ASD).

Treatment of autistic spectrum disorder with insulin-like growth factors

There are no treatments for the core symptoms of autistic spectrum disorder (ASD), but there is now more knowledge on emerging mechanisms and on mechanism-based therapies. In autism there are altered synapses: genes affected are commonly related to synaptic and immune function. Dysregulation of activity-dependent signaling networks may have a key role the etiology of autism. There is an over-activation of IGF-AKT-mTor in autism spectrum disorders. Morphological and electro-physiological defects of the cerebellum are linked to system-wide ASD-like behavior defects. The molecular basis for a cerebellar contribution has been demonstrated in a mouse model. These have led to a potential mechanism-based use of drug targets and mouse models. Neurotrophic factors are potential candidates for the treatment. Insulin-like growth factor-1 (IGF-1) is altered in autism. It reduces neuro-inflammation: by causing changes of cytokines such as IL-6 and microglial function. IGF-1 reduces the defects in the synapse. It alleviates NMDA-induced neurotoxicity via the IGF-AKT-mTor pathway in microglia. IGF-1 may rescue function in Rett syndrome and ASD caused by changes of the SCHANK3 gene. There are recently pilot studies of the treatment of Rett syndrome and of SCHANK3 gene deficiency syndromes. The FDA has granted Orphan drug designations for Fragile X syndrome, SCHANK3 gene deficiency syndrome and Rett syndrome.

IGF-I in the clinics: Use in retinopathy of prematurity

Retinopathy of prematurity is a potentially blinding disease, which is associated with low neonatal IGF-I serum concentrations and poor growth. In severe cases impaired retinal vessel growth is followed by pathologic neovascularization, which may lead to retinal detachment. IGF-I may promote growth even in catabolic states. Treating preterm infants with recombinant human (rh) IGF-I to concentrations normally found during gestation has been suggested to have a preventative effect on ROP. A recent phase 2 study treating infants (gestational age between 23weeks+0days and 27weeks +6days) with rhIGF-I/IGF binding protein-3 until 30 postmenstrual weeks showed no effect on ROP but a 53% reduction in severe bronchopulmonary dysplasia and 44% reduction in severe intraventricular hemorrhage. Oxygen is a major risk factor for ROP and during the phase 2 study oxygen saturation targets were increased to 90-95%, due to national guidelines, which might have affected ROP rate and severity making increased IGF-I a weaker preventative factor for ROP.

Role of Insulinlike Growth Factor 1 in Fetal Development and in the Early Postnatal Life of Premature Infants

The neonatal period of very preterm infants is often characterized by a difficult adjustment to extrauterine life, with an inadequate nutrient supply and insufficient levels of growth factors, resulting in poor growth and a high morbidity rate. Long-term multisystem complications include cognitive, behavioral, and motor dysfunction as a result of brain damage as well as visual and hearing deficits and metabolic disorders that persist into adulthood. Insulinlike growth factor 1 (IGF-1) is a major regulator of fetal growth and development of most organs especially the central nervous system including the retina. Glucose metabolism in the developing brain is controlled by IGF-1 which also stimulates differentiation and prevents apoptosis. Serum concentrations of IGF-1 decrease to very low levels after very preterm birth and remain low for most of the perinatal development. Strong correlations have been found between low neonatal serum concentrations of IGF-1 and poor brain and retinal growth as well as poor general growth with multiorgan morbidities, such as intraventricular hemorrhage, retinopathy of prematurity, bronchopulmonary dysplasia, and necrotizing enterocolitis. Experimental and clinical studies indicate that early supplementation with IGF-1 can improve growth in catabolic states and reduce brain injury after hypoxic/ischemic events. A multicenter phase II study is currently underway to determine whether intravenous replacement of human recombinant IGF-1 up to normal intrauterine serum concentrations can improve growth and development and reduce prematurity-associated morbidities.

The therapeutic potential of insulin-like growth factor-1 in central nervous system disorders

Central nervous system (CNS) development is a finely tuned process that relies on multiple factors and intricate pathways to ensure proper neuronal differentiation, maturation, and connectivity. Disruption of this process can cause significant impairments in CNS functioning and lead to debilitating disorders that impact motor and language skills, behavior, and cognitive functioning. Recent studies focused on understanding the underlying cellular mechanisms of neurodevelopmental disorders have identified a crucial role for insulin-like growth factor-1 (IGF-1) in normal CNS development. Work in model systems has demonstrated rescue of pathophysiological and behavioral abnormalities when IGF-1 is administered, and several clinical studies have shown promise of efficacy in disorders of the CNS, including autism spectrum disorder (ASD). In this review, we explore the molecular pathways and downstream effects of IGF-1 and summarize the results of completed and ongoing pre-clinical and clinical trials using IGF-1 as a pharmacologic intervention in various CNS disorders. This aim of this review is to provide evidence for the potential of IGF-1 as a treatment for neurodevelopmental disorders and ASD.

Fingerprint changes in CSF composition associated with different aetiologies in human neonatal hydrocephalus: inflammatory cytokines

PURPOSE

Hydrocephalus (HC) has a multifactorial and complex picture of pathophysiology due to aetiology, age at and duration since onset. We have previously identified distinctions in markers of cell death associated with different aetiologies. Here, we examined cerebrospinal fluid (CSF) from human HC neonates for cytokines to identify further distinguishing features of different aetiologies.

METHODS

CSF was collected during routine lumbar puncture or ventricular tap from neonates with hydrocephalus, or with no neurological condition (normal controls). Total protein, Fas receptor, Fas ligand, stem cell factor (SCF), hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), insulin growth factor-1 (IGF-1), tumour necrosis factor alpha (TNF-α) and interleukin 6 (IL-6) were measured and compared between 8 unaffected and 28 HC neonatal CSF samples.

RESULTS

Total protein was significantly (P < 0.05) raised in late-onset hydrocephalus (LOH). Fas receptor was raised (P < 0.05) in post-haemorrhagic hydrocephalus (PHH) and spina bifida with hydrocephalus (SB/HC), but no difference in Fas ligand was found. SCF was raised (P < 0.05) in SB/HC. HGF was found in all HC and was increased (P < 0.01) in PHH. Increased VEGF was found in PHH (P < 0.01) and SB/HC (P < 0.05). Variable levels of IL-6, TNF-α and IGF-1 were found in all HC groups compared with none in normal.

CONCLUSIONS

LOH was unusual with significantly raised total protein indicating an inflammatory state. Increased Fas receptor, VEGF, IGF-1 and HGF suggest anti-apoptotic and repair mechanism activation. By contrast, elevated TNF-α and IL-6 indicate inflammatory processes in these neonatal brains. Taken with our previous study, these data indicate that different pathophysiology, inflammation and repair are occurring in HC of different aetiologies and that additional treatment strategies may benefit these infants in addition to fluid diversion.

Engrailed2 modulates cerebellar granule neuron precursor proliferation, differentiation and insulin-like growth factor 1 signaling during postnatal development

Background

The homeobox transcription factor Engrailed2 (En2) has been studied extensively in neurodevelopment, particularly in the midbrain/hindbrain region and cerebellum, where it exhibits dynamic patterns of expression and regulates cell patterning and morphogenesis. Because of its roles in regulating cerebellar development and evidence of cerebellar pathology in autism spectrum disorder (ASD), we previously examined an ENGRAILED2 association and found evidence to support EN2 as a susceptibility gene, a finding replicated by several other investigators. However, its functions at the cell biological level remain undefined. In the mouse, En2 gene is expressed in granule neuron precursors (GNPs) just as they exit the cell cycle and begin to differentiate, raising the possibility that En2 may modulate these developmental processes.

Methods

To define En2 functions, we examined proliferation, differentiation and signaling pathway activation in En2 knockout (KO) and wild-type (WT) GNPs in response to a variety of extracellular growth factors and following En2 cDNA overexpression in cell culture. In vivo analyses of cerebellar GNP proliferation as well as responses to insulin-like growth factor-1 (IGF1) treatment were also conducted.

Results

Proliferation markers were increased in KO GNPs in vivo and in 24-h cultures, suggesting En2 normally serves to promote cell cycle exit. Significantly, IGF1 stimulated greater DNA synthesis in KO than WT cells in culture, a finding associated with markedly increased phospho-S6 kinase activation. Similarly, there was three-fold greater DNA synthesis in the KO cerebellum in response to IGF1 in vivo. On the other hand, KO GNPs exhibited reduced neurite outgrowth and differentiation. Conversely, En2 overexpression increased cell cycle exit and promoted neuronal differentiation.

Conclusions

In aggregate, our observations suggest that the ASD-associated gene En2 promotes GNP cell cycle exit and differentiation, and modulates IGF1 activity during postnatal cerebellar development. Thus, genetic/epigenetic alterations of EN2 expression may impact proliferation, differentiation and IGF1 signaling as possible mechanisms that may contribute to ASD pathogenesis.

Revisiting the timing hypothesis: biomarkers that define the therapeutic window of estrogen for stroke

Significantly extended life expectancy coupled with contemporary sedentary lifestyles and poor nutrition has created a global epidemic of cardiovascular disease and stroke. For women, this issue is complicated by the discrepant outcomes of hormone therapy (HT) for stroke incidence and severity as well as the therapeutic complications for stroke associated with advancing age. Here we propose that the impact of estrogen therapy cannot be considered in isolation, but should include age-related changes in endocrine, immune, and nucleic acid mediators that collaborate with estrogen to produce neuroprotective effects commonly seen in younger, healthier demographics. Due to their role as modulators of ischemic cell death, the post-stroke inflammatory response, and neuronal survival and regeneration, this review proposes that Insulin-like Growth Factor (IGF)-1, Vitamin D, and discrete members of the family of non-coding RNA peptides called microRNAs (miRNAs) may be crucial biochemical markers that help determine the neuroprotective "window" of HT. Specifically, IGF-1 confers neuroprotection in concert with, and independently of, estrogen and failure of the insulin/IGF-1 axis is associated with metabolic disturbances that increase the risk for stroke. Vitamin D and miRNAs regulate and complement IGF-1 mediated function and neuroprotective efficacy via modulation of IGF-1 availability and neural stem cell and immune cell proliferation, differentiation and secretions. Together, age-related decline of these factors differentially affects stroke risk, severity, and outcome, and may provide a novel therapeutic adjunct to traditional HT practices.

Native and Complexed IGF-1: Biodistribution and Pharmacokinetics in Infantile Neuronal Ceroid Lipofuscinosis

Infantile neuronal ceroid lipofuscinosis (INCL) is a severe neurodegenerative disorder of childhood characterized by selective death of cortical neurons. Insulin-like growth factor 1 (IGF-1) is important in embryonic development and is considered as a potential therapeutic agent for several disorders of peripheral and central nervous systems. In circulation IGF-1 is mainly bound to its carrier protein IGFBP-3. As a therapeutic agent IGF-1 has shown to be more active as free than complexed form. However, this may cause side effects during the prolonged treatment. In addition to IGFBP-3 the bioavailability of IGF-1 can be modulated by using mesoporous silicon nanoparticles (NPs) which are optimal carriers for sustained release of unstable peptide hormones like IGF-1. In this study we compared biodistribution, pharmacokinetics, and bioavailability of radiolabeled free IGF-1, IGF-1/IGFBP-3, and IGF-1/NP complexes in a Cln1-/- knockout mouse model. IGF-1/NP was mainly accumulated in liver and spleen in all studied time points, whereas minor and more constant amounts were measured in other organs compared to free IGF-1 or IGF-1/IGFBP-3. Also concentration of IGF-1/NP in blood was relatively high and stable during studied time points suggesting continuous release of IGF-1 from the particles.

AF4 is a critical regulator of the IGF-1 signaling pathway during Purkinje cell development

Deregulation of the insulin-like growth factor 1 (IGF-1) signaling pathway is a recurrent finding in mouse models and human patients with cerebellar ataxia and thus represents a common pathological cascade in neuronal cell death that may be targeted for therapy. We have previously identified a point mutation in AF4, a transcription cofactor of RNA polymerase II elongation and chromatin remodeling, that causes progressive and highly specific Purkinje cell (PC) death in the ataxic mouse mutant robotic, leading to the accumulation of AF4 in PCs. Here we confirm that the spatiotemporal pattern of PC degeneration in the robotic cerebellum correlates with the specific profile of AF4 upregulation. To identify the underlying molecular pathways, we performed microarray gene expression analysis of PCs obtained by laser capture microdissection (LCM) at the onset of degeneration. Igf-1 was significantly downregulated in robotic PCs compared with wild-type controls before and throughout the degenerative process. Consistently, we observed a decrease in the activation of downstream signaling molecules including type 1 IGF receptor (IGF-1R) and the extracellular signal-regulated kinase (ERK) 1 and ERK2. Chromatin immunoprecipitation confirmed that Igf-1 is a direct and the first validated target of the AF4 transcriptional regulatory complex, and treatment of presymptomatic robotic mice with IGF-1 indeed markedly delayed the progression of PC death. This study demonstrates that small changes in the levels of a single transcriptional cofactor can deleteriously affect normal cerebellum function and opens new avenues of research for the manipulation of the IGF-1 pathway in the treatment of cerebellar ataxia in humans.

Amyotrophic lateral sclerosis: from current developments in the laboratory to clinical implications

Amyotrophic lateral sclerosis (ALS) is a late-onset progressive degeneration of motor neurons occurring both as a sporadic and a familial disease. The etiology of ALS remains unknown, but one fifth of instances are due to specific gene defects, the best characterized of which is point mutations in the gene coding for Cu/Zn superoxide dismutase (SOD1). Because sporadic and familial ALS affect the same neurons with similar pathology, it is hoped that understanding these gene defects will help in devising therapies effective in both forms. A wealth of evidence has been collected in rodents made transgenic for mutant SOD1, which represent the best available models for familial ALS. Mutant SOD1 likely induces selective vulnerability of motor neurons through a combination of several mechanisms, including protein misfolding, mitochondrial dysfunction, oxidative damage, cytoskeletal abnormalities and defective axonal transport, excitotoxicity, inadequate growth factor signaling, and inflammation. Damage within motor neurons is enhanced by noxious signals originating from nonneuronal neighboring cells, where mutant SOD1 induces an inflammatory response that accelerates disease progression. The clinical implication of these findings is that promising therapeutic approaches can be derived from multidrug treatments aimed at the simultaneous interception of damage in both motor neurons and nonmotor neuronal cells.

Modulation by insulin-like growth factor I of the phosphatase PTEN in astrocytes

Characterization of intracellular pathways underlying the pleiotropic actions of insulin-like growth factor-I (IGF-I) on brain cells is incomplete. We analyzed IGF-I signalling on astrocytes through the canonical phosphatidylinositol 3-kinase (PI3K)/Akt pathway and focused on possible changes in PTEN, a phosphatase that modulates IGF-I signalling by inhibiting Akt activation and, in turn is positively regulated by PI3K. After exposure of astrocytes to IGF-I, PTEN mRNA and protein levels were reduced and its phosphatase activity diminished. Inhibition of PTEN involved activation of a PI3K/protein kinase C (PKC) pathway that decreased in a proteasome-dependent step the levels of the transcription factor Egr-1, a key regulator of PTEN levels in astrocytes, causing decreased binding of Egr-1 to the PTEN promoter. Enhanced mitogenesis in PTEN siRNA-transduced astrocytes after IGF-I suggested that reduced PTEN may be a permissive factor for the mitogenic activity of IGF-I. Subsequent recovery of reduced PTEN required also activation by IGF-I of PI3K to recruit in this case protein kinase A (PKA) which stimulated Egr-1 levels and, consequently PTEN synthesis. Because basal levels of PTEN in astrocytes are also governed by PI3K, IGF-I appears to modulate PTEN in astrocytes by redirecting its homeostasic control through PI3K in a timed fashion.

Intranasal administration of IGF-I improves behavior and Purkinje cell pathology in SCA1 mice

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disease caused by the expansion of polyglutamine repeat within ataxin-1 protein. Cerebellar Purkinje cells are the primary targets of SCA1 pathology. These cells synthesize insulin-like growth factor-I (IGF-I) and express its receptors during their entire life. The aim of present study was to determine if intranasally administered IGF-I to SCA1 transgenic mice suppresses toxic effects of ataxin-1. Two-week old SCA1 heterozygous animals were randomly divided into two treatment groups of IGF-I (30 and 60 microg IGF-I/animal) and a vehicle-treated control group. The wildtype animals served as normal controls. IGF-I or vehicle was administered at 48 h intervals for the total of 10 doses. Animals were then subjected to rotarod test, sacrificed, brains removed and processed for immunohistochemical and Western blot analysis. Radiolabeled IGF-I and bioactive TAT peptide accumulated in the brains of SCA1 mice following intranasal administration validating the use of intranasal route. SCA1 mice showed SCA1 pathology with impaired motor function and downregulation of calcium binding proteins as compared to wildtype mice. However, 30 and 60 microg IGF-I-treated animals showed improved performance on the rotarod as compared to vehicle-treated SCA1 mice with significant improvement (p < 0.05) on day 3 in 60 microg IGF-I group. The immunohistochemical data further showed partial recovery in the expression of calbindin D28k and protein kinase C-gamma in Purkinje cells in IGF-I-treated SCA1 animals. Our results indicate that suppression of ataxin-1-mediated adverse effects by intranasal IGF-I treatment may be of a therapeutic value to treat SCA1.

Potential of progenitors from postnatal cerebellar neuroepithelium and white matter: lineage specified vs. multipotent fate

Progenitors that migrate through the white matter of the postnatal cerebellum give rise to interneurons, astrocytes, and oligodendrocytes. To investigate the lineage potential of progenitors from the neuroepithelium and the white matter, we performed an in vitro clonal analysis in the presence or absence of various growth factors. Clonal progeny of cells labeled with a green fluorescent protein (GFP)-expressing retrovirus was characterized using morphological features and lineage markers. The large majority of clones were homogeneous, containing astrocytes, oligodendrocytes, neurons, or hybrid progenitors-cells labeled with markers for astrocytes and oligodendrocytes. Heterogeneous clones consisted of astrocytes and oligodendrocytes, with only a few mixed glial-neuronal clones. The neuroepithelium contains a higher number of multipotent progenitors than the white matter, pointing to a lineage specification of most of the cerebellar progenitors before their migration to the white matter.

Expression of Mcl-1 in cerebellar granule neurons is regulated by IGF-I in a developmentally specific fashion

Transgenic (Tg) mice that overexpress IGF-I during postnatal brain development exhibit remarkable cerebellar overgrowth characterized by significant increases in granule cell number that is predominantly due to IGF-I anti-apoptotic actions. Using these mice as a model to define the gene expression profile underlying the pro-survival actions of IGF-I, we screened 243 apoptosis-related genes by cDNA arrays and found that Mcl-1 was down-regulated in cerebella of IGF-I Tg mice. Contrary to the results obtained by cDNA array, Northern blot analyses showed that the Mcl-1 mRNA abundance in the cerebella of IGF-I Tg mice at postnatal day 14 (P14) was five times more than that of wild-type (Wt) controls. The increase in Mcl-1 mRNA expression in IGF-I Tg mice was detected as early as P8, peaked at P14, and remained detectable at P20. Both IGF-I Tg and Wt mice showed a similar expression pattern of Mcl-1 mRNA which coincided with the post-mitotic migration and the post-migratory maturation of granule cells. We measured the relative abundance of Mcl-1 protein in the cerebellum by immunoblots and found that anti-apoptotic Mcl-1L was the predominant form, while pro-apoptotic Mcl-1S was minimally detectable. Cerebellar Mcl-1L was 2.6-fold more abundant in IGF-I Tg mice compared with that in their Wt littermates. Using laser capture microdissection followed by RT-PCR, we determined that Mcl-1 mRNA was expressed in granule cells, but not in Purkinje cells. In summary, these findings show that the anti-apoptotic Mcl-1 isoform is expressed in cerebellar granule neurons, which undergo apoptosis during postnatal cerebellar cortical lamination, and Mcl-1 expression is up-regulated by IGF-I overexpression in a developmentally specific manner. These data suggest that anti-apoptotic Mcl-1 may mediate IGF-I pro-survival actions on granule neurons during the development of cerebellar cortex. They also point out pitfalls of cDNA array analyses.

Corticotropin releasing factor enhances survival of cultured GABAergic cerebellar neurons after exposure to a neurotoxin

Corticotropin-releasing factor (CRF), in addition to its role as a hormone in the stress response, functions as a neuromodulator in the cerebellum, where it enhances both the spontaneous and amino acid induced firing rate of Purkinje cells. In the cerebellum, CRF and its two types of receptors (CRF-R(1) and CRF-R(2)) are present during cerebellar development at ages that precede the onset of afferent ingrowth and synaptogenesis, suggesting a distinct role during early cerebellar development. The present study was undertaken to determine whether CRF enhances the survival of cerebellar neurons, in particular GABAergic neurons. Primary cultures of cerebellar neurons obtained from embryonic day 18 mice were composed primarily, but not exclusively, of GABAergic neurons. Although CRF-R(1) is present in most neurons in this culture system, when CRF was added to the medium, no significant change in neuronal survival was observed when compared to control cultures. It is possible that a role for CRF is not seen in growth-promoting culture medium at the plating density chosen for this study and may only be evident when the cells have been exposed to conditions that reduce the likelihood of survival, such as exposure to neurotoxins such as AraC. We propose that, because AraC increases the number of cleaved caspase-3 positive cells, indicating apoptosis, it is possible that a CRF effect involves an inhibition of the apoptotic pathway. Cultures treated with AraC had a decrease in the total number of GABAergic neurons and an increase in apoptotic cells as measured with the apoptotic marker cleaved caspase-3. Co-treatment with CRF rescued many GABAergic neurons. It is interesting to note that apoptotic cells do not exhibit GABA or c-fos positive immunolabeling. Thus, these data support the concept that CRF plays a neuroprotective role in the survival of GABAergic cerebellar neurons in culture after exposure to a neurotoxin.

An investigation into signal transduction mechanisms involved in insulin-induced long-term depression in the CA1 region of the hippocampus

Recent work has demonstrated that brief application of insulin to hippocampal slices can induce a novel form of long-term depression (insulin-LTD) in the CA1 region of the hippocampus; however, the molecular details of how insulin triggers LTD remain unclear. Using electrophysiological and biochemical approaches in the hippocampal slices, we show here that insulin-LTD (i) is specific to 3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor- but not NMDA receptor-mediated synaptic transmission; (ii) is induced and expressed postsynaptically but does not require the activation of ionotropic and metabotropic glutamate receptors; (iii) requires a concomitant Ca(2+) influx through l-type voltage-activated Ca(2+) channels (VACCs) and the release of Ca(2+) from intracellular stores; (iv) requires the series of protein kinases, including protein tyrosine kinase (PTK), phosphatidylinositol 3-kinase (PI3K), and protein kinase C (PKC); (v) is mechanistically distinct from low-frequency stimulation-induced LTD (LFS-LTD) and independent on protein phosphatase 1/2 A (PP1/2 A) and PP2B activation; (vi) is dependent on a rapamycin-sensitive local translation of dendritic mRNA, and (vii) is associated with a persistent decrease in the surface expression of GluR2 subunit. These results suggest that a PI3K/PKC-dependent insulin signaling, which controls postsynaptic surface AMPA receptor numbers through PP-independent endocytosis, may be a major expression mechanism of insulin-LTD in hippocampal CA1 neurons.

Insulin-like growth factor-I improves cerebellar dysfunction but does not prevent cerebellar neurodegeneration in the calcium channel mutant mouse, leaner

The effects of insulin-like growth factor-I (IGF-I) on cerebellar dysfunction and neurodegeneration were investigated in leaner mice, which exhibit cerebellar ataxia and neurodegeneration related to P/Q-type calcium channel mutations. Leaner mice showed significantly reduced serum and cerebellar IGF-I concentrations compared to wild-type mice at postnatal day 30. Behavioral assessment of leaner mice injected with IGF-I subcutaneously for 4 weeks showed partially improved cerebellar function. Histological analysis of IGF-I treated leaner cerebella showed no difference in the number of dying Purkinje cells compared to control leaner cerebella. These results further support potential use of IGF-I as a therapeutic aid for cerebellar ataxia related to calcium channel mutations. Nonetheless, IGF-I administration does not rescue dying cerebellar neurons, which suggests that the beneficial effects of IGF-I may have been achieved through surviving cerebellar neurons.

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.

Neurotrophic factors in the pathogenesis of Rett syndrome

Rett syndrome is characterized by disruption of a period of vigorous brain growth with synapse development. Neurotrophic factors are important regulators of neuronal growth, differentiation, and survival during early brain development. The aims of this study were to study the role of neurotrophic factors in Rett syndrome, specifically whether Rett syndrome has abnormal levels of specific neurotrophic factors in serum and cerebrospinal fluid and whether the changes differ from other neuropediatric patients, for example, those with infantile autism. Four neurotrophic factors were measured: nerve growth factor, brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, and insulin-like growth factor 1 from the frozen cerebrospinal fluid and from serum (except glial cell line-derived neurotrophic factor) by enzyme-linked immunosorbent assay and cerebrospinal fluid glutamate and aspartate by high-performance liquid chromatography (HPLC) method in patients with Rett syndrome. Insulin-like growth factor 1 was measured from the cerebrospinal fluid of patients with infantile autism. We found low concentrations of cerebrospinal fluid nerve growth factor in patients with Rett syndrome compared with control patients. The serum levels and other cerebrospinal fluid neurotrophic factor levels of the patients did not differ from the controls. Patients with Rett syndrome had high cerebrospinal fluid glutamate levels. Patients with infantile autism had low cerebrospinal fluid insulin-like growth factor 1 levels. Nerve growth factor acts especially on cholinergic neurons of the basal forebrain, whereas insulin-like growth factor 1 acts on cerebellar neurons. In Rett syndrome, the forebrain is more severely affected than the other cortical areas. In autism, many studies show hippocampal or cerebellar pathology. Our findings are in agreement with the different morphologic and neurochemical findings (brain growth, affected brain areas, neurotransmitter metabolism) in the two syndromes. Impairment in dendritic development in Rett syndrome could be the consequence of cholinergic deficiency and of neurotrophic factor/glutamate imbalance. Cholinergic gene expression might be influenced by the Rett syndrome gene directly or via the neurotrophic factor system.

Down-regulation of 14-3-3 eta gene expression by IGF-I in mouse cerebellum during postnatal development

Insulin-like growth factor I (IGF-I) overexpression in the postnatal cerebellum of transgenic (Tg) mice results in remarkable cerebellar overgrowth characterized by a near doubling of granule cell number that is predominantly due to inhibition of apoptosis. Using this Tg model we set out to investigate IGF-I anti-apoptotic mechanisms by defining the influence of IGF-I on gene expression. Using a cDNA array technique, we screened a total of 243 mouse apoptosis-related genes, and found that 14-3-3 eta gene expression was significantly reduced in the cerebella of Tg mice compared with their wild-type (Wt) littermates. Using Northern blot analysis to corroborate our microarray finding, we showed that 14-3-3 eta mRNA abundance was decreased from postnatal day P5 through P17. Nonetheless, the expression pattern of 14-3-3 eta in Tg mice followed the same pattern observed in Wt mice, and was indistinguishable from that in Wt mice at P20 and P23. 14-3-3 eta protein abundance, as determined by Western immunoblot analyses, showed similar decreases in the cerebella of Tg mice. In situ hybridization demonstrated that 14-3-3 eta was predominantly, if not exclusively, expressed and regulated in Purkinje cells. 14-3-3 proteins have multiple functions, including participation in pathways that favor cell survival. Our finding of IGF-I-induced down-regulation of 14-3-3 eta expression in Purkinje cell at a time when IGF-I promotes granule cell survival leads us to speculate that down-regulation of 14-3-3 eta may: (a) serve a negative feedback role to modulate Purkinje cell survival, i.e. limit Purkinje cell number, and/or (b) function as part of a distinct signaling mechanism, perhaps one that augments the capacity of Purkinje cells to promote granule cell survival.

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.

Effects of insulin-like growth factor I on climbing fibre synapse elimination during cerebellar development

Functional neural circuit formation includes the process by which redundant synaptic connections formed earlier during development are subsequently eliminated. We report that insulin-like growth factor I (IGF-I) is a candidate factor that influences the developmental transition from multiple to mono innervation of cerebellar Purkinje cells (PCs) by climbing fibres (CFs). Continuous local application of exogenous IGF-I to the mouse cerebellum by means of ethylene-vinyl acetate copolymer (Elvax) significantly increased the degree of multiple CF innervation, when the IGF-I containing Elvax was implanted at postnatal day 8 (P8). In contrast, the IGF-I application starting at P12 had no effect on CF innervation. Conversely, continuous local application of antisera against IGF-I and its receptor significantly decreased the degree of multiple CF innervation when the application started at P8. We found that chronic treatment of exogenous IGF-I from P8 significantly enhanced the CF-mediated excitatory postsynaptic currents (CF-EPSCs). This effect was manifest for the smaller CF-EPSCs but not for the largest CF-EPSC of the multiple-innervated PCs. Conversely, chronic application of antisera from P8 caused attenuation of the largest CF-EPSCs. Other parameters for basic synaptic functions and cerebellar morphology were largely normal after the IGF-I or antisera treatment. These results suggest that IGF-I enhances the strength of developing CF synapses and may promote their survival, whereas the shortage of IGF-I impairs the development of CF synapses and, as a result, may facilitate their elimination. Thus, IGF-I is a potentially important factor among various signalling molecules that can influence CF synapse elimination during cerebellar development.

Intracerebroventricular infusion of insulin-like growth factor-I ameliorates the age-related decline in hippocampal neurogenesis

The dentate gyrus of the hippocampus is one of few regions in the adult mammalian brain characterized by ongoing neurogenesis. Significantly, recent studies indicate that the rate of neurogenesis in the hippocampus declines with age, perhaps contributing to age-related cognitive changes. Although a variety of factors may influence the addition of new neurons in the adult dentate gyrus, the mechanisms responsible for the age-related reduction remain to be established. Insulin-like growth factor-I (IGF-I) is one promising candidate to regulate neurogenesis in the adult and aging brain since it influences neuronal production during development and since, like the rate of neurogenesis, it decreases with age. In the current study, we used bromodeoxyuridine labeling and multilabel immunofluorescence to assess age-related changes in neuronal production in the dentate gyrus of adult Brown Norway x Fischer 344 rats. In addition, we investigated the relationship between changes in neurogenesis and the age-dependent reduction in IGF-I by evaluating the effect of i.c.v. infusion of IGF-I on neurogenesis in the senescent dentate gyrus. The analyses revealed an age-dependent reduction in the number of newly generated cells in the adult dentate subgranular proliferative zone and, in addition, a 60% reduction in the differentiation of newborn cells into neurons. Restoration of IGF-I levels in senescent rats significantly restored neurogenesis through an approximately three-fold increase in neuronal production. The results of this study suggest that IGF-I may be an important regulator of neurogenesis in the adult and aging hippocampus and that an age-related decline in IGF-I-dependent neurogenesis could contribute to age-related cognitive changes.

The PEHO syndrome

The progressive encephalopathy with edema, hypsarrhythmia, and optic atrophy (PEHO) syndrome is a pediatric disorder of unknown origin, characterized by a combination of postnatally progressive encephalopathy, hypsarrhythmia, and optic atrophy. The pathological findings are early progressive atrophy of the cerebellum, brainstem, and optic nerves. Nitric acid (NO) has recently been implicated in the mechanisms of seizure activity and neurodegeneration, which are both very active in the PEHO syndrome. However, recent studies have provided evidence that insulin-like growth factor 1 (IGF-1) may prevent the NO-mediated neuronal damage and is essential for the survival of the cerebellar granule cells. These cells will degenerate in the PEHO syndrome. In this study, we set out to test the hypothesis that NO production is activated in the PEHO syndrome and that NO production may be correlated with the reduced production of IGF-1 in the brain. Cerebrospinal fluid IGF-1 was determined with an RIA kit and NO metabolites by the Griess calorimetric method. In patients with the PEHO syndrome, as compared with controls, the levels of IGF-1 were reduced and the levels of nitrite/nitrate were markedly elevated. Defective production of IGF-1 probably reflects the underlying neurodegeneration and the increase in NO production probably reflects the seizure activity and/or neurodegeneration. These are the first biochemical abnormalities found in the PEHO syndrome and their study may lead to a better understanding of this devasting disease.

Expression of cerebellar long-term depression requires postsynaptic clathrin-mediated endocytosis

Cerebellar long-term depression (LTD) is a cellular model system of information storage that may underlie certain forms of motor learning. While cerebellar LTD is expressed as a selective modification of postsynaptic AMPA receptors, this might involve changes in receptor number/distribution, unitary conductance, kinetics, or glutamate affinity. The observation that GluR2-containing synaptic AMPA receptors could be internalized by regulated clathrin-mediated endocytosis suggested that this process could underlie LTD expression. To test this hypothesis, we postsynaptically applied dynamin and amphiphysin peptides that interfere with the clathrin endocytotic complex and found that they blocked LTD expression in cultured Purkinje neurons. In addition, induction of LTD and attenuation of AMPA responses by stimulation of clathrin-mediated endocytosis occluded each other. These findings suggest that the expression of cerebellar LTD requires clathrin-mediated internalization of postsynaptic AMPA receptors.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Neuroprotective actions of peripherally administered insulin-like growth factor I in the injured olivo-cerebellar pathway

Exogenous administration of insulin-like growth factor I (IGF-I) restores motor function in rats with neurotoxin-induced cerebellar deafferentation. We first determined that endogenous IGFs are directly involved in the recovery process because infusion of an IGF-I receptor antagonist into the lateral ventricle blocks gradual recovery of limb coordination that spontaneously occurs after partial deafferentation of the olivo-cerebellar circuitry. We then analysed mechanisms whereby exogenous IGF-I restores motor function in rats with complete damage of the olivo-cerebellar pathway. Treatment with IGF-I normalized several markers of cell function in the cerebellum, including calbindin, glutamate receptor 1 (GluR1), gamma-aminobutyric acid (GABA) and glutamate, which are all depressed after 3-acetylpyridine (3AP)-induced deafferentation. IGF-I also promoted functional reinnervation of the cerebellar cortex by inferior olive (IO) axons. In the IO, increased expression of bax in neurons and bcl-X in astrocytes after 3AP was significantly reduced by IGF-I treatment. On the contrary, IGF-I prevented the decrease in poly-sialic-acid neural cell adhesion molecule (PSA-NCAM) and GAP-43 expression induced by 3AP in IO cells. IGF-I also significantly increased the number of neurons expressing bcl-2 in brainstem areas surrounding the IO. Altogether, these results indicate that subcutaneous IGF-I therapy promotes functional recovery of the olivo-cerebellar pathway by acting at two sites within this circuitry: (i) by modulating death- and plasticity-related proteins in IO neurons; and (ii) by impinging on homeostatic mechanisms leading to normalization of cell function in the cerebellum. These results provide insight into the neuroprotective actions of IGF-I and may be of practical consequence in the design of new therapeutic approaches for neurodegenerative diseases.