Insulin regulates soluble amyloid precursor protein release via phosphatidyl inositol 3 kinase-dependent pathway

Ginsenoside Rg1 promotes nonamyloidgenic cleavage of APP via estrogen receptor signaling to MAPK/ERK and PI3K/Akt

BACKGROUND

The pathogenic accumulation of amyloid β peptide (Aβ), a natural occurring peptide processed from beta-amyloid precursor protein (APP), is considered to play a key role in the development of Alzheimer's disease (AD). Ginsenoside Rg1, an active component in ginseng, has been identified as a phytoestrogen and also found to be neuroprotective. However, it is unknown whether Rg1-induced estrogenic activity intervenes in APP processing, and improves memory performance.

METHODS

Using HT22 cells and SH-SY5Y cells stably expressing the Swedish mutant APP (APPsw), this study investigated whether Rg1 intervened in APP metabolism through estrogenic activity. Using the ovariectomized (OVX) rats to mimic age-related changes in postmenopausal females, this study also tested the long-term effect of Rg1 on APP metabolism.

RESULTS

The in vitro study demonstrated that Rg1 increased extracellular secretion of soluble amyloid precursor protein α (sAPPα), enhanced α-secretase activity and decreased extracellular release of Aβ. These effects of Rg1 could be prevented by inhibitors of protein kinase C (PKC), Extracellular-Signal Regulated Kinase/Mitogen-Activated Protein Kinase (ERK/MAPK) and Phosphoinositide-3 kinase (PI3K)/Akt pathways. Inhibition of endogenous estrogen receptor (ER) activity abrogated Rg1-triggered release of sAPPα, increase of α-secretase activity, and activation of ERK and Akt signaling. In addition, Rg1 promoted phosphorylation of ERα at Ser118 residue. The in vivo study demonstrated that 8-week Rg1 treatment of OVX rats increased sAPPα levels and decreased Aβ content in the hippocampi, and improved the spatial learning and memory.

GENERAL SIGNIFICANCE

Rg1 might be used to slow or prevent AD, in particular in postmenopausal females.

Activation of α-secretase cleavage

Alpha-secretase-mediated cleavage of the amyloid precursor protein (APP) releases the neuroprotective APP fragment sαAPP and prevents amyloid β peptide (Aβ) generation. Moreover, α-secretase-like cleavage of the Aβ transporter 'receptor for advanced glycation end products' counteracts the import of blood Aβ into the brain. Assuming that Aβ is responsible for the development of Alzheimer's disease (AD), activation of α-secretase should be preventive. α-Secretase-mediated APP cleavage can be activated via several G protein-coupled receptors and receptor tyrosine kinases. Protein kinase C, mitogen-activated protein kinases, phosphatidylinositol 3-kinase, cAMP and calcium are activators of receptor-induced α-secretase cleavage. Selective targeting of receptor subtypes expressed in brain regions affected by AD appears reasonable. Therefore, the PACAP receptor PAC1 and possibly the serotonin 5-HT(6) receptor subtype are promising targets. Activation of APP α-secretase cleavage also occurs upon blockade of cholesterol synthesis by statins or zaragozic acid A. Under physiological statin concentrations, the brain cholesterol content is not influenced. Statins likely inhibit Aβ production in the blood by α-secretase activation which is possibly sufficient to inhibit AD development. A disintegrin and metalloproteinase 10 (ADAM10) acts as α-secretase on APP. By targeting the nuclear retinoic acid receptor β, the expression of ADAM10 and non-amyloidogenic APP processing can be enhanced. Excessive activation of ADAM10 should be avoided because ADAM10 and also ADAM17 are not APP-specific. Both ADAM proteins cleave various substrates, and therefore have been associated with tumorigenesis and tumor progression.

The Phosphatidylinositol 3-Kinase/mTor Pathway as a Therapeutic Target for Brain Aging and Neurodegeneration

Many pathological conditions are associated with phosphatidylinositol 3-kinase (PI3K) dysfunction, providing an incentive for the study of the effects of PI3K modulation in different aspects of diabetes, cancer, and aging. The PI3K/AKT/mTOR pathway is a key transducer of brain metabolic and mitogenic signals involved in neuronal proliferation, differentiation, and survival. In several models of neurodegenerative diseases associated with aging, the PI3K/AKT pathway has been found to be dysregulated, suggesting that two or more initiating events may trigger disease formation in an age-related manner. The search for chemical compounds able to modulate the activity of the PI3K/AKT/mTOR pathway is emerging as a potential therapeutic strategy for the treatment and/or prevention of some metabolic defects associated with brain aging. In the current review, we summarize some of the critical actions of PI3K in brain function as well as the evidence of its involvement in aging and Alzheimer's disease.

Caloric intake, dietary lifestyles, macronutrient composition, and alzheimer' disease dementia

Alzheimer's disease is a devastating neurodegenerative condition currently affecting over 5 million elderly individuals in the United States. There is much evidence suggesting that certain dietary lifestyles can help to prevent and possibly treat Alzheimer's disease. In this paper, we discuss how certain cardiovascular and diabetic conditions can induce an increased susceptibility for Alzheimer's disease and the mechanisms through which this occurs. We further discuss how the consumption of certain foods or food components can help to reduce one's risk for Alzheimer's disease and may possibly be developed as a therapeutic agent.

Mechanisms of brain aging regulation by insulin: implications for neurodegeneration in late-onset Alzheimer's disease

Insulin and IGF seem to be important players in modulating brain aging. Neurons share more similarities with islet cells than any other human cell type. Insulin and insulin receptors are diffusely found in the brain, especially so in the hippocampus. Caloric restriction decreases insulin resistance, and it is the only proven mechanism to expand lifespan. Conversely, insulin resistance increases with age, obesity, and sedentarism, all of which have been shown to be risk factors for late-onset Alzheimer's disease (AD). Hyperphagia and obesity potentiate the production of oxidative reactive species (ROS), and chronic hyperglycemia accelerates the formation of advanced glucose end products (AGEs) in (pre)diabetes—both mechanisms favoring a neurodegenerative milieu. Prolonged high cerebral insulin concentrations cause microvascular endothelium proliferation, chronic hypoperfusion, and energy deficit, triggering β-amyloid oligomerization and tau hyperphosphorylation. Insulin-degrading enzyme (IDE) seems to be the main mechanism in clearing β-amyloid from the brain. Hyperinsulinemic states may deviate IDE utilization towards insulin processing, decreasing β-amyloid degradation.

Insulin-resistant brain state: the culprit in sporadic Alzheimer's disease?

Severe abnormalities in brain glucose/energy metabolism and insulin signaling have been documented to take a pivotal role in early sporadic Alzheimer's disease (sAD) pathology. Indeed, the "insulin-resistant brain state" has been hypothesized to form the core of the neurodegenerative events that occur in sAD. In this vein, intracerebroventricular administration of subdiabetogenic doses of streptozotocin (STZ) in rats can induce an insulin-resistant brain state, which is proposed as a suitable experimental model of sAD. This review highlights the involvement of disturbed brain insulin metabolism in sAD etiopathogenesis. Furthermore, current knowledge demonstrates that central STZ administration produces brain pathology and behavioral changes that resemble changes found in sAD patients. The STZ-intracerebroventricularly treated rat represents a promising experimental tool in this field by providing new insights concerning early brain alterations in sAD, which can be translated in novel etiopathogenic and therapeutic approaches in this disease.

IGF-1 reduces BACE-1 expression in PC12 cells via activation of PI3-K/Akt and MAPK/ERK1/2 signaling pathways

Insulin-like growth factor 1 (IGF-1) stimulates α-secretase processing of amyloid precursor protein (APP) and decreases Aβ production. Little is known about the relationship between IGF-1 and β-site amyloid precursor protein cleaving enzyme 1 (BACE-1), the protease essential for the production of β-amyloid peptides (Aβ). Here, we investigated the effect of IGF-1 on BACE-1 in PC12 cells. Quantitative polymerase chain reaction analysis and western blot showed that treatment of cells with IGF-1 significantly decreased the levels of BACE-1 mRNA and protein. Furthermore, IGF-1 increased the phosphorylation of Akt and ERK1/2. The presence of the phosphatidylinositol 3-kinase (PI3-K) inhibitor LY294002 and the mitogen-activated protein kinase kinases (MEK) inhibitor PD98059 blocked the effect of IGF-1 on BACE-1. Our data indicated that IGF-1-induced reduction of BACE-1 might involve the PI3-K/Akt and MAPK/ERK1/2 signaling pathways.

Dietary fatty acids in dementia and predementia syndromes: epidemiological evidence and possible underlying mechanisms

Drugs currently used in the treatment of cognitive impairment and dementia have a very limited therapeutic value, suggesting the necessity to potentially individualize new strategies able to prevent and to slow down the progression of predementia and dementia syndromes. An increasing body of epidemiological evidence suggested that elevated saturated fatty acids (SFA) could have negative effects on age-related cognitive decline (ARCD) and mild cognitive impairment (MCI). Furthermore, a clear reduction of risk for cognitive decline has been found in population samples with elevated fish consumption, high intake of monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), particularly n-3 PUFA. Epidemiological findings demonstrated that high PUFA intake appeared to have borderline non-significant trend for a protective effect against the development of MCI. Several hypotheses could explain the association between dietary unsaturated fatty acids and cognitive functioning, including mechanisms through the co-presence of antioxidant compounds in food groups rich in fatty acids, via atherosclerosis and thrombosis, inflammation, accumulation of b-amyloid, or via an effect in maintaining the structural integrity of neuronal membranes, determining the fluidity of synaptosomal membranes that thereby regulate neuronal transmission. However, recent findings from clinical trials with n-3 PUFA supplementation showed efficacy on depressive symptoms only in non-apolipoprotein E (APOE) epsilon4 carriers, and on cognitive symptoms only in very mild Alzheimer's disease (AD) subgroups, MCI patients, and cognitively unimpaired subjects non-APOE epsilon4 carriers. These data together with epidemiological evidence support a possible role of fatty acid intake in maintaining adequate cognitive functioning and possibly for the prevention and management of cognitive decline and dementia, but not when the AD process has already taken over.

Insulin-like growth factor-1 (IGF-1)-induced processing of amyloid-beta precursor protein (APP) and APP-like protein 2 is mediated by different metalloproteinases

alpha-Secretase cleavage of the amyloid precursor protein (APP) is of great interest because it prevents the formation of the Alzheimer-linked amyloid-beta peptide. APP belongs to a conserved gene family including the two paralogues APP-like protein (APLP) 1 and 2. Insulin-like growth factor-1 (IGF-1) stimulates the shedding of all three proteins. IGF-1-induced shedding of both APP and APLP1 is dependent on phosphatidylinositol 3-kinase (PI3-K), whereas APLP2 shedding is independent of this signaling pathway. Here, we used human neuroblastoma SH-SY5Y cells to investigate the involvement of protein kinase C (PKC) in the proteolytic processing of endogenously expressed members of the APP family. Processing was induced by IGF-1 or retinoic acid, another known stimulator of APP alpha-secretase shedding. Our results show that stimulation of APP and APLP1 processing involves multiple signaling pathways, whereas APLP2 processing is mainly dependent on PKC. Next, we wanted to investigate whether the difference in the regulation of APLP2 shedding compared with APP shedding could be due to involvement of different processing enzymes. We focused on the two major alpha-secretase candidates ADAM10 and TACE, which both are members of the ADAM (a disintegrin and metalloprotease) family. Shedding was analyzed in the presence of the ADAM10 inhibitor GI254023X, or after transfection with small interfering RNAs targeted against TACE. The results clearly demonstrate that different alpha-secretases are involved in IGF-1-induced processing. APP is mainly cleaved by ADAM10, whereas APLP2 processing is mediated by TACE. Finally, we also show that IGF-1 induces PKC-dependent phosphorylation of TACE.

The effects of cardiovascular risk factors on cognitive compromise

As life expectancy in the United States continues to increase, the projected numbers of elderly people who will develop dementia will grow rapidly. This paper reviews four well-established cardiovascular risk factors (type 2 diabetes, hypertension, cholesterol, and inflammation), for which there is longitudinal epidemiological evidence of increased risk of dementia, Alzheimer's disease, mild cognitive impairment, and cognitive decline. These risk factors are of special interest because of their potential modif lability, which may affect the course of cognitive compromise. Diabetes is the cardiovascular risk factor (CvRF) most consistently associated with cognition. Hypertension in midlife is consistently associated with cognition, but its associations with late-life hypertension are less clear. Total cholesterol is not consistently associated with cognition, interleukin-6 and C-reactive protein are inflammatory markers relatively consistently associated with cognition. Composites of the CvRFs increase the risk for dementia in a dose-dependent fashion, suggesting a cumulative effect of these factors on neuronal stress. In the relatively few studies that have reported interactions of risk factors, they potentiate each other. The effect of each of these risk factors varies according to apolipoprotein E genotype, it may be that the effect of these risk factors varies according to the presence of the others, and these complex relationships underlie the biological mechanisms of cognitive compromise. This may be crucial for understanding the effects on cognition of druqs and other approaches, such as lifestyle chanqe, for treatinq these risk factors.

Constitutively active Akt inhibits trafficking of amyloid precursor protein and amyloid precursor protein metabolites through feedback inhibition of phosphoinositide 3-kinase

Amyloid-beta (Abeta) peptides, generated through sequential proteolytic cleavage of amyloid precursor protein (APP), aggregate to form amyloid plaques in Alzheimer's disease (AD). Understanding the regulation of Abeta generation and cellular secretion is critical to our understanding of AD pathophysiology. In the present study, we examined the role of the insulin/insulin-like growth factor-1 (IGF-1) signaling pathway in regulating APP trafficking and Abeta secretion. Previous studies have demonstrated that insulin or IGF-1 stimulation can increase Abeta and APP secretion in a phosphoinositide 3-kinase (PI3K) dependent manner. To expand upon these studies and better understand the molecular targets responsible for alterations in APP secretion, we constitutively activated Akt, a downstream component of the insulin/IGF-1 signaling pathway. Counterintuitively, constitutively active Akt (myr-Akt) overexpression produced an opposite effect to insulin/IGF-1 stimulation and inhibited secretion of APP and APP metabolites in multiple cell lines. Myr-Akt overexpression also resulted in increased APP protein stability. Since the insulin/IGF-1 signaling pathway is tightly regulated by feedback inhibition pathways, we hypothesized that myr-Akt overexpression may be inducing feedback inhibition of PI3K, resulting in impaired APP trafficking. In support of this hypothesis, myr-Akt acted at a known node of PI3K inhibition and decreased insulin receptor substrate 1 (IRS1) protein levels. Our studies provide further support for PI3K as a modulator of APP trafficking and demonstrate that overactivation of the insulin/IGF-1 signaling pathway may result in feedback inhibition of PI3K through IRS1 and reduce APP trafficking and Abeta secretion.

Deletion of Irs2 reduces amyloid deposition and rescues behavioural deficits in APP transgenic mice

As impaired insulin signalling (IIS) is a risk factor for Alzheimer’s disease we crossed mice (Tg2576) over-expressing human amyloid precursor protein (APP), with insulin receptor substrate 2 null (Irs2^−/−) mice which develop insulin resistance. The resulting Tg2576/Irs2^−/− animals had increased tau phosphorylation but a paradoxical amelioration of Aβ pathology. An increase of the Aβ binding protein transthyretin suggests that increased clearance of Aβ underlies the reduction in plaques. Increased tau phosphorylation correlated with reduced tau-phosphatase PP2A, despite an inhibition of the tau-kinase glycogen synthase kinase-3. Our findings demonstrate that disruption of IIS in Tg2576 mice has divergent effects on pathological processes—a reduction in aggregated Aβ but an increase in tau phosphorylation. However, as these effects are accompanied by improvement in behavioural deficits, our findings suggest a novel protective effect of disrupting IRS2 signalling in AD which may be a useful therapeutic strategy for this condition.

Interleukin-1beta enhances nucleotide-induced and alpha-secretase-dependent amyloid precursor protein processing in rat primary cortical neurons via up-regulation of the P2Y(2) receptor

The heterologous expression and activation of the human P2Y(2) nucleotide receptor (P2Y(2)R) in human 1321N1 astrocytoma cells stimulates alpha-secretase-dependent cleavage of the amyloid precursor protein (APP), causing extracellular release of the non-amyloidogenic protein secreted amyloid precursor protein (sAPPalpha). To determine whether a similar response occurs in a neuronal cell, we analyzed whether P2Y(2)R-mediated production of sAPPalpha occurs in rat primary cortical neurons (rPCNs). In rPCNs, P2Y(2)R mRNA and receptor activity were virtually absent in quiescent cells, whereas overnight treatment with the pro-inflammatory cytokine interleukin-1beta (IL-1beta) up-regulated both P2Y(2)R mRNA expression and receptor activity by four-fold. The up-regulation of the P2Y(2)R was abrogated by pre-incubation with Bay 11-7085, an IkappaB-alpha phosphorylation inhibitor, which suggests that P2Y(2)R mRNA transcript levels are regulated through nuclear factor-kappa-B (NFkappaB) signaling. Furthermore, the P2Y(2)R agonist Uridine-5'-triphosphate (UTP) enhanced the release of sAPPalpha in rPCNs treated with IL-1beta or transfected with P2Y(2)R cDNA. UTP-induced release of sAPPalpha from rPCNs was completely inhibited by pre-treatment of the cells with the metalloproteinase inhibitor TACE inhibitor (TAPI-2) or the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002, and was partially inhibited by the MAPK/extracellular signal-regulated kinase inhibitor U0126 and the protein kinase C inhibitor GF109203. These data suggest that P2Y(2)R-mediated release of sAPPalpha from cortical neurons is directly dependent on a disintegrin and metalloproteinase (ADAM) 10/17 and PI3K activity, whereas extracellular signal-regulated kinase 1/2 and PI3K activity may indirectly regulate APP processing. These results demonstrate that elevated levels of pro-inflammatory cytokines associated with neurodegenerative diseases, such as IL-1beta, can enhance non-amyloidogenic APP processing through up-regulation of the P2Y(2)R in neurons.

Insulin in combination with other diabetes medication is associated with less Alzheimer neuropathology

OBJECTIVE

To examine the association between treatment for diabetes and Alzheimer disease (AD) neuropathology.

METHODS

This postmortem study matched 124 subjects with diabetes to 124 without diabetes from the Mount Sinai School of Medicine Brain Bank, on age (mean = 81.2 + 9.3), sex (57.3% F), and severity of dementia (Clinical Dementia Rating [CDR] 2.4 + 1.7). Densities of neuritic plaques (NPs) and of neurofibrillary tangles (NFTs) were assessed in several neocortical regions and in the hippocampus, entorhinal cortex, and amygdala. Diabetic subjects were classified according to their recorded lifetime antidiabetic medications: none (n = 29), insulin only (n = 49), diabetes medications other than insulin only (n = 28), or concomitant use of both insulin and any oral antidiabetic medications (n = 18). For each dependent variable, analysis of covariance controlling for age at death, sex, and CDR distinguished among the nondiabetic patients and four diabetic subgroups.

RESULTS

There were differences among the five groups for NP ratings in the entorhinal cortex (p = 0.003), amygdala (p = 0.009), and overall NP (p = 0.014) as well as counts of NPs in all regions examined (p values ranging from 0.009 to 0.04). NP ratings in the hippocampus (p = 0.057) and the combined neocortical measure (p = 0.052) approached significance. In each analysis, the concomitant medication group had significantly fewer NPs (approximately 20%) than any of the other groups, which were relatively similar. No significant NFT differences were found.

CONCLUSION

The results of this study suggest that the combination of insulin with other diabetes medication is associated with substantially lower neuritic plaque density consistent with the effects of both on the neurobiology of insulin.

ERK1/2 mediate wounding- and G-protein-coupled receptor ligands-induced EGFR activation via regulating ADAM17 and HB-EGF shedding

PURPOSE

Previous studies have shown that wounding of human corneal epithelial cells (HCECs) results in the release of G-protein-coupled receptor ligands such as ATP and lysophosphatidic acid (LPA), which in turn transactivate epidermal growth factor (EGF) receptor (EGFR) through ectodomain shedding of heparin-binding EGF-like growth factor (HB-EGF). In the present study, the role of extracellular signal-regulated kinases 1/2 (ERK1/2) in regulating EGFR transactivation was investigated.

METHODS

SV40-immortalized HCECs were wounded or stimulated with ATP and LPA. EGFR and ADAM17 activation was analyzed by immunoprecipitation followed by Western blot analysis with phospho-tyrosine or phospho-serine antibodies, respectively. Phosphorylation of ERK and AKT was analyzed by Western blot analysis. HB-EGF shedding was assessed by measuring the release of alkaline phosphatase (AP) in a stably transfected human corneal epithelial (THCE) cell line expressing HB-EGF-AP. ADAM17 and ERK interaction was determined by coimmunoprecipitation.

RESULTS

Early, but not late, ERK1/2 phosphorylation in response to wounding, LPA, and ATP was EGFR independent, but sensitive to the inhibitors of calcium influx, protein kinase C and Src kinase. Wounding-, LPA-, and ATP-induced HB-EGF shedding and EGFR activation were attenuated by the MAPK/ERK kinase (MEK) inhibitors PD98059 and U0126, as well as by ADAM10 and -17 inhibitors. ADAM17 was found to be physically associated with active ERK and phosphorylated at serine residues in an ERK-dependent manner in wounded cells.

CONCLUSIONS

Taken together, our data suggest that in addition to functioning as an EGFR downstream effector, ERK1/2 also mediates ADAM-dependent HB-EGF shedding and subsequent EGFR transactivation in response to a variety of stimuli, including wounding and GPCR ligands.

Amyloid precursor protein expression is upregulated in adipocytes in obesity

The aim of this study was to determine whether amyloid precursor protein (APP) is expressed in human adipose tissue, dysregulated in obesity, and related to insulin resistance and inflammation. APP expression was examined by microarray expression profiling of subcutaneous abdominal adipocytes (SAC) and cultured preadipocytes from obese and nonobese subjects. Quantitative real-time PCR (QPCR) was performed to confirm differences in APP expression in SAC and to compare APP expression levels in adipose tissue, adipocytes, and stromal vascular cells (SVCs) from subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) specimens. Adipose tissue samples were also examined by western blot and immunofluorescence confocal microscopy. Microarray studies demonstrated that APP mRNA expression levels were higher in SAC (approximately 2.5-fold) and preadipocytes (approximately 1.4) from obese subjects. Real-time PCR confirmed increased APP expression in SAC in a separate group of obese compared with nonobese subjects (P=0.02). APP expression correlated to in vivo indices of insulin resistance independently of BMI and with the expression of proinflammatory genes, such as monocyte chemoattractant protein-1 (MCP-1) (R=0.62, P=0.004), macrophage inflammatory protein-1alpha (MIP-1alpha) (R=0.60, P=0.005), and interleukin-6 (IL-6) (R=0.71, P=0.0005). Full-length APP protein was detected in adipocytes by western blotting and APP and its cleavage peptides, Abeta40 and Abeta42, were observed in SAT and VAT by immunofluorescence confocal microscopy. In summary, APP is highly expressed in adipose tissue, upregulated in obesity, and expression levels correlate with insulin resistance and adipocyte cytokine expression levels. These data suggest a possible role for APP and/or Abeta in the development of obesity-related insulin resistance and adipose tissue inflammation.

Nitric oxide isoenzymes regulate lipopolysaccharide-enhanced insulin transport across the blood-brain barrier

Insulin transported across the blood-brain barrier (BBB) has many effects within the central nervous system. Insulin transport is not static but altered by obesity and inflammation. Lipopolysaccharide (LPS), derived from the cell walls of Gram-negative bacteria, enhances insulin transport across the BBB but also releases nitric oxide (NO), which opposes LPS-enhanced insulin transport. Here we determined the role of NO synthase (NOS) in mediating the effects of LPS on insulin BBB transport. The activity of all three NOS isoenzymes was stimulated in vivo by LPS. Endothelial NOS and inducible NOS together mediated the LPS-enhanced transport of insulin, whereas neuronal NOS (nNOS) opposed LPS-enhanced insulin transport. This dual pattern of NOS action was found in most brain regions with the exception of the striatum, which did not respond to LPS, and the parietal cortex, hippocampus, and pons medulla, which did not respond to nNOS inhibition. In vitro studies of a brain endothelial cell (BEC) monolayer BBB model showed that LPS did not directly affect insulin transport, whereas NO inhibited insulin transport. This suggests that the stimulatory effect of LPS and NOS on insulin transport is mediated through cells of the neurovascular unit other than BECs. Protein and mRNA levels of the isoenzymes indicated that the effects of LPS are mainly posttranslational. In conclusion, LPS affects insulin transport across the BBB by modulating NOS isoenzyme activity. NO released by endothelial NOS and inducible NOS acts indirectly to stimulate insulin transport, whereas NO released by nNOS acts directly on BECs to inhibit insulin transport.

Dicholine salt of succinic acid, a neuronal insulin sensitizer, ameliorates cognitive deficits in rodent models of normal aging, chronic cerebral hypoperfusion, and beta-amyloid peptide-(25-35)-induced amnesia

BACKGROUND

Accumulated evidence suggests that insulin resistance and impairments in cerebral insulin receptor signaling may contribute to age-related cognitive deficits and Alzheimer's disease. The enhancement of insulin receptor signaling is, therefore, a promising strategy for the treatment of age-related cognitive disorders. The mitochondrial respiratory chain, being involved in insulin-stimulated H2O2 production, has been identified recently as a potential target for the enhancement of insulin signaling. The aim of the present study is to examine: (1) whether a specific respiratory substrate, dicholine salt of succinic acid (CS), can enhance insulin-stimulated insulin receptor autophosphorylation in neurons, and (2) whether CS can ameliorate cognitive deficits of various origins in animal models.

RESULTS

In a primary culture of cerebellar granule neurons, CS significantly enhanced insulin-stimulated insulin receptor autophosphorylation. In animal models, CS significantly ameliorated cognitive deficits, when administered intraperitoneally for 7 days. In 16-month-old middle-aged C57Bl/6 mice (a model of normal aging), CS enhanced spatial learning in the Morris water maze, spontaneous locomotor activity, passive avoidance performance, and increased brain N-acetylaspartate/creatine levels, as compared to the age-matched control (saline). In rats with chronic cerebral hypoperfusion, CS enhanced spatial learning, passive avoidance performance, and increased brain N-acetylaspartate/creatine levels, as compared to control rats (saline). In rats with beta-amyloid peptide-(25-35)-induced amnesia, CS enhanced passive avoidance performance and increased activity of brain choline acetyltransferase, as compared to control rats (saline). In all used models, CS effects lasted beyond the seven-day treatment period and were found to be significant about two weeks following the treatment.

CONCLUSION

The results of the present study suggest that dicholine salt of succinic acid, a novel neuronal insulin sensitizer, ameliorates cognitive deficits and neuronal dysfunctions in animal models relevant to age-related cognitive impairments, vascular dementia, and Alzheimer's disease.

The GSK3 hypothesis of Alzheimer's disease

Glycogen synthase kinase 3 (GSK3) is a constitutively active, proline-directed serine/threonine kinase that plays a part in a number of physiological processes ranging from glycogen metabolism to gene transcription. GSK3 also plays a pivotal and central role in the pathogenesis of both sporadic and familial forms of Alzheimer's disease (AD), an observation that has led us to coin the ‘GSK3 hypothesis of AD’. According to this hypothesis, over-activity of GSK3 accounts for memory impairment, tau hyper-phosphorylation, increased β-amyloid production and local plaque-associated microglial-mediated inflammatory responses; all of which are hallmark characteristics of AD. If our ‘GSK3 hypothesis of AD’ is substantiated and GSK3 is indeed a causal mediator of AD then inhibitors of GSK3 would provide a novel avenue for therapeutic intervention in this devastating disorder.

Insulin stimulates the cleavage and release of the extracellular domain of Klotho by ADAM10 and ADAM17

Cleavage and release (shedding) of membrane proteins is a critical regulatory step in many normal and pathological processes. Evidence suggests that the antiaging transmembrane protein Klotho (KL) is shed from the cell surface by proteolytic cleavage. In this study, we attempted to identify the enzymes responsible for the shedding of KL by treating KL-transfected COS-7 cells with a panel of proteinase inhibitors and measuring cleavage products by Western blot. We report that metalloproteinase inhibitors, including EDTA, EGTA, and TAPI-1, inhibit the shedding of KL, whereas insulin increases shedding. The effects of the inhibitors in KL-transfected COS-7 cells were repeated in studies on rat kidney slices ex vivo, which validates the use of COS-7 cells as our model system. Tissue inhibitor of metalloproteinase (Timp)-3 effectively inhibits KL cleavage, whereas Timp-1 and Timp-2 do not, a profile that indicates the involvement of members of the A Desintegrin and Metalloproteinase (ADAM) family. Cotransfection of KL with either ADAM10 or ADAM17 enhances KL cleavage, whereas cotransfection of KL with small interference RNAs specific to ADAM10 and ADAM17 inhibits KL secretion. These results indicate that KL shedding is mediated mainly by ADAM10 and ADAM17 in KL-transfected COS-7 cells. The effect of insulin is abolished when ADAM10 or ADAM17 are silenced. Furthermore, we demonstrate that the effect of insulin on KL shedding is inhibited by wortmannin, showing that insulin acts through a PI3K-dependent pathway. Insulin enhances KL shedding without increasing ADAM10 and ADAM17 mRNA and protein levels, suggesting that it acts by stimulating their proteolytic activities.

New protein kinase C activator regulates amyloid precursor protein processing in vitro by increasing alpha-secretase activity

The beta amyloid (Abeta) cascade has been at the forefront of the hypothesis used to describe the pathogenesis of Alzheimer's disease (AD). It is generally accepted that drugs that can regulate the processing of the amyloid precursor protein (APP) toward the non-amyloidogenic pathway may have a therapeutic potential. Previous studies have shown that protein kinase C (PKC) hypofunction has an important role in AD pathophysiology. Therefore, the effects of a new PKC activator, alpha-APP modulator [(2S,5S)-(E,E)-8-(5-(4-(trifluoromethyl)phenyl)-2,4-pentadienoylamino)benzolactam (TPPB)], on APP processing were investigated. Using PC12 cells and SH-SY5Y(APP695) cells, it was found that TPPB promoted the secretion of sAPPalpha without affecting full-length expression of APP. The increase in sAPPalpha by TPPB was blocked by inhibitors of PKC, extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and tyrosine kinase, suggesting the involvement of these signal transduction pathways. TPPB increased alpha-secretase activity [a disintegrin and metalloproteinase (ADAM)10 and 17], as shown by direct fluorescence activity detection and Western blot analysis. TPPB-induced sAPPalpha release was blocked by the metalloproteinase inhibitor TAPI-2, furin inhibitor CMK and by the protein-trafficking inhibitor brefeldin. The results also showed that TPPB decreased beta-secretase activity, Abeta40 release and beta site APP-cleaving enzyme 1 (BACE1) expression, but did not significantly affect neprilysin (NEP) and insulin-degrading enzyme (IDE) expression. Our data indicate that TPPB could direct APP processing towards the non-amyloidogenic pathway by increasing alpha-secretase activity, and suggest its therapeutic potential in AD.

Upregulation of amyloid precursor protein by platelet-derived growth factor in hippocampal precursor cells

Amyloid precursor protein generates the secreted amyloid precursor protein alpha, which protects hippocampal neurons from ischemic injury and facilitates neuronal survival and synaptogenesis in the developing nervous system. Here, we examined whether platelet-derived growth factor regulates the generation of secreted amyloid precursor protein alpha during the neuronal differentiation of hippocampal precursor cells, HiB5. We showed that platelet-derived growth factor promoted amyloid precursor protein production and secreted amyloid precursor protein alpha secretion. These effects of platelet-derived growth factor were diminished by the PI3K-specific inhibitor wortmannin and the protein kinase C-specific inhibitor GF109203X, suggesting the involvement of the PI3K and protein kinase C-signaling pathway. Furthermore, the conditioned media enriched with secreted amyloid precursor protein alpha promoted the survival of HiB5 cells during neuronal differentiation. These results suggest that the neurotrophic effect of platelet-derived growth factor is mediated in part via upregulation of the expression and release of secreted amyloid precursor protein alpha.

Alzheimer-like changes in rat models of spontaneous diabetes

OBJECTIVE

To examine whether changes characteristic of Alzheimer's disease occur in two rat models with spontaneous onset of type 1 and type 2 diabetes.

RESEARCH DESIGN AND METHODS

The frontal cortices of 8-month-diabetic rats were examined with respect to neuronal densities, neurite degeneration, expression, and/or immunolocalization of amyloid precursor protein (APP), beta-secretase, beta-amyloid, COOH-terminal fragment (CTF), insulin receptor, IGF-1 receptor, glycogen synthase kinase 3-beta (GSK-3beta), protein kinase B (Akt), phosphorylated tau (phospho-tau), synaptophysin, and phosphorylated neurofilaments (SMI-31).

RESULTS

Neuronal loss occurred in both models, significantly more so in type 2 diabetic BBZDR/Wor rats compared with type 1 diabetic BB/Wor rats and was associated with a ninefold increase of dystrophic neurites. APP, beta-secretase, beta-amyloid, and CTF were significantly increased in type 2 diabetic rats, as was phospho-tau. The insulin receptor expression was decreased in type 1 diabetes, whereas IGF-1 receptor was decreased in both models, as were Akt and GSK-3beta expression.

CONCLUSIONS

The data show that beta-amyloid and phospho-tau accumulation occur in experimental diabetes and that this is associated with neurite degeneration and neuronal loss. The changes were more severe in the type 2 diabetic model and appear to be associated with insulin resistance and possibly hypercholesterolemia. The two models will provide useful tools to unravel further mechanistic associations between diabetes and Alzheimer's disease.

Effects of Alzheimer's amyloid-beta and tau protein on mitochondrial function -- role of glucose metabolism and insulin signalling

Alzheimer's disease (AD) is the most frequent form of dementia among the elderly and is characterized by neuropathological hallmarks of extracellular amyloid-beta (Abeta) plaques and intracellular neurofibrillary tangles composed of abnormally hyperphosphorylated microtubular protein tau in the brains of AD patients. Of note, current data illustrate a complex interplay between the amyloid and tau pathology during the course of the disease. We hypothesize a direct impact of abnormally phosphorylated tau and Abeta on proteins/enzymes involved in metabolism, respiratory chain function and cellular detoxification. Probably at the level of mitochondria, both Alzheimer proteins exhibit synergistic effects finally leading to/accelerating neurodegenerative mechanisms. Moreover, accumulating evidence that mitochondria failure, reduced glucose utilization and deficient energy metabolism occur already very early in the course of the disease suggests a role of impaired insulin signalling in the pathogenesis of AD. Thus, this review addresses also the question if mitochondrial dysfunction may represent a link between diabetes and AD.

IGF-1-induced Processing of the Amyloid Precursor Protein Family Is Mediated by Different Signaling Pathways

The mammalian amyloid precursor protein (APP) protein family consists of the APP and the amyloid precursor-like proteins 1 and 2 (APLP1 and APLP2). The neurotoxic amyloid beta-peptide (Abeta) originates from APP, which is the only member of this protein family implicated in Alzheimer disease. However, the three homologous proteins have been proposed to be processed in similar ways and to have essential and overlapping functions. Therefore, it is also important to take into account the effects on the processing and function of the APP-like proteins in the development of therapeutic drugs aimed at decreasing the production of Abeta. Insulin and insulin-like growth factor-1 (IGF-1) have been shown to regulate APP processing and the levels of Abeta in the brain. In the present study, we show that IGF-1 increases alpha-secretase processing of endogenous APP and also increases ectodomain shedding of APLP1 and APLP2 in human SH-SY5Y neuroblastoma cells. We also investigated the role of different IGF-1-induced signaling pathways, using specific inhibitors for phosphatidylinositol 3-kinase and mitogen-activated protein kinase (MAPK). Our results indicate that phosphatidylinositol 3-kinase is involved in ectodomain shedding of APP and APLP1, but not APLP2, and that MAPK is involved only in the ectodomain shedding of APLP1.

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.

Diabetes and Alzheimer's disease - is there a connection?

It has been known for some time that diabetes may be associated with impaired cognitive function. During the last decade, epidemiological data have emerged suggesting a linkage between diabetes, particularly type 2 diabetes, and Alzheimer's disease (AD). There is evidence to suggest that impaired activities of neurotrophic factors such as insulin, IGF-1 and NGF, which occur in both diabetes and AD, may provide a mechanistic link between the two disorders. An additional probable factor that has been less evaluated to date is hypercholesterolemia, a common accompaniment to type 2 diabetes. Increased cholesterol availability is believed to play a crucial role in the abnormal metabolism of amyloid precursor protein leading to accumulation of amyloid-beta. Impaired insulin signaling in particular appears to be involved in hyperphosphorylation of the tau protein, which constitutes neurofibrillary tangles in AD. The linkage between abnormal amyloid metabolism and phosphor-tau is likely to be provided by the activation of caspases both by increased amyloid-beta and by impaired insulin signaling. Although the details of many of these components still await evaluation, it appears clear that commonalities exist in the underlying pathogenesis of diabetes and Alzheimer's disease. In this review we provide a brief update on linkages between these two diverse but common disorders.

Abeta(1-42) modulation of Akt phosphorylation via alpha7 nAChR and NMDA receptors

Elevated Abeta and its deposition as senile plaques are pathogenic features of Alzheimer's disease. Abeta has been shown to be toxic to neurons and to inhibit long-term potentiation yet, the intracellular signalling pathways underlying these actions are unknown. We report for the first time that acute exposure of primary mouse neurons to 400nM Abeta(1-42) increased Akt phosphorylation in an alpha(7) nicotinic receptor and NMDA receptor dependant manner. However, prolonged incubation resulted in Akt phosphorylation returning to baseline consistent with the action of a physiological regulator. Analysis of an APP transgenic mouse (TAS10) revealed a significant deficit in hippocampal Akt phosphorylation at 13 months. This time point corresponds to the emergence of plaque formation and memory impairments in these mice. The present study suggests that Abeta(1-42) regulates Akt phosphorylation in a complex manner. Acutely, Abeta(1-42) stimulates Akt phosphorylation however, chronic exposure to Abeta in TAS10 mice resulted in a downregulation of Akt phosphorylation consistent with abnormalities in excitatory neurotransmission in these mice and with recent reports of Abeta(1-42) driven internalisation of NMDA receptors.

Central insulin resistance as a trigger for sporadic Alzheimer-like pathology: an experimental approach

A growing body of evidence implicates impairments in brain insulin signaling in early sporadic Alzheimer disease (sAD) pathology. However, the most widely accepted hypothesis for AD aetiology stipulates that pathological aggregations of the amyloid beta (Abeta) peptide are the cause of all forms of Alzheimer's disease. Streptozotocin-intracerebroventricularly (STZ-icv) treated rats are proposed as a probable experimental model of sAD. The current work reviews evidence obtained from this model indicating that central STZ administration induces brain pathology and behavioural alterations resembling those in sAD patients. Recently, alterations of the brain insulin system resembling those in sAD have been found in the STZ-icv rat model and are associated with tau protein hyperphosphorylation and Abeta-like aggregations in meningeal vessels. In line with these findings the hypothesis has been proposed that insulin resistance in the brain might be the primary event which precedes the Abeta pathology in sAD.

Alois Alzheimer revisited: differences in origin of the disease carrying his name

Based on the means of his time, Alois Alzheimer supposed that the disease, later carrying his name, is a disease of older age, and that the pathomorphological structures he described are due to disturbances in brain metabolism. In this contribution, it is discussed which cellular metabolic abnormalities may be representative for age-related sporadic Alzheimer disease (SAD) the predominant form of SAD in contrast to the very rare hereditary early-onset form. In focus are disturbances in glucose/energy metabolism which involve the deficits in acetylcholine, cholesterol and UDP-N-acetylglucosamine beside ATP. Another leading abnormality is the defect in cell membrane composition. The interrelation between abnormal glucose/energy metabolism and membrane defect may be assumed to form the basis for the induction of both the perturbed metabolism of the amyloid precursor protein leading to increased formation of beta-amyloid and hyperphosphorylation of tau-protein destroying cell structures. Alois Alzheimer may have been so prescient to assume most of this 100 years ago.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

Apolipoprotein E, cholesterol metabolism, diabetes, and the convergence of risk factors for Alzheimer's disease and cardiovascular disease

High fat diets and sedentary lifestyles are becoming major concerns for Western countries. They have led to a growing incidence of obesity, dyslipidemia, high blood pressure, and a condition known as the insulin-resistance syndrome or metabolic syndrome. These health conditions are well known to develop along with, or be precursors to atherosclerosis, cardiovascular disease, and diabetes. Recent studies have found that most of these disorders can also be linked to an increased risk of Alzheimer's disease (AD). To complicate matters, possession of one or more apolipoprotein E epsilon4 (APOE epsilon4) alleles further increases the risk or severity of many of these conditions, including AD. ApoE has roles in cholesterol metabolism and Abeta clearance, both of which are thought to be significant in AD pathogenesis. The apparent inadequacies of ApoE epsilon4 in these roles may explain the increased risk of AD in subjects carrying one or more APOE epsilon4 alleles. This review describes some of the physiological and biochemical changes that the above conditions cause, and how they are related to the risk of AD. A diversity of topics is covered, including cholesterol metabolism, glucose regulation, diabetes, insulin, ApoE function, amyloid precursor protein metabolism, and in particular their relevance to AD. It can be seen that abnormal lipid, cholesterol and glucose metabolism are consistently indicated as central in the pathophysiology, and possibly the pathogenesis of AD. As diagnosis of mild cognitive impairment and early AD are becoming more reliable, and as evidence is accumulating that health conditions such as diabetes, obesity, and coronary artery disease are risk factors for AD, appropriate changes to diets and lifestyles will likely reduce AD risk, and also improve the prognosis for people already suffering from such conditions.

Metabolism of amyloid beta peptide and pathogenesis of Alzheimer's disease. Towards presymptomatic diagnosis, prevention and therapy

The conversion of what has been interpreted as "normal brain aging" to Alzheimer's disease (AD) via a transition state, i.e. mild cognitive impairment, appears to be a continuous process caused primarily by aging-dependent accumulation of amyloid beta peptide (Abeta) in the brain. This notion give us a hope that, by manipulating the Abeta levels in the brain, we may be able not only to prevent and cure the disease but also to partially control some very significant aspects of brain aging. Abeta is constantly produced from its precursor and immediately catabolized under normal conditions, whereas dysmetabolism of Abeta seems to lead to pathological deposition upon aging. We have focused our attention on elucidation of the unresolved mechanism of Abeta catabolism in the brain. In this review, we describe a new approach to prevent AD development by reducing Abeta burdens in aging brains through up-regulation the catabolic mechanism involving neprilysin that can degrade both monomeric and oligomeric forms of Abeta. The strategy of combining presymptomatic diagnosis with preventive medicine seems to be the most pragmatic in both medical and socio-economical terms. We also introduce a novel non-invasive amyloid imaging approach using a high-power magnetic resonance imaging (MRI) for the presymptomatic diagnosis of AD.

Short-term interleukin-1β increases the release of secreted APPα via MEK1/2-dependent and JNK-dependent α-secretase cleavage in neuroglioma U251 cells

Several lines of neuroimmunological evidence correlate the development of the inflammatory responses of the brain with the formation of amyloid plaques associated with the pathogenesis of neurodegenerative disorders such as Alzheimer's disease. Within this context, we tested the ability of interleukin-1beta (IL-1beta) to regulate the processing of beta-amyloid precursor protein (beta-APP) in neuroglioma U251 cells. Our findings have shown that short-term treatment with IL-1beta (2 hr) resulted in a concentration-dependent decrease in the amount of the cell-associated form of beta-APP in U251 cells as compared to untreated cells, whereas a 2-hr treatment with IL-1beta led to increased release of secreted APP(alpha) fragment (sAPP(alpha)) into the conditioned media of the cells. The fact that sAPP(alpha) is an alpha-secretase cleavage metabolite of the cell-associated form of beta-APP, and the observation that IL-1beta-induced sAPP(alpha) release could be blocked by tissue inhibitors of metalloproteinases-1 (alpha-secretase inhibitors), suggested that alpha-secretase might be involved in IL-1beta-induced-sAPP(alpha) release. Moreover, to determine whether an intracellular signaling pathway mediates the IL-1beta-induced increase in sAPP(alpha) secretion, we used various specific signaling inhibitors and found that sAPP(alpha) release is significantly blocked by the mitogen-activated protein kinase (MEK1/2) inhibitor PD98059 and the c-Jun N-terminal kinase inhibitor SP600125. These findings suggested that the mechanism of IL-1beta-induced-sAPP(alpha) release is dependent on MEK1/2- and JNK-activated alpha-secretase cleavage in neuroglioma U251 cells.

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.

Glycogen synthase kinase-3 is increased in white cells early in Alzheimer's disease

Alzheimer's disease (AD) is a disorder without a molecular marker in peripheral tissues or a disease modifying treatment. As increasing evidence has suggested a role for glycogen synthase kinase-3 (GSK-3) in the pathogenesis of the condition we measured total GSK-3 protein (alpha and beta isoforms) and GSK-3 activity (serine 9 phosphorylation) in a group of healthy elderly people, in AD and in mild cognitive impairment (MCI). Total GSK-3 protein was increased in both AD and in MCI without a compensatory decrease in activity. These data suggest that GSK-3 assays might be a useful diagnostic marker in a readily available tissue and moreover that GSK-3 activity is increased in the prodromal phase of the disorder suggesting that inhibition of GSK-3 might be a useful therapeutic strategy.

The non-amyloidogenic pathway: structure and function of alpha-secretases

The amyloid cascade hypothesis is the most accepted explanation for the pathogenesis of Alzheimer's disease (AD). APP is the precursor of the amyloid beta peptide (Abeta), the principal proteinaceous component of amyloid plaques in brains of Alzheimer's disease patients. Proteolytic cleavage of APP by the alpha-secretase within the Abeta sequence precludes formation of amyloidogenic peptides and leads to a release of soluble APPsalpha which has neuroprotective properties. In several studies, a decreased amount of APPsalpha in the cerebrospinal fluid of AD patients has been observed. Three members of the ADAM family (a disintegrin and metalloproteinase) ADAM-10, ADAM-17 (TACE) and ADAM-9 have been proposed as alpha-secretases. We review the evidence for each of these enzymes acting as a physiologically relevant alpha-secretase. In particular, we focus on ADAM-10, which recently was shown in a transgenic mouse model for AD, to act as an alpha-secretase in vivo. We also discuss the pharmacological up-regulation of alpha-secretases as a possible therapeutic treatment for AD.

Mitogen-activated protein kinases and the evolution of Alzheimer's: a revolutionary neurogenetic axis for therapeutic intervention?

Alzheimer's disease (AD) is a neurogenetic condition that affects the processes via which the brain functions. Major observable hallmarks of AD are accumulated clusters of proteins in the brain. These clusters, termed neurofibrillary tangles (NFT), resemble pairs of threads wound around each other in a helix fashion accumulating within neurons. These tangles consist of a protein called Tau, which binds to tubulin, thus forming microtubules. Unlike NFTs, deposits of amyloid precursor protein (beta-APP) gather in the spaces between nerve cells. The nearby neurons often look swollen and deformed, and the clusters of protein are usually accompanied by reactive inflammatory cells, microglia, which are part of the brain's immune system responsible for degrading and removing damaged neurons or plaques. Since phosphorylation/dephosphorylation mechanisms are crucial in the regulation of Tau and beta-APP, a superfamily of mitogen-activated protein kinases (MAPKs) has recently emerged as key regulators of the formation of plagues, eventually leading to dementia and AD. The complex molecular interactions between MAPKs and proteins (plagues) associated with the evolution of AD form a cornerstone in the knowledge of a still burgeoning field of neurodegenerative diseases and ageing. This review overviews current understanding of the molecular pathways related to MAPKs and their role in the development of AD and, possibly, dementia. MAPKs, therefore, may constitute a neurogenetic, therapeutic target for the diagnosis and evolution of a preventative medical strategy for early detection, and likely treatment, of Alzheimer's.

Glucose metabolism and insulin receptor signal transduction in Alzheimer disease

Nosologically, Alzheimer disease is not a single disorder in spite of a common clinical phenotype. Etiologically, two different types or even more exist. (1) In a minority of about 5% or less of all cases, Alzheimer disease is due to mutations of three genes, resulting in the permanent generation of betaA4. (2) The great majority (95% or more) of cases of Alzheimer disease are sporadic in origin, with old age as main risk factor, supporting the view that susceptibility genes and aging contribute to age-related sporadic Alzheimer disease. However, disturbances in the neuronal insulin signal transduction pathway may be of central pathophysiological significance. In early-onset familial Alzheimer disease, the inhibition of neuronal insulin receptor function may be due to competitive binding of amyloid beta (Abeta) to the insulin receptor. In late-onset sporadic Alzheimer disease, the neuronal insulin receptor may be desensitized by inhibition of receptor function at different sites by noradrenaline and/or cortisol, the levels of which both increase with increasing age. The consequences of the inhibition of neuronal insulin signal transduction may be largely identical to those of disturbances of oxidative energy metabolism and related metabolism, and of hyperphosphorylation of tau-protein. As far as the metabolism of amyloid precursor protein (APP) in late-onset sporadic Alzheimer disease is concerned, neuronal insulin receptor dysfunction may result in the intracellular accumulation of Abeta and in subsequent cellular damage. In this context, the desensitization of the neuronal insulin receptor in late-onset sporadic Alzheimer disease is different from that occurring in normal aging and early-onset familial Alzheimer disease. In late-onset sporadic Alzheimer disease changes in the brain are similar to those caused by non-insulin-dependent diabetes mellitus.

Beta-secretase BACE1 is differentially controlled through muscarinic acetylcholine receptor signaling

The beta-amyloid peptides derived by proteolytic cleavage from the amyloid precursor protein (APP) play a major role in the pathogenesis of Alzheimer's disease (AD) by forming aggregated, fibrillary complexes that have been shown to be neurotoxic. The beta-site APP-cleaving enzyme (BACE1) has been identified as the key enzyme leading to beta-amyloid formation, and cholinergic mechanisms have been shown to control APP processing. The present study sought to determine whether BACE1 expression is controlled by muscarinic acetylcholine receptor (mAChR) subtypes in the neuroblastoma cell line SK-SH-SY5Y. Stimulation of cells with the M1/M3-selective mAChR agonist talsaclidine for 1 hr resulted in a dose-dependent increase in BACE1 expression up to twofold over basal levels. Similar effects of BACE1 up-regulation were observed when protein kinase C was directly activated by phorbol esters. However, when the MAP kinases MEK/ERK were inhibited, BACE1 expression was no longer up-regulated by the activation of M1-mAChR. In contrast, BACE1 expression was suppressed by stimulation of M2-mediated pathways via selective M2-agonist binding or direct activation of adenylate cyclase with forskolin, an effect that was prevented by inhibiting protein kinase A. These results may explain the observed deterioration of AD patients after initial improvements with AChE inhibitor or M1-mAChR agonist treatment.

Oxidative stress affects synaptosomal gamma-aminobutyric acid and glutamate transport in diabetic rats: the role of insulin

Evidence suggests that oxidative stress is involved in the pathophysiology of diabetic complications and that insulin has a neuroprotective role in oxidative stress conditions. In this study, we evaluated the in vitro effect of insulin in the susceptibility to oxidative stress and in the transport of the amino acid neurotransmitters gamma-aminobutyric acid (GABA) and glutamate in a synaptosomal fraction isolated from male type 2 diabetic Goto-Kakizaki (GK) rat brain cortex. The ascorbate/Fe(2+)-induced increase in thiobarbituric acid reactive substances (TBARSs) was similar in Wistar and GK rats and was not reverted by insulin (1 micromol/l), suggesting that other mechanisms, rather than a direct effect in membrane lipid peroxidation, may mediate insulin neuroprotection. Diabetes did not affect GABA and glutamate transport, despite the significant decrease in membrane potential and ATP/ADP ratio, and insulin increased the uptake of both GABA and glutamate in GK rats. Upon oxidation, there was a decrease in the uptake of both neurotransmitters and an increase in extrasynaptosomal glutamate levels and in ATP/ADP ratio in GK rats. Insulin treatment reverted the ascorbate/Fe(2+)-induced decrease in GABA accumulation, with a decrease in extrasynaptosomal GABA. These results suggest that insulin modulates synaptosomal GABA and/or glutamate transport, thus having a neuroprotective role under oxidizing and/or diabetic conditions.

Causes and consequences of disturbances of cerebral glucose metabolism in sporadic Alzheimer disease: therapeutic implications

Alzheimer disease is not a single disorder. Etiologically, two different types or even diseases exist: inheritance in 5% to 10% of all Alzheimer cases versus 90% to 95% AD cases whith sporadic origin (SAD). Different susceptibility genes along with adult lifestyle risk-factors- in the case of SAD the risk factor aging- may be assumed to cause the latter disorder. There is evidence that a disturbance in the insulin signal transduction pathway may be a central and early pathophysiologic event in SAD. Both, hypercortisolemia and increased adrenergic activity, in both old age and SAD may render the function of the neuronal insulin receptor vulnerable resulting in a diminished production of ATP. The reduced availability of ATP may damage the function of the endoplasmic reticulum/Golgi apparatus/trans Golgi network generating misfolded and malfolded proteins retained in the cell. In SAD, amyloid precursor protein is found to accumulate intracellularly thus not representing the cause but a driving force in the pathogenesis of SAD. Additionally, both disturbed insulin signaling and reduced ATP forward the hyperphosphorylation of tau protein. Thus, abnormalities in oxidative brain metabolism lead to the formation of two main morphologic hallmarks of SAD: senile plaques and neurofibrillary tangles. Therefore, the therapeutic goal in SAD should be the improvement of the neuronal energy state. Findings from both basic and clinical studies showed that Ginkgo biloba extract (EGb 761) may be appropiate to approach that goal.

Accumulation of the amyloid precursor-like protein APLP2 and reduction of APLP1 in retinoic acid-differentiated human neuroblastoma cells upon curcumin-induced neurite retraction

Amyloid precursor protein (APP) belongs to a conserved gene family, also including the amyloid precursor-like proteins, APLP1 and APLP2. The function of these three proteins is not yet fully understood. One of the proposed roles of APP is to promote neurite outgrowth. The aim of this study was to investigate the regulation of the expression levels of APP family members during neurite outgrowth. We observed that retinoic acid (RA)-induced neuronal differentiation of human SH-SY5Y cells resulted in increased expression of APP, APLP1 and APLP2. We also examined the effect of the NFkappaB, AP-1 and c-Jun N-terminal kinase inhibitor curcumin (diferuloylmethane) on the RA-induced expression levels of these proteins. We found that treatment with curcumin counteracted the RA-induced mRNA expression of all APP family members. In addition, we observed that curcumin treatment resulted in neurite retraction without any effect on cell viability. Surprisingly, curcumin had differential effects on the APLP protein levels in RA-differentiated cells. RA-induced APLP1 protein expression was blocked by curcumin, while the APLP2 protein levels were further increased. APP protein levels were not affected by curcumin treatment. We propose that the sustained levels of APP and the elevated levels of APLP2, in spite of the reduced mRNA expression, are due to altered proteolytic processing of these proteins. Furthermore, our results suggest that APLP1 does not undergo the same type of regulated processing as APP and APLP2.

Insulin receptor substrate-2 deficiency impairs brain growth and promotes tau phosphorylation

Insulin resistance and diabetes might promote neurodegenerative disease, but a molecular link between these disorders is unknown. Many factors are responsible for brain growth, patterning, and survival, including the insulin-insulin-like growth factor (IGF)-signaling cascades that are mediated by tyrosine phosphorylation of insulin receptor substrate (IRS) proteins. Irs2 signaling mediates peripheral insulin action and pancreatic beta-cell function, and its failure causes diabetes in mice. In this study, we reveal two important roles for Irs2 signaling in the mouse brain. First, disruption of the Irs2 gene reduced neuronal proliferation during development by 50%, which dissociated brain growth from Irs1-dependent body growth. Second, neurofibrillary tangles containing phosphorylated tau accumulated in the hippocampus of old Irs2 knock-out mice, suggesting that Irs2 signaling is neuroprotective. Thus, dysregulation of the Irs2 branch of the insulin-Igf-signaling cascade reveals a molecular link between diabetes and neurodegenerative disease.

Insulin dose-response effects on memory and plasma amyloid precursor protein in Alzheimer's disease: interactions with apolipoprotein E genotype

In previous studies, adults with Alzheimer's disease (AD) showed memory enhancement when plasma insulin levels were raised to 85 microU/ml, whereas normal adults' memory was unchanged. Degree of memory enhancement was also related to apolipoprotein E (apoE) genotype status for AD patients. Response differences between normal and AD groups could reflect dose-response differences for insulin. To examine this question, 22 adults with AD and 15 normal adults received five doses of insulin on separate days in counterbalanced order, resulting in five plasma insulin levels (10, 25, 35, 85 and 135 microU/ml), while plasma glucose levels of ~100 mg/dl were maintained. Cognitive performance and plasma APP levels were measured after 120 min of infusion. Relative to baseline, AD patients who were not apoE- epsilon 4 homozygotes had improved memory at higher insulin levels of 35 and 85 microuU/ml, whereas normal adults and AD patients who were epsilon 4 homozygotes showed improved memory at insulin levels of 25 microU/ml. Normal adults' memory was also improved at insulin levels of 85 microU/ml. Plasma APP was lowered for adults with AD without the epsilon 4 allele at higher levels (85 microU/ml) than for normal adults and epsilon 4 homozygotes, who showed decreased APP at the 35 microU/ml level. AD patients with a single epsilon 4 allele showed a different pattern of insulin effects on APP than did other subjects. In general, few effects of insulin were seen at the highest dose for any subject group. These results support a role for insulin in normal memory and APP modulation that follows a curvilinear response pattern, and suggest that AD patients who are not epsilon 4 homozygotes have reduced sensitivity to insulin that may interfere with such modulation.