Insulin-like growth factor I (IGF-I) protects cells from apoptosis by Alzheimer's V642I mutant amyloid precursor protein through IGF-I receptor in an IGF-binding protein-sensitive manner

Effect of gastrodin against cognitive impairment and neurodegeneration in APP/PS1 mice via regulating gut microbiota-gut-brain axis

Gastrodin (Gas) has exhibited protective activity in neurological disorders. Here, we investigated the neuroprotective effect and potential mechanisms of Gas against cognitive impairment via regulating gut microbiota. APPswe/PSEN1dE9 transgenic (APP/PS1) mice were treated intragastrically with Gas for 4 weeks, and then cognitive deficits, deposits of amyloid-β (Aβ) and phosphorylation of tau were analyzed. The expression levels of insulin-like growth factor-1 (IGF-1) pathway-related proteins, such as cAMP response element-binding protein (CREB), were detected. Meanwhile, gut microbiota composition was evaluated. Our results showed that Gas treatment significantly improved cognitive deficits and Aβ deposition in APP/PS1 mice. Moreover, Gas treatment increased the level of Bcl-2 and decreased level of Bax and ultimately inhibited neuronal apoptosis. Gas treatment markedly increased the expression levels of IGF-1 and CREB in APP/PS1 mice. Moreover, Gas treatment improved abnormal composition and structure of gut microbiota in APP/PS1 mice. These findings revealed that Gas actively participated in regulating the IGF-1 pathway to inhibit neuronal apoptosis via the gut-brain axis and that it can be considered a new therapeutic strategy against Alzheimer's disease.

Identification of the molecular mechanism of insulin-like growth factor-1 (IGF-1): a promising therapeutic target for neurodegenerative diseases associated with metabolic syndrome

Neurodegenerative disorders are accompanied by neuronal degeneration and glial dysfunction, resulting in cognitive, psychomotor, and behavioral impairment. Multiple factors including genetic, environmental, metabolic, and oxidant overload contribute to disease progression. Recent evidences suggest that metabolic syndrome is linked to various neurodegenerative diseases. Metabolic syndrome (MetS) is known to be accompanied by symptoms such as hyperglycemia, abdominal obesity, hypertriglyceridemia, and hypertension. Despite advances in knowledge about the pathogenesis of neurodegenerative disorders, effective treatments to combat neurodegenerative disorders caused by MetS have not been developed to date. Insulin growth factor-1 (IGF-1) deficiency has been associated with MetS-related pathologies both in-vivo and in-vitro. IGF-1 is essential for embryonic and adult neurogenesis, neuronal plasticity, neurotropism, angiogenesis, metabolic function, and protein clearance in the brain. Here, we review the evidence for the potential therapeutic effects of IGF-1 in the neurodegeneration related to metabolic syndrome. We elucidate how IGF-1 may be involved in molecular signaling defects that occurs in MetS-related neurodegenerative disorders and highlight the importance of IGF-1 as a potential therapeutic target in MetS-related neurological diseases.

Cyclic Glycine-Proline Improves Memory and Reduces Amyloid Plaque Load in APP/PS1 Transgenic Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative condition that is pathologically characterized by the presence of amyloid plaques and neurofibrillary tangles. Animal models of AD have been useful in understanding the disease process and in investigating the effects of compounds on pathology and behavior. APP/PS1 mice develop amyloid plaques and show memory impairment. Cyclic glycine-proline (cGP) is a cyclic dipeptide that is likely produced from a tripeptide, glycine-proline-glutamate, which itself is generated after proteolytic cleavage of insulin-like growth factor-1. Here, we show that cGP improves spatial memory and reduces amyloid plaque burden in APP/PS1 mice. The results thus suggest that cGP could potentially provide beneficial effects in AD.

Down-Regulation of Insulin Like Growth Factor 1 Involved in Alzheimer’s Disease via MAPK, Ras, and FoxO Signaling Pathways

The inability to halt or even delay the course of Alzheimer's disease (AD) forces the development of new molecular signatures and therapeutic strategies. Insulin like growth factor 1 (IGF1) is a promising target for AD treatment, yet exact mechanisms of AD ascribed to IGF1 remain elusive. Herein, gene expression profiles of 195 samples were analyzed and 19,245 background genes were generated, among which 4,424 differentially expressed genes (DEGs) were overlapped between AD/control and IGF1-low/high groups. Based on such DEGs, seven co-expression modules were established by weight gene correlation network analysis (WGCNA). The turquoise module had the strongest correlation with AD and IGF1-low expression, the DEGs of which were enriched in GABAergic synapse, long-term potentiation, mitogen-activated protein kinase (MAPK), Ras, and forkhead box O (FoxO) signaling pathways. Furthermore, cross-talking pathways of IGF1, including MAPK, Ras, and FoxO signaling pathways were identified in the protein-protein interaction network. According to the area under the curve (AUC) analysis, down-regulation of IGF1 exhibited good diagnostic performance in AD prediction. Collectively, our findings highlight the involvement of low IGF1 in AD pathogenesis via MAPK, Ras, and FoxO signaling pathways, which might advance strategies for the prevention and therapy of AD based on IGF1 target.

Dysregulation of IGF-1/GLP-1 signaling in the progression of ALS: potential target activators and influences on neurological dysfunctions

The prominent causes for motor neuron diseases like ALS are demyelination, immune dysregulation, and neuroinflammation. Numerous research studies indicate that the downregulation of IGF-1 and GLP-1 signaling pathways plays a significant role in the progression of ALS pathogenesis and other neurological disorders. In the current review, we discussed the dysregulation of IGF-1/GLP-1 signaling in neurodegenerative manifestations of ALS like a genetic anomaly, oligodendrocyte degradation, demyelination, glial overactivation, immune deregulation, and neuroexcitation. In addition, the current review reveals the IGF-1 and GLP-1 activators based on the premise that the restoration of abnormal IGF-1/GLP-1 signaling could result in neuroprotection and neurotrophic effects for the clinical-pathological presentation of ALS and other brain diseases. Thus, the potential benefits of IGF-1/GLP-1 signal upregulation in the development of disease-modifying therapeutic strategies may prevent ALS and associated neurocomplications.

Mitochondrial Peptide Humanin Protects Silver Nanoparticles-Induced Neurotoxicity in Human Neuroblastoma Cancer Cells (SH-SY5Y)

The extensive usage of silver nanoparticles (AgNPs) as medical products such as antimicrobial and anticancer agents has raised concerns about their harmful effects on human beings. AgNPs can potentially induce oxidative stress and apoptosis in cells. However, humanin (HN) is a small secreted peptide that has cytoprotective and neuroprotective cellular effects. The aim of this study was to assess the harmful effects of AgNPs on human neuroblastoma SH-SY5Y cells and also to investigate the protective effect of HN from AgNPs-induced cell death, mitochondrial dysfunctions, DNA damage, and apoptosis. AgNPs were prepared with an average size of 18 nm diameter to study their interaction with SH-SY5Y cells. AgNPs caused a dose-dependent decrease of cell viability and proliferation, induced loss of plasma-membrane integrity, oxidative stress, loss of mitochondrial membrane potential (MMP), and loss of ATP content, amongst other effects. Pretreatment or co-treatment of HN with AgNPs protected cells from several of these AgNPs induced adverse effects. Thus, this study demonstrated for the first time that HN protected neuroblastoma cells against AgNPs-induced neurotoxicity. The mechanisms of the HN-mediated protective effect on neuroblastoma cells may provide further insights for the development of novel therapeutic agents against neurodegenerative diseases.

Distinctive Effects of Aerobic and Resistance Exercise Modes on Neurocognitive and Biochemical Changes in Individuals with Mild Cognitive Impairment

Background:

Decreased levels of the neuroprotective growth factors, low-grade inflammation, and reduced neurocognitive functions during aging are associated with neurodegenerative diseases, such as Alzheimer’s disease. Physical exercise modifies these disadvantageous phenomena while a sedentary lifestyle promotes them.

Purpose:

The purposes of the present study included investigating whether both aerobic and resistance exercise produce divergent effects on the neuroprotective growth factors, inflammatory cytokines, and neurocognitive performance, and further exploring whether changes in the levels of these molecular biomarkers are associated with alterations in neurocognitive performance.

Methods:

Fifty-five older adults with amnestic MCI (aMCI) were recruited and randomly assigned to an aerobic exercise (AE) group, a resistance exercise (RE) group, or a control group. The assessment included neurocognitive measures [e.g., behavior and event-related potential (ERP)] during a task-switching paradigm, as well as circulating neuroprotective growth factors (e.g., BDNF, IGF-1, VEGF, and FGF-2) and inflammatory cytokine (e.g., TNF-α, IL-1β, IL-6, IL-8, and IL-15) levels at baseline and after either a 16-week aerobic or resistance exercise intervention program or a control period.

Results:

Aerobic and resistance exercise could effectively partially facilitate neurocognitive performance [e.g., accuracy rates (ARs), reaction times during the heterogeneous condition, global switching cost, and ERP P3 amplitude] when the participants performed the task switching paradigm although the ERP P2 components and P3 latency could not be changed. In terms of the circulating molecular biomarkers, the 16-week exercise interventions did not change some parameters (e.g., leptin, VEGF, FGF-2, IL-1β, IL-6, and IL-8). However, the peripheral serum BDNF level was significantly increased, and the levels of insulin, TNF-α, and IL-15 levels were significantly decreased in the AE group, whereas the RE group showed significantly increased IGF-1 levels and decreased IL-15 levels. The relationships between the changes in neurocognitive performance (AR and P3 amplitudes) and the changes in the levels of neurotrophins (BDNF and IGF-1)/inflammatory cytokines (TNF-α) only approached significance.

Conclusion:

These findings suggested that in older adults with aMCI, not only aerobic but also resistance exercise is effective with regard to increasing neurotrophins, reducing some inflammatory cytokines, and facilitating neurocognitive performance. However, the aerobic and resistance exercise modes likely employed divergent molecular mechanisms on neurocognitive facilitation.

Sevoflurane downregulates IGF‑1 via microRNA‑98

Insulin-like growth factor (IGF)-1 functions as a neuroprotective hormone and may protect against cognitive impairment, which may occur as a result of sevoflurane exposure. The aim of the present study was to assess the effect of sevoflurane on the production of IGF‑1 and investigate the molecular mechanisms underlying this regulation. The BRL rat hepatocyte cell line and adult mice were exposed to 1 or 2 minimal alveolar concentrations sevoflurane for 4 or 8 h. IGF‑1 and microRNA‑98 levels were quantified using an enzyme‑linked immunosorbent assay, western blotting and reverse transcription‑quantitative polymerase chain reaction analyses. The importance of microRNA‑98 in the regulation of IGF‑1 by sevoflurane was investigated using a microRNA‑98 inhibitor. Sevoflurane treatment reduced IGF‑1 levels and simultaneously upregulated microRNA‑98 expression levelsin rat hepatocytes and adult mice. Inhibition of microRNA‑98 attenuated this effect. Therefore, sevoflurane may reduce the synthesis of IGF‑1 by upregulating microRNA‑98 expression.

Humanin: Functional Interfaces with IGF-I

Humanin is the first newly discovered peptide encoded in the mitochondrial genome in over three decades. It is the first member of a novel class of mitochondrial derived peptides. This small, 24 amino acid peptide was initially discovered to have neuroprotective effects and subsequent experiments have shown that it is beneficial in a diverse number of disease models including stroke, cardiovascular disease, and cancer. Over a decade ago, our lab found that humanin bound IGFBP-3 and more recent studies have found it to decrease circulating IGF-I levels. In turn, IGF-I also seems to regulate humanin levels and in this review, we cover the known interaction between humanin and IGF-I. Although the exact mechanism for how humanin and IGF-I regulate each other still needs to be elucidated, it is clear that humanin is a new player in IGF-I signaling.

The participation of insulin-like growth factor-binding protein 3 released by astrocytes in the pathology of Alzheimer's disease

Background

Alzheimer’s disease (AD) is characterized by senile plaques, extracellular deposits composed primarily of amyloid–beta (Aβ), and neurofibrillary tangles, which are abnormal intracellular inclusions containing hyperphosphorylated tau.

The amyloid cascade hypothesis posits that the deposition of Aβ in the brain parenchyma initiates a sequence of events that leads to dementia. However, the molecular process by which the extracellular accumulation of Aβ peptides promotes intracellular pathologic changes in tau filaments remains unclear. To elucidate this process, we presumed that astrocytes might trigger neuronal reactions, leading to tau phosphorylation. In this study, we examined AD pathology from the perspective of the astrocyte-neuron interaction.

Results

A cytokine-array analysis revealed that Aβ stimulates astrocytes to release several chemical mediators that are primarily related to inflammation and cell adhesion. Among those mediators, insulin-like growth factor (IGF)-binding protein 3 (IGFBP-3) was highly upregulated.

In AD brains, the expression of IGFBP-3 was found to be increased by western blot analysis, and increased expression of IGFBP-3 was observed in astrocytes via fluorescence microscopy.

In addition, we reproduced the increase in IGFBP-3 after treatment with Aβ using human astrocytoma cell lines and found that IGFBP-3 was expressed via calcineurin. In AD brains, the activated forms of calcineurin were found to be increased by western blot analysis, and increased expression of calcineurin was observed in astrocytes via fluorescence microscopy.

When Ser9 of glycogen synthase kinase-3β (GSK-3β) is phosphorylated, GSK-3β is controlled and tau phosphorylation is suppressed. Aβ suppresses the phosphorylation of GSK-3β, leading to tau phosphorylation. In this study, we found that IGF-Ι suppressed tau phosphorylation induced by Aβ, although IGFBP-3 inhibited this property of IGF-Ι. As a result, IGFBP-3 contributed to tau phosphorylation and cell death induced by Aβ.

Conclusions

Our study suggested that calcineurin in astrocytes was activated by Aβ, leading to IGFBP-3 release. We further demonstrated that IGFBP-3 produced by astrocytes induced tau phosphorylation in neurons. Our study provides novel insights into the role of astrocytes in the induction of tau phosphorylation and suggests that IGFBP-3 could be an important link between Aβ and tau pathology and an important therapeutic target.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0174-2) contains supplementary material, which is available to authorized users.

Amyloid beta-mediated epigenetic alteration of insulin-like growth factor binding protein 3 controls cell survival in Alzheimer's disease

Swedish double mutation (KM670/671NL) of amyloid precursor protein (APP) is reported to increase toxic amyloid β (Aβ) production via aberrant cleavage at the β-secretase site and thereby cause early-onset Alzheimer's disease (AD). However, the underlying molecular mechanisms leading to AD pathogenesis remains largely unknown. Previously, our transcriptome sequence analyses revealed global expressional modifications of over 600 genes in APP-Swedish mutant-expressing H4 (H4-sw) cells compared to wild type H4 cells. Insulin-like growth factor binding protein 3 (IGFBP3) is one gene that showed significantly decreased mRNA expression in H4-sw cells. In this study, we investigated the functional role of IGFBP3 in AD pathogenesis and elucidated the mechanisms regulating its expression. We observed decreased IGFBP3 expression in the H4-sw cell line as well as the hippocampus of AD model transgenic mice. Treatment with exogenous IGFBP3 protein inhibited Aβ_1–42- induced cell death and caspase-3 activity, whereas siRNA-mediated suppression of IGFBP3 expression induced cell death and caspase-3 cleavage. In primary hippocampal neurons, administration of IGFBP3 protein blocked apoptotic cell death due to Aβ_1–42 toxicity. These data implicate a protective role for IGFBP3 against Aβ_1–42-mediated apoptosis. Next, we investigated the regulatory mechanisms of IGFBP3 expression in AD pathogenesis. We observed abnormal IGFBP3 hypermethylation within the promoter CpG island in H4-sw cells. Treatment with the DNA methyltransferase inhibitor 5-aza-2′-deoxycytidine restored IGFBP3 expression at both the mRNA and protein levels. Chronic exposure to Aβ_1–42 induced IGFBP3 hypermethylation at CpGs, particularly at loci −164 and −173, and subsequently suppressed IGFBP3 expression. Therefore, we demonstrate that expression of anti-apoptotic IGFBP3 is regulated by epigenetic DNA methylation, suggesting a mechanism that contributes to AD pathogenesis.

Changes in cerebrospinal fluid and blood plasma levels of IGF-II and its binding proteins in Alzheimer's disease: an observational study

Background

The Insulin-like Growth Factor (IGF)-related system is implicated in neuroregeneration and cell repair, as well as regulating lifespan. IGF-II, one component of this system, has also been found to affect memory functions in a rat model. In this study we explored changes in the IGF-related system in patients with Alzheimer’s disease (AD), including changes in IGF-II levels.

Methods

We measured blood plasma and cerebrospinal fluid (CSF) levels of IGF-I, IGF-II, IGFBP-2 and IGFBP-3 in 72 healthy controls and 92 patients with AD.

Results

We found significantly lower blood plasma levels of IGF-II and IGFBP-3 in patients with AD, compared with controls. The levels of IGF-II and IGFBP-2 were significantly elevated in the CSF from patients with AD. We also found correlations between established CSF biomarkers for AD (tau and P-tau) and components of the IGF system.

Conclusions

CSF and blood plasma levels of IGF-II and some of its binding proteins are changed in patients with AD. Further investigation into this area may unravel important clues to the nature of this disease.

IGF-1 induces iNOS expression via the p38 MAPK signal pathway in the anti-apoptotic process in pulmonary artery smooth muscle cells during PAH

Apoptosis and cell proliferation are two important cellular processes that determine the accumulation of pulmonary artery smooth muscle cells (PASMC) during pulmonary arterial hypertension (PAH). Insulin-like growth factor 1 (IGF-1) is an endocrine and autocrine/paracrine growth factor that circulates at high levels in the plasma and is expressed in most cell types. IGF-1 has major effects on development, cell growth and differentiation, also tissue repair. Inducible nitric oxide synthase (iNOS) has been shown to serve many vasoprotective roles in vascular smooth muscle cells (VSMCs) including inhibition of VSMC proliferation and migration and stimulation of endothelial cell growth. In this study, we investigated the involvement of iNOS in the process of IGF-1-induced inhibition of PASMC apoptosis. We also examined the role of p38 mitogen-activated protein kinase (MAPK) in the IGF-1-induced iNOS activation. Our results show that exogenous IGF-1 induced the up-regulation of iNOS in PASMC. Immunofluorescence of IGF-1 and iNOS showed a decreased immunostaining of both IGF-1 and iNOS in the cytoplasm and the perinucleus under serum deprivation condition. iNOS inhibition in PASMC in vitro markedly induced IGF-1-mediated anti-apoptosis as assessed by the cell viability measurement, Western blot, mitochondrial potential analysis and nuclear morphology determination. A p38 MAPK inhibitor blocked all the effects of IGF-1 on iNOS. Our findings suggest that IGF-1 inhibits cells apoptosis in PASMC by activating the p38 MAPK-iNOS transduction pathway. This mechanism may contribute to the accumulation of PASMC in early human PAH.

Insulin, IGF-1 and GLP-1 signaling in neurodegenerative disorders: targets for disease modification?

Insulin and Insulin Growth Factor-1 (IGF-1) play a major role in body homeostasis and glucose regulation. They also have paracrine/autocrine functions in the brain. The Insulin/IGF-1 signaling pathway contributes to the control of neuronal excitability, nerve cell metabolism and cell survival. Glucagon like peptide-1 (GLP-1), known as an insulinotropic hormone has similar functions and growth like properties as insulin/IGF-1. Growing evidence suggests that dysfunction of these pathways contribute to the progressive loss of neurons in Alzheimer's disease (AD) and Parkinson's disease (PD), the two most frequent neurodegenerative disorders. These findings have led to numerous studies in preclinical models of neurodegenerative disorders targeting insulin/IGF-1 and GLP-1 signaling with currently available anti-diabetics. These studies have shown that administration of insulin, IGF-1 and GLP-1 agonists reverses signaling abnormalities and has positive effects on surrogate markers of neurodegeneration and behavioral outcomes. Several proof-of-concept studies are underway that attempt to translate the encouraging preclinical results to patients suffering from AD and PD. In the first part of this review, we discuss physiological functions of insulin/IGF-1 and GLP-1 signaling pathways including downstream targets and receptors distribution within the brain. In the second part, we undertake a comprehensive overview of preclinical studies targeting insulin/IGF-1 or GLP-1 signaling for treating AD and PD. We then detail the design of clinical trials that have used anti-diabetics for treating AD and PD patients. We close with future considerations that treat relevant issues for successful translation of these encouraging preclinical results into treatments for patients with AD and PD.

Insulin-like growth factor I and Alzheimer´s disease: therapeutic prospects?

The search for a cure of Alzheimer's dementia is restless. In recent years, unexpected epidemiological data showing a protective effect of anti-inflammatory and cholesterol-lowering drugs gave way to clinical trials with these compounds. Now, a newly described mechanism indicating that brain amyloid clearance is modulated by serum insulin-like growth factor I may also lead to new trials with this growth factor. Insulin-like growth factor I is an abundant circulating hormone with potent central actions whose levels in serum appear to be altered in Alzheimer's patients. Amyloid clearance, a potential therapeutic target in Alzheimer's disease was mostly neglected until recent antiamyloid therapies proved to involve a peripheral amyloid sink. Although more work in animal models are required, the evidence available strongly indicates that insulin-like growth factor I therapy in Alzheimer's dementia may be addressing pathogenic processes.

Dietary vitamin E modulates differential gene expression in the rat hippocampus: Potential implications for its neuroprotective properties

A wide range of cell culture, animal and human epidemiological studies are suggestive of a role of vitamin E (VE) in brain function and in the prevention of neurodegeneration. However, the underlying molecular mechanisms remain largely unknown. In the current investigation Affymetrix gene chip technology was utilised to establish the impact of chronic VE deficiency on hippocampal genes expression. Male albino rats were fed either a VE deficient or standard diet (60 mg/kg feed) for a period of 9 months. Rats were sacrificed, the hippocampus removed and genes expression established in individual animals. VE deficiency showed to have a strong impact on genes expression in the hippocampus. An important number of genes found to be regulated by VE was associated with hormones and hormone metabolism, nerve growth factor, apoptosis, dopaminergic neurotransmission, and clearance of amyloid-beta and advanced glycated endproducts. In particular, VE strongly affected the expression of an array of genes encoding for proteins directly or indirectly involved in the clearance of amyloid beta, changes which are consistent with a protective effect of VE on Alzheimer's disease progression.

Insulin-like growth factor-I and insulin-like growth factor binding protein-3 in Alzheimer's disease

CONTEXT

Few large studies have been conducted to assess the relationship between circulating IGF and late-life cognition.

OBJECTIVE

The aim of the study was to assess the relationship between IGF-I and IGF binding protein-3 (IGFBP-3) serum levels and cognitive impairment, including Alzheimer's disease (AD).

METHODS

In this multicentric cross-sectional study, 694 elderly subjects (218 men, 476 women; 78.6 ± 6.7 yr old) were included; 481 had memory complaints and were diagnosed, after comprehensive cognitive assessment, with AD (n = 224) or mild cognitive impairment (MCI) (n = 257). The control group was comprised of 213 subjects without memory complaint and with normal cognition (recruited among patients' caregivers). IGF-I and IGFBP-3 serum levels were determined by ELISA.

RESULTS

IGF-I and IGFBP-3 serum levels were significantly associated with cognitive status in men (IGF-I, 137 ± 69 ng/ml for AD vs. 178 ± 88 ng/ml for MCI and 172 ± 91 ng/ml for controls, P = 0.01; IGFBP-3, 3675 ± 1542 ng/ml for AD vs. 4143 ± 1828 ng/ml for MCI and 4488 ± 1893 ng/ml for controls, P = 0.04). In women, IGFBP-3 was significantly associated with cognitive status (3781 ± 1351 ng/ml for AD vs. 4190 ± 1408 ng/ml for MCI and 4390 ± 1552 ng/ml for controls; P < 0.001), but no significant differences between groups for IGF-I occurred. After adjustment for confounding variables (age, educational level, body mass index, diabetes, apolipoprotein E ε4 status), logistic regression indicated that IGF-I [odds ratio (95% confidence interval) = 0.48 (0.26-0.88)] and IGFBP-3 [odds ratio (95% confidence interval) = 0.71 (0.52-0.97)] serum levels were independently associated with AD in men, but not in women.

CONCLUSIONS

We report a significant association between low IGF-I and IGFBP-3 serum levels and AD in men, but not in women.

Alzheimer's disease: pathological mechanisms and the beneficial role of melatonin

Alzheimer's disease (AD) is a highly complex neurodegenerative disorder of the aged that has multiple factors which contribute to its etiology in terms of initiation and progression. This review summarizes these diverse aspects of this form of dementia. Several hypotheses, often with overlapping features, have been formulated to explain this debilitating condition. Perhaps the best-known hypothesis to explain AD is that which involves the role of the accumulation of amyloid-β peptide in the brain. Other theories that have been invoked to explain AD and summarized in this review include the cholinergic hypothesis, the role of neuroinflammation, the calcium hypothesis, the insulin resistance hypothesis, and the association of AD with peroxidation of brain lipids. In addition to summarizing each of the theories that have been used to explain the structural neural changes and the pathophysiology of AD, the potential role of melatonin in influencing each of the theoretical processes involved is discussed. Melatonin is an endogenously produced and multifunctioning molecule that could theoretically intervene at any of a number of sites to abate the changes associated with the development of AD. Production of this indoleamine diminishes with increasing age, coincident with the onset of AD. In addition to its potent antioxidant and anti-inflammatory activities, melatonin has a multitude of other functions that could assist in explaining each of the hypotheses summarized above. The intent of this review is to stimulate interest in melatonin as a potentially useful agent in attenuating and/or delaying AD.

IGF-I gene variability is associated with an increased risk for AD

Insulin-like growth factor I (IGF-I), a neuroprotective factor with a wide spectrum of actions in the adult brain, is involved in the pathogenesis of Alzheimer's disease (AD). Circulating levels of IGF-I change in AD patients and are implicated in the clearance of brain amyloid beta (Aβ) complexes. To investigate this hypothesis, we screened the IGF-I gene for various well known single nucleotide polymorphisms (SNPs) covering % of the gene variability in a population of 2352 individuals. Genetic analysis indicated different distribution of genotypes of 1 single nucleotide polymorphism, and 1 extended haplotype in the AD population compared with healthy control subjects. In particular, the frequency of rs972936 GG genotype was significantly greater in AD patients than in control subjects (63% vs. 55%). The rs972936 GG genotype was associated with an increased risk for disease, independently of apolipoprotein E genotype, and with enhanced circulating levels of IGF-I. These findings suggest that polymorphisms within the IGF-I gene could infer greater risk for AD through their effect on IGF-I levels, and confirm the physiological role IGF-I in the pathogenesis of AD.

Critical Role for Sphingosine Kinase-1 in Regulating Survival of Neuroblastoma Cells Exposed to Amyloid-β Peptide

We examined the role of sphingosine kinase-1 (SphK1), a critical regulator of the ceramide/sphingosine 1-phosphate (S1P) biostat, in the regulation of death and survival of SH-SY5Y neuroblastoma cells in response to amyloid beta (Abeta) peptide (25-35). Upon incubation with Abeta, SH-SY5Y cells displayed a marked down-regulation of SphK1 activity coupled with an increase in the ceramide/S1P ratio followed by cell death. This mechanism was redox-sensitive; N-acetylcysteine totally abrogated the down-regulation of SphK1 activity and strongly inhibited Abeta-induced cell death. SphK1 overexpression impaired the cytotoxicity of Abeta, whereas SphK1 silencing by RNA interference mimicked Abeta-induced cell death, thereby establishing a critical role for SphK1. We further demonstrated that SphK1 could mediate the well established cytoprotective action of insulin-like growth factor (IGF-I) against Abeta toxicity. A dominant-negative form of SphK1 or its pharmacological inhibition not only abrogated IGF-I-triggered stimulation of SphK1 but also hampered IGF-I protective effect. Similarly to IGF-I, the neuroprotective action of TGF-beta1 was also dependent on SphK1 activity; activation of SphK1 as well as cell survival were impeded by a dominant-negative form of SphK1. Taken together, these results provide the first illustration of SphK1 role as a critical regulator of death and survival of Abeta-treated cells.

Brain mitochondrial dysfunction as a link between Alzheimer's disease and diabetes

It has been argued that in late-onset Alzheimer's disease a disturbance in the control of neuronal glucose metabolism consequent to impaired insulin signalling strongly resembles the pathophysiology of type 2 diabetes in non-neural tissue. The fact that mitochondria are the major generators and direct targets of reactive oxygen species led several investigators to foster the idea that oxidative stress and damage in mitochondria are contributory factors to several disorders including Alzheimer's disease and diabetes. Since brain possesses high energetic requirements, any decline in brain mitochondria electron chain could have a severe impact on brain function and particularly on the etiology of neurodegenerative diseases. This review is primarily focused in the discussion of brain mitochondrial dysfunction as a link between diabetes and Alzheimer's disease.

Cognitive and physical activity differently modulate disease progression in the amyloid precursor protein (APP)-23 model of Alzheimer's disease

BACKGROUND

In aging mice, activity maintains hippocampal plasticity and adult hippocampal neurogenesis at a level corresponding to a younger age. Here we studied whether physical exercise and environmental enrichment would also affect brain plasticity in a mouse model of Alzheimer's disease (AD).

METHODS

Amyloid precursor protein (APP)-23 mice were housed under standard or enriched conditions or in cages equipped with a running wheel. We assessed beta-amyloid plaque load, adult hippocampal neurogenesis, spatial learning, and mRNA levels of trophic factors in the brain.

RESULTS

Despite stable beta-amyloid plaque load, enriched-living mice showed improved water maze performance, an up-regulation of hippocampal neurotrophin (NT-3) and brain-derived neurotrophic factor (BDNF) and increased hippocampal neurogenesis. In contrast, despite increased bodily fitness, wheel-running APP23 mice showed no change in spatial learning and no change in adult hippocampal neurogenesis but a down-regulation of hippocampal and cortical growth factors.

CONCLUSIONS

We conclude that structural and molecular prerequisites for activity-dependent plasticity are preserved in mutant mice with an AD-like pathology. Our study might help explain benefits of activity for the aging brain but also demonstrates differences between physical and more cognitive activity. It also suggests a possible cellular correlate for the dissociation between structural and functional pathology often found in AD.

Neuronal life-and-death signaling, apoptosis, and neurodegenerative disorders

When subjected to excessive oxidative stress, neurons may respond adaptively to overcome the stress, or they may activate a programmed cell death pathway called apoptosis. Apoptosis is characterized by alterations in mitochondria and the endoplasmic reticulum and activation of cysteine proteases called caspases. Increasing evidence suggests that apoptotic biochemical cascades are involved in the dysfunction and death of neurons in neurodegenerative disorders such as Alzheimer's, Parkinson, and Huntington's diseases. Studies of normal aging, of genetic mutations that cause disease, and of environmental factors that affect disease risk are revealing cellular and molecular alterations that may cause excessive oxidative stress and trigger neuronal apoptosis. Accumulation of self-aggregating proteins such as amyloid beta-peptide, tau, alpha-synuclein, and huntingtin may be involved in apoptosis both upstream and downstream of oxidative stress. Membrane-associated oxidative stress resulting in perturbed lipid metabolism and disruption of cellular calcium homeostasis may trigger apoptosis in several different neurodegenerative disorders. Counteracting neurodegenerative processes are an array of mechanisms including neurotrophic factor signaling, antioxidant enzymes, protein chaperones, antiapoptotic proteins, and ionostatic systems. Emerging findings suggest that the resistance of neurons to death during aging can be enhanced by modifications of diet and lifestyle.

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 protects against amyloid beta-peptide toxicity in brain mitochondria of diabetic rats

This study compared the status of brain mitochondria isolated from 12-week streptozotocin (STZ)-diabetic rats versus STZ-diabetic animals treated with insulin during a period of 4 weeks. Brain mitochondria isolated from 12-week citrate (vehicle)-treated rats were used as control. For that purpose, several mitochondrial parameters were evaluated: respiratory indexes (respiratory control ratio (RCR) and ADP/O ratio), transmembrane potential (DeltaPsim), repolarization lag phase, repolarization level, ATP, glutathione and coenzyme Q (CoQ) contents, production of H2O2, ATPase activity, and the capacity of mitochondria to accumulate Ca2+. Furthermore, the effect of Abeta1-40 was also analyzed. We observed that STZ-induced diabetes promoted a significant decrease in mitochondrial CoQ9, ATPase activity, and a lower capacity of mitochondria to accumulate Ca2+ when compared with control and insulin-treated diabetic rats. The presence of 4 microM Abeta1-40 induced a significant decrease in RCR in the three groups of rats. However, this peptide induced a significant increase in the repolarization lag phase and a significant decrease in the repolarization level in control and diabetic animals without insulin treatment. Furthermore, this peptide exacerbated significantly the production of H2O2 in STZ-diabetic rats, this effect being avoided by insulin treatment. Our data show that although diabetes induces some alterations in brain mitochondrial activity, those alterations do not interfere significantly with mitochondria functional efficiency. Similarly, insulin does not affect basal mitochondria function. However, in the presence of amyloid beta-peptide, insulin seems to prevent the decline in mitochondrial oxidative phosphorylation efficiency and avoids an increase in oxidative stress, improving or preserving the function of neurons under adverse conditions, such as Alzheimer's disease.

Neuroprotective effect of activity-dependent neurotrophic factor against toxicity from familial amyotrophic lateral sclerosis-linked mutant SOD1 in vitro and in vivo

Amyotrophic lateral sclerosis (ALS) is the most common fatal motor neuron disease, affecting mostly middle-aged people. There are no curative therapies for ALS. Several lines of evidence have supported the notion that the proapoptotic property of familial ALS (FALS)-linked mutant Cu/Zn-superoxide dismutase-1 (SOD1) genes may play an important role in the pathogenesis of some FALS cases. Here we found that activity-dependent neurotrophic factor (ADNF), a neurotrophic factor originally identified to have the anti-Alzheimer's disease (AD) activity, protected against neuronal cell death caused by FALS-linked A4T-, G85R- and G93R-SOD1 in a dose-responsive fashion. Notably, ADNF-mediated complete suppression of SOD1 mutant-induced neuronal cell death occurs at concentrations as low as 100 fM. ADNF maintains the neuroprotective activity even at concentrations of more than 1 nM. This is in clear contrast to the previous finding that ADNF loses its protective activity against neurotoxicity induced by AD-relevant insults, including some familial AD genes and amyloid beta peptide at concentrations of more than 1 nM. Characterization of the neuroprotective activity of ADNF against cell death caused by SOD1 mutants revealed that CaMKIV and certain tyrosine kinases are involved in ADNF-mediated neuroprotection. Moreover, in vivo studies showed that intracerebroventricularly administered ADNF significantly improved motor performance of G93A-SOD1 transgenic mice, a widely used model of FALS, although survival was extended only marginally. Thus, the neuroprotective activity of ADNF provides a novel insight into the development of curative drugs for ALS.

Protective effects of insulin-like growth factor-I on the somatostatinergic system in the temporal cortex of beta-amyloid-treated rats

Insulin-like growth factor-I (IGF-I) has protective effects against beta-amyloid (Abeta)-induced neuronal cell death. Because alterations of the somatostatinergic system have been described in Alzheimer's disease, we investigated the effects of the Abeta peptide and the possible protective role of IGF-I on the somatostatinergic system of the rat temporal cortex and on cell death and phosphorylated (p)-Akt levels in this area. Abeta25-35 was administered intracerebroventricularly to male rats via an osmotic minipump over 14 days (300 pmol/day). Another group received a subcutaneous IGF-I infusion (50 microg/kg/day), concomitant with Abeta25-35 administration, whereas a third group received IGF-I alone. Abeta25-35 significantly decreased the somatostatin (SRIF)-like immunoreactive content and the SRIF receptor density, as a result of a decrease in the levels of the SRIF receptor subtype 2. The inhibitory effect of SRIF on adenylyl cyclase activity was significantly lower after Abeta25-35 infusion, whereas the levels of the inhibitory G protein subunit Gialpha1, Gialpha2 or Gialpha3 were unaltered. Cell death was increased and p-Akt levels decreased in Abeta25-35-treated animals. IGF-I administration increased immunoreactive IGF-I levels in the temporal cortex and restored all parameters affected by Abeta25-35 to baseline values. These findings suggest that IGF-I prevents the deleterious effect of Abeta25-35 on the somatostatinergic system.

Cytotoxic mechanisms by M239V presenilin 2, a little-analyzed Alzheimer's disease-causative mutant

Although neurotoxic functions are well characterized in familial Alzheimer's disease (FAD)-linked N141I mutant of presenilin (PS)2, little has been known about M239V-PS2, another established FAD-causative mutant. We found that expression of M239V-PS2 caused neuronal cytotoxicity. M239V-PS2 exerted three forms of cytotoxicity: one was sensitive to both an antioxidant glutathione-ethyl-ester (GEE) and a caspase inhibitor Ac-DEVD-CHO (DEVD); the second was sensitive to GEE but resistant to DEVD; and the third was resistant to both. The GEE/DEVD-sensitive cytotoxicity by M239V-PS2 was likely through NADPH oxidase and the GEE-sensitive/DEVD-resistant cytotoxicity through xanthine oxidase (XO). Both mechanisms by M239V-PS2 were suppressed by pertussis toxin (PTX) and were mediated by Galpha(o), but not by Galpha(i). Although Abeta1-43 itself induced no cytotoxicity, Abeta1-43 potentiated all three components of M239V-PS2 cytotoxicity. As these cytotoxic mechanisms by M239V-PS2 are fully shared with N141I-PS2, they are most likely implicated in the pathomechanism of FAD by PS2 mutations. Notably, cytotoxicity by M239V-PS2 could be inhibited by the combination of two clinically usable inhibitors of superoxide-generating enzymes, apocynin and oxypurinol.

Humanin antagonists: mutants that interfere with dimerization inhibit neuroprotection by Humanin

The 24-residue peptide Humanin (HN) protects neuronal cells from insults of various Alzheimer's disease (AD) genes and Abeta by forming a homodimer. We have previously shown that P3A, S7A, C8A, L9A, L12A, T13A, S14A and P19A mutations nullify the neuroprotective function of HN [Yamagishi, Y., Hashimoto, Y., Niikura, T. & Nishimoto, I. (2003) Peptides, 24, 585-595]. Here we examined whether any of these 'null' mutants could function as dominant-negative mutants. Homodimerization-defective mutants, P3A-, L12A-, S14A- and P19A-HN, specifically blocked neuroprotection by HN, but not by activity-dependent neurotrophic factor. Furthermore, insertion of S7A, the mutation that blocks the homodimerization of HN, but not insertion of G5A abolished the antagonizing function of L12A-HN. While L12A-HN and G5A/L12A-HN actually inhibited HN homodimerization, S7A/L12A-HN had no effect. These data indicate that P3A-, L12A-, S14A- and P19A-HN function as HN antagonists by forming an inactive dimer with HN. This study provides a novel insight into the understanding of the in vivo function of HN, as well as into the development of clinically applicable HN neutralizers.

The Gtx Homeodomain Transcription Factor Exerts Neuroprotection Using Its Homeodomain

Certain cases of familial Alzheimer's disease are caused by mutants of amyloid-beta precursor protein (AbetaPP), including V642I-AbetaPP, K595N/M596L-AbetaPP (NL-AbetaPP), A617G-AbetaPP, and L648P-AbetaPP. By using an unbiased functional screening with transfection and expression of a human brain cDNA library, we searched for genes that protect neuronal cells from toxicity by V642I-AbetaPP. One protective clone was identical to the human GTX, a neuronal homeobox gene. Human Gtx (hGtx) inhibited caspase inhibitor-sensitive neuronal cell death not only by V642I-AbetaPP but also by L648P-, NL-, A617G-AbetaPP, apolipoprotein E4, and Abeta. The region of hGtx responsible for this rescue function was specified to be its homeodomain (Lys148-His207). The rescue function was shared by DLX4, a distal-less family gene with a homeodomain only 38.3% homologous to that of hGtx, suggesting that this function would be generally shared by homeodomains. The neuroprotective function of hGtx was attributable to hGtx-stimulated production and secretion of insulin-like growth factor-I. This study provides molecular clues to understand how neuronal cells developmentally regulate themselves against cell death as well as to develop reagents effective in curative therapeutics of Alzheimer's disease.

Characterization of V642I-A?PP-induced cytotoxicity in primary neurons

Amyloid precursor protein (AbetaPP), a precursor of amyloid beta (Abeta) peptide, is one of the molecules involved in the pathogenesis of Alzheimer's disease (AD). Specific mutations in AbetaPP have been found in patients inheriting familial AD (FAD). These mutant AbetaPP proteins cause cell death in neuronal cell lines in vitro, but the molecular mechanism of cytotoxicity has not yet been clarified completely. We analyzed the cytotoxic mechanisms of the London-type AbetaPP mutant, V642I-AbetaPP, in primary cortical neurons utilizing an adenovirus-mediated gene transfer system. Expression of V642I-AbetaPP protein induced degeneration of the primary neurons. This cytotoxicity was blocked by pertussis toxin, a specific inhibitor for heterotrimeric G proteins, Go/i, and was suppressed by an inhibitor of caspase-3/7 and an antioxidant, glutathione ethyl ester. A specific inhibitor for NADPH oxidase, apocynin, but not a xanthine oxidase inhibitor or a nitric oxide inhibitor, blocked V642I-AbetaPP-induced cytotoxicity. Among mitogen-activated protein kinase (MAPK) family proteins, c-Jun N-terminal kinase (JNK) and p38MAPK, but not extracellular regulated kinase (ERK), were involved in this cytotoxic pathway. The V642I-AbetaPP-induced cytotoxicity was not suppressed by two secretase inhibitors, suggesting that Abeta does not play a major role in this cytotoxicity. Two neuroprotective factors, insulin-like growth factor I (IGF-I) and Humanin, protected these primary neurons from V642I-AbetaPP-induced cytotoxicity. Furthermore, interleukin-6 and -11 also attenuated this cytotoxicity. This study demonstrated that the signaling pathway activated by mutated AbetaPP in the primary neurons is the same as that by the other artificial insults such as antibody binding to AbetaPP and the artificial dimerization of cytoplasmic domain of AbetaPP. The potential of neurotrophic factors and cytokines in AD therapy is also indicated.

Amino- and carboxyl-terminal mutants of presenilin 1 cause neuronal cell death through distinct toxic mechanisms: Study of 27 different presenilin 1 mutants

Presenilin (PS)1 and its mutants, which consist of the N-terminal and C-terminal fragments, cause certain familial forms of Alzheimer's disease (FAD). Our earlier studies found that FAD-linked M146L-PS1 causes neuronal cell death through nitrogen oxide synthase (NOS) and that FAD-linked N141I-PS2, another member of the PS family, causes neuronal cell death through NADPH oxidase. In this study, we examined 27 different FAD-linked mutants of PS1, and found that PS1 mutants with mutations in the N-terminal fragment caused NOS inhibitor (NOSI)-sensitive neuronal cell death; in contrast, the PS1 mutants with mutations in the C-terminal fragment caused NOSI-resistant neuronal cell death. The former toxicity was resistant to the specific NADPH oxidase inhibitor apocynin and was inhibited by Humanin (HN), a newly identified neuroprotective factor against Alzheimer's disease (AD)-relevant insults, but not by insulin-like growth factor-I (IGF-I). In contrast, the latter toxicity was sensitive to apocynin and inhibited by both IGF-I and HN. This study indicates for the first time that N- and C-terminal fragment PS1 mutants can generate distinct neurotoxic signals, which will provide an important clue to the understanding of the entire array of neurotoxic signals generated by FAD-causative mutations of PS1.

Preservation of motor neuron Ca2+ channel sensitivity to insulin-like growth factor-1 in brain motor cortex from senescent rat

Despite the multiple effects on mammals during development, the effectiveness of the insulin-like growth factor-1 (IGF-1) to sustain cell function and structure in the brain of senescent mammals is almost completely unknown. To address this issue, we investigated whether the effects of IGF-1 on specific targets are preserved at later stages of life. Voltage-gated Ca2+ channels (VGCC) are well-characterized targets of IGF-1. VGCC regulate membrane excitability and gene transcription along with other functions that have been found to be impaired in the brain of senescent rodents. As the voluntary control of movement has been reported to be altered in the elderly, we investigated the expression, function and responsiveness of high (HVA)- and low-voltage-activated (LVA) Ca2+ channels to IGF-1, using the whole-cell configuration of the patch-clamp and RT-PCR in the specific region of the rat motor cortex that controls hindlimb muscle movement. We detected the expression of alpha 1A, alpha 1B and alpha 1E genes encoding the HVA Ca2+ channels P/Q, N and R, respectively, but not alpha 1C, alpha 1D, alpha 1S encoding the L-type Ca2+ channel in this region of the brain cortex. IGF-1 enhanced Ca2+ channel currents through P/Q- and N-type channels but not significantly through the R-type or LVA channels. IGF-1 enhanced the amplitude but did not modify the voltage dependence of Ca2+ channel currents in young (2- to 4-week-old), young adult (7-month-old) and senescent (28- to 29-month-old) rats. These results support the concept that despite the reported decrease in circulating (liver) and local (central nervous system) production of IGF-1 with ageing, key neuronal targets such as the VGCC remain responsive to the growth factor throughout life.

Interaction between the Alzheimer's survival peptide humanin and insulin-like growth factor-binding protein 3 regulates cell survival and apoptosis

Insulin-like growth factor-binding protein-3 (IGFBP-3) regulates IGF bioactivity and also independently modulates cell growth and survival. By using a yeast two-hybrid screen to identify IGFBP-3-interacting proteins, we cloned humanin (HN) as an IGFBP-3-binding partner. HN is a 24-aa peptide that has been shown to specifically inhibit neuronal cell death induced by familial Alzheimer's disease mutant genes and amyloid-beta (Abeta). The physical interaction of HN with IGFBP-3 was determined to be of high affinity and specificity and was confirmed by yeast mating, displaceable pull-down experiments with (His)-6-tagged HN, and ligand blot experiments. Co-immunoprecipitation of IGFBP-3 and HN from mouse testes confirmed the interaction in vivo. In cross-linking experiments, HN bound IGFBP-3 but did not compete with IGF-I-IGFBP-3 binding; competitive ligand dot blot experiments revealed the 18-aa heparin-binding domain of IGFBP-3 as the binding site for HN. Alanine scanning determined that F6A-HN mutant does not bind IGFBP-3. HN but not F6A-HN inhibited IGFBP-3-induced apoptosis in human glioblastoma-A172. In contrast, HN did not suppress IGFBP-3 response in SH-SY5Y neuroblastoma and mouse cortical primary neurons. In primary neurons, IGFBP-3 markedly potentiated HN rescue ability from Abeta1-43 toxicity. In summary, we have identified an interaction between the survival peptide HN and IGFBP-3 that is pleiotrophic in nature and is capable of both synergistic and antagonistic interaction. This interaction may prove to be important in neurological disease processes and could provide important targets for drug development.

The cytoplasmic domain of Alzheimer's amyloid-beta protein precursor causes sustained apoptosis signal-regulating kinase 1/c-Jun NH2-terminal kinase-mediated neurotoxic signal via dimerization

The biological function of full-length amyloid-beta protein precursor (AbetaPP), the precursor of Abeta, is not fully understood. Multiple laboratories have reported that antibody binding to cell surface AbetaPP causes neuronal cell death. Here we examined whether induced dimerization of the cytoplasmic domain of AbetaPP (AbetaPPCD) triggers neuronal cell death. In neurohybrid cells expressing fusion constructs of the epidermal growth factor (EGF) receptor with AbetaPPCD (EGFR/AbetaPP hybrids), EGF drastically enhanced neuronal cell death in a manner sensitive to acetyl-l-aspartyl-l-glutamyl-l-valyl-l-aspartyl-aldehyde (Ac-DEVD-CHO; DEVD), GSH-ethyl ester (GEE), and pertussis toxin (PTX). Dominant-negative apoptosis signal-regulating kinase 1 (ASK1) blocked this neuronal cell death, but not alpha-synuclein-induced cell death. Constitutively active ASK1 (caASK1) caused DEVD/GEE-sensitive cell death in a manner resistant to PTX and sensitive to Humanin, which also suppressed neuronal cell death by EGFR/AbetaPP hybrid. ASK1 formed a complex with AbetaPPCD via JIP-1b, the c-Jun N-terminal kinase (JNK)-interacting protein. EGFR/AbetaPP hybrid-induced and caASK1-induced neuronal cell deaths were specifically blocked by SP600125 (anthra[1,9-cd]pyrazol-6(2H)-one), a specific JNK inhibitor. Combined with our earlier study, these data indicate that dimerization of AbetaPPCD triggers ASK1/JNK-mediated neuronal cell death. We also noticed a potential role of ASK1/JNK in sustaining the activity of this mechanism after initial activation by AbetaPP, which allows for the achievement of cell death by short-term anti-AbetaPP antibody treatment. Understanding the function of AbetaPPCD and its downstream pathway should lead to effective anti-Alzheimer's disease therapeutics.

Two serine residues distinctly regulate the rescue function of Humanin, an inhibiting factor of Alzheimer's disease-related neurotoxicity: functional potentiation by isomerization and dimerization

The 24-residue peptide Humanin (HN), containing two Ser residues at positions 7 and 14, protects neuronal cells from insults of various Alzheimer's disease (AD) genes and A beta. It was not known why the rescue function of (S14G)HN is more potent than HN by two to three orders of magnitude. Investigating the possibility that the post-translational modification of Ser14 might play a role, we found that HN with D-Ser at position 14 exerts neuroprotection more potently than HN by two to three orders of magnitude, whereas D-Ser7 substitution does not affect the rescue function of HN. On the other hand, S7A substitution nullified the HN function. Multiple series of experiments indicated that Ser7 is necessary for self-dimerization of HN, which is essential for neuroprotection by this factor. These findings indicate that the rescue function of HN is quantitatively modulated by d-isomerization of Ser14 and Ser7-relevant dimerization, allowing for the construction of a very potent HN derivative that was fully neuroprotective at 10 pM against 25 microM A beta1-43. This study provides important clues to the understanding of the neuroprotective mechanism of HN, as well as to the development of novel AD therapeutics.

Identification of essential amino acids in Humanin, a neuroprotective factor against Alzheimer's disease-relevant insults

Humanin (HN) is a secretory peptide that inhibits neurotoxicity by various Alzheimer's disease-relevant insults. We have so far identified that the substitution of Leu9 for Arg nullifies the extracellular secretion of HN. Here we comprehensively investigate the amino acid requirement of HN essential for its secretion and for its neuroprotective function. Intracellulary expressed HN-EGFP (EGFP N-terminally fused with HN) was extracellularly secreted, whereas neither EGFP nor (L9R)HN-EGFP was secreted at all. While Ala substitution of neither residue affected HN secretion, Arg substitution revealed that the two structures-Leu9-Leu11 and Pro19-Va120-were essential for the secretion of full-length HN. In the Leu9-Leu11 domain, the Leu10 residue turned out to play a central role in this function, because the Asp substitution of Leu10, but not Leu9 or Leu11, nullified the secretion of HN. Utilizing Ala-scanned HN constructs, we also investigated a comprehensive structure-function relationship for the neuroprotective function of full-length HN, which revealed (i) that Pro3, Ser7, Cys8, Leu9, Leu12, Thr13, Ser14, and Pro19 were essential for this function and (ii) that Ser7 and Leu9 were essential for self-dimerization of HN. These findings indicate that HN has activity similar to a signal peptide, for which the Leu9-Leu11 region, particularly Leu10, functions as a core domain, and suggest that self-dimerization of HN is a process essential for its neuroprotective function.

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

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

Insulin-like growth factor binding protein 5 and apoptosis in mammary epithelial cells

Insulin-like growth factors (IGFs) are important survival signals that can protect a range of cell types from apoptosis. Although IGF bioavailability is modulated by high affinity interactions with IGF-binding proteins (IGFBPs), there is currently no experimental evidence that IGFBPs regulate the survival function of IGFs in the mammary gland. We have examined IGFBP expression during mammary gland development and studied the effects of IGFBPs on IGF-mediated survival and signalling in mammary epithelial cells in culture. IGFBP-5 protein was greatly increased during days 1-3 of mammary gland involution, when levels of apoptosis are dramatically elevated to remodel the gland after lactation. Primary cultures of mammary epithelial cells (MECs) expressed IGFBP-5 from their basal surface suggesting that IGFBP-5 is suitably located to inhibit IGF signalling. Addition of exogenous IGFBP-5 and IGFBP-3 to MECs suppressed IGF-I-mediated survival, resulting in threefold greater apoptosis in cells incubated with IGF-I and IGFBP-5 compared with IGF-I alone. Examination of signalling pathways involved in apoptosis revealed that phosphorylation of PKB and the forkhead transcription factor, FKHRL1, was induced by IGFs, but that phosphorylation was blocked by IGFBP-5 and IGFBP-3. This study provides evidence that IGFBP-5 plays an important role in the regulation of apoptosis in the mammary gland.

Insulin-like growth factor-I inhibits endogenous acetylcholine release from the rat hippocampal formation: possible involvement of GABA in mediating the effects

Evidence suggests that insulin-like growth factor-I (IGF-I) plays an important role during brain development and in the maintenance of normal as well as activity-dependent functioning of the adult brain. Apart from its trophic effects, IGF-I has also been implicated in the regulation of brain neurotransmitter release thus indicating a neuromodulatory role for this growth factor in the central nervous system. Using in vitro slice preparations, we have earlier reported that IGF-I potently inhibits K(+)-evoked endogenous acetylcholine (ACh) release from the adult rat hippocampus and cortex but not from the striatum. The effects of IGF-I on hippocampal ACh release was sensitive to the Na(+) channel blocker tetrodotoxin, suggesting that IGF-I might act indirectly via the release of other transmitters/modulators. In the present study, we have characterized the possible involvement of GABA in IGF-I-mediated inhibition of ACh release and measured the effects of this growth factor on choline acetyltransferase (ChAT) activity and high-affinity choline uptake in the hippocampus of the adult rat brain. Prototypical agonists of GABA(A) and GABA(B) receptors (i.e. 10 microM muscimol and 10 microM baclofen) inhibited, whereas the antagonists of the respective receptors (i.e. 10 microM bicuculline and 10 microM phaclofen) potentiated K(+)-evoked ACh release from rat hippocampal slices. IGF-I (10 nM) inhibited K(+)- as well as veratridine-evoked ACh release from rat hippocampal slices and the effect is possibly mediated via the activation of a typical IGF-I receptor and the subsequent phosphorylation of the insulin receptor substrate-1 (IRS-1). The inhibitory effects of IGF-I on hippocampal ACh release were not additive to those of either muscimol or baclofen, but were attenuated by GABA antagonists, bicuculline and phaclofen. Additionally, in contrast to ACh release, IGF-I did not alter either the activity of the enzyme ChAT or the uptake of choline in the hippocampus. These results, taken together, indicate that IGF-I, under acute conditions, can decrease hippocampal ACh release by acting on the typical IGF-I/IRS receptor complex while having no direct effect on ChAT activity or the uptake of choline. Furthermore, the evidence that effects of IGF-I could be modulated, at least in part, by GABA antagonists suggest that the release of GABA and the activation of its receptors may possibly be involved in mediating the inhibitory effects of IGF-I on hippocampal ACh release.

Insulin-like growth factor-1 activates Akt and Jun N-terminal kinases (JNKs) in promoting the survival of T lymphocytes

Insulin-like growth factor 1 receptor (IGF-1R) expression is augmented on T cells upon ligation of CD28, and this promotes IGF-1-mediated protection from Fas-induced cell death for up to 6 days. To determine the mechanism of action of IGF-1R in T-cell expansion, we investigated the signalling pathways activated by IGF-1 in T cells and in Jurkat cells. We found that IGF-1 transiently induces Akt, jun N-terminal kinases (JNK), and c-Jun phosphorylation in activated T cells, with JNK and c-Jun phosphorylation occurring faster than Akt phosphorylation. To mimic IGF-1R expression levels in CD28-stimulated Jurkat cells these cells were stably transfected to over-express the IGF-1R. Jurkat/IGF-1R cells exhibited enhanced constitutive Akt phosphorylation compared with mock-transfected controls, but IGF-1 induced transient phosphorylation of MKK4, JNKs, and c-Jun. Inhibition of PI-3 kinase activity and Akt phosphorylation with LY294002 totally suppressed IGF-1-mediated protection from Fas killing in activated T cells, but only partially suppressed IGF-1-mediated protection in Jurkat/IGF-1R cells. However, either dicumarol in T cells or a dominant negative JNK1 (APF) in Jurkat/IGF-1R cells greatly suppressed IGF-1-mediated protection from Fas killing. Together, these data demonstrate that IGF-1-mediated activation of JNKs and PI-3 kinase contributes to normal T-cell survival, whereas the JNK pathway may be more important in Jurkat leukaemia cells.

Serum insulin-like growth factor I regulates brain amyloid-beta levels

Levels of insulin-like growth factor I (IGF-I), a neuroprotective hormone, decrease in serum during aging, whereas amyloid-beta (Abeta), which is involved in the pathogenesis of Alzheimer disease, accumulates in the brain. High brain Abeta levels are found at an early age in mutant mice with low circulating IGF-I, and Abeta burden can be reduced in aging rats by increasing serum IGF-I. This opposing relationship between serum IGF-I and brain Abeta levels reflects the ability of IGF-I to induce clearance of brain Abeta, probably by enhancing transport of Abeta carrier proteins such as albumin and transthyretin into the brain. This effect is antagonized by tumor necrosis factor-alpha, a pro-inflammatory cytokine putatively involved in dementia and aging. Because IGF-I treatment of mice overexpressing mutant amyloid markedly reduces their brain Abeta burden, we consider that circulating IGF-I is a physiological regulator of brain amyloid levels with therapeutic potential.

Death and survival of neuronal cells exposed to Alzheimer's insults

Neuronal cell death is the central abnormality occurring in brains suffering from Alzheimer's disease (AD). The notion that AD is a disease caused by loss of neurons points toward suppression of neuronal death as the most important therapeutic target. Nevertheless, the mechanisms for neuronal death in AD are still relatively unclear. Three known mutant genes cause familial AD (FAD): amyloid precursor protein, presenilin 1, and presenilin 2. Detailed analysis of cytotoxic mechanisms of the FAD-linked mutant genes reveals that they cause neuronal cell death at physiologically low expression levels. Unexpectedly, cytotoxic mechanisms vary depending on the type of mutations and genes, suggesting that various mechanisms for neuronal cell death are involved in AD patients. In support of this, activity-dependent neurotrophic factor, basic fibroblast growth factor, and insulin-like growth factor-I can completely protect neurons from beta-amyloid (A beta) cytotoxicity but exhibit incomplete or little effect on cytotoxicity by FAD mutant genes. By contrast, Humanin, a newly identified 24-residue peptide, suppresses neuronal cell death by various FAD mutants and A beta, whereas this factor has no effect on cytotoxicity from AD-irrelevant insults. Studies investigating death and survival of neuronal cells exposed to AD insults will open a new horizon in developing therapy aimed at neuroprotection.

Signaling events in amyloid beta-peptide-induced neuronal death and insulin-like growth factor I protection

Amyloid beta-peptide (Abeta) is implicated as the toxic agent in Alzheimer's disease and is the major component of brain amyloid plaques. In vitro, Abeta causes cell death, but the molecular mechanisms are unclear. We analyzed the early signaling mechanisms involved in Abeta toxicity using the SH-SY5Y neuroblastoma cell line. Abeta caused cell death and induced a 2- to 3-fold activation of JNK. JNK activation and cell death were inhibited by overexpression of a dominant-negative SEK1 (SEK1-AL) construct. Butyrolactone I, a cdk5 inhibitor, had an additional protective effect against Abeta toxicity in these SEK1-AL-expressing cells suggesting that cdk5 and JNK activation independently contributed to this toxicity. Abeta also weakly activated ERK and Akt but had no effect on p38 kinase. Inhibitors of ERK and phosphoinositide 3-kinase (PI3K) pathways did not affect Abeta-induced cell death, suggesting that these pathways were not important in Abeta toxicity. Insulin-like growth factor I protected against Abeta toxicity by strongly activating ERK and Akt and blocking JNK activation in a PI3K-dependent manner. Pertussis toxin also blocked Abeta-induced cell death and JNK activation suggesting that G(i/o) proteins were upstream activators of JNK. The results suggest that activation of the JNK pathway and cdk5 may be initial signaling cascades in Abeta-induced cell death.

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.