Pigment epithelium-derived factor maintains retinal pigment epithelium function by inhibiting vascular endothelial growth factor-R2 signaling through gamma-secretase.

Wet age-related macular degeneration (AMD) attacks the integrity of the retinal pigment epithelium (RPE) barrier system. The pathogenic process was hypothesized to be mediated by vascular endothelial growth factor (VEGF) and antagonized by pigment epithelium-derived factor (PEDF). To dissect these functional interactions, monolayer cultures of RPE cells were established, and changes in transepithelial resistance were evaluated after administration of PEDF, placenta growth factor (VEGF-R1 agonist), and VEGF-E (VEGF-R2 agonist). A recently described mechanism of VEGF inhibition in endothelia required the release of VEGF-R1 intracellular domain by gamma-secretase. To evaluate this pathway in the RPE, cells were pretreated with inhibitors DAPT or LY411575. Processing of VEGF receptors was assessed by Western blot analysis. Administration of VEGF-E rapidly increased RPE permeability, and PEDF inhibited the VEGF-E response dose-dependently. Both gamma-secretase antagonists prevented the inhibitory effects of PEDF. The co-administration of PEDF and VEGF-E depleted the amount of VEGF-R2 in the membrane and increased the amount of VEGF-R2 ectodomain in the media. Therefore, the inhibitory effect of PEDF appears to be mediated via the processing of VEGF-R2 by gamma-secretase. gamma-Secretase generates the amyloid-beta (Abeta) peptide of Alzheimer disease from its precursor (amyloid precursor protein). This peptide is also a component of drusen in dry AMD. The results support the hypothesis that misregulation of gamma-secretase may not only lead to Abeta deposits in dry AMD but can also be damaging to RPE function by blocking the protective effects of PEDF to prevent VEGF from driving the dry to wet AMD transition.

Geranylgeranyl pyrophosphate stimulates gamma-secretase to increase the generation of Abeta and APP-CTFgamma.

Cleavage of the amyloid precursor protein (APP) by beta- and gamma-secretases results in generation of the amyloid-beta protein (Abeta), which is characteristically deposited in the brain of Alzheimer's disease patients. Inhibitors of 3-hydroxy-3-methyl-glutaryl (HMG)-CoA reductase (the statins) reduce levels of cholesterol and isoprenoids such as geranylgeranyl pyrophosphate (GGPP). Previous studies have demonstrated that cholesterol increases and statins reduce Abeta levels mostly by regulating beta-secretase activity. In this study, we focused on the role of geranylgeranyl isoprenoids GGPP and geranylgeraniol (GGOH) in regulating Abeta production. Our data show that the inhibition of GGPP synthesis by statins plays an important role in statin-mediated reduction of Abeta secretion. Consistent with this finding, the geranylgeranyl isoprenoids preferentially increase the yield of Abeta of 42 residues (Abeta42) in a dose-dependent manner. Our studies further demonstrated that geranylgeranyl isoprenoids increase the yield of APP-CTFgamma (a.k.a. AICD) as well as Abeta by stimulating gamma-secretase-mediated cleavage of APP-CTFalpha and APP-CTFbeta in vitro. Furthermore, GGOH increases the levels of the active gamma-secretase complex in the detergent-insoluble membrane fraction along with its substrates, APP-CTFalpha and APP-CTFbeta. Our results indicate that geranylgeranyl isoprenoids may be an important physiological facilitator of gamma-secretase activity that can foster production of the pathologically important Abeta42.

High cholesterol-induced neuroinflammation and amyloid precursor protein processing correlate with loss of working memory in mice.

Recent findings suggest that hypercholesterolemia may contribute to the onset of Alzheimer's disease-like dementia but the underlying mechanisms remain unknown. In this study, we evaluated the cognitive performance in rodent models of hypercholesterolemia in relation to neuroinflammatory changes and amyloid precursor protein (APP) processing, the two key parameters of Alzheimer's disease pathogenesis. Groups of normal C57BL/6 and low density lipoprotein receptor (LDLR)-deficient mice were fed a high fat/cholesterol diet for an 8-week period and tested for memory in a radial arm maze. It was found that the C57BL/6 mice receiving a high fat diet were deficient in handling an increasing working memory load compared with counterparts receiving a control diet while the hypercholesterolemic LDLR-/- mice showed impaired working memory regardless of diet. Immunohistochemical analysis revealed the presence of activated microglia and astrocytes in the hippocampi from high fat-fed C57BL/6 mice and LDLR-/- mice. Consistent with a neuroinflammatory response, the hyperlipidemic mice showed increased expression of cytokines/mediators including tumor necrosis factor-alpha, interleukin-1beta and -6, nitric oxide synthase 2, and cycloxygenase 2. There was also an induced expression of the key APP processing enzyme i.e. beta-site APP cleaving enzyme 1 in both high fat/cholesterol-fed C57BL/6 and LDLR-/- mice accompanied by an increased generation of C-terminal fragments of APP. Although ELISA for beta-amyloid failed to record significant changes in the non-transgenic mice, a threefold increase in beta-amyloid 40 accumulation was apparent in a strain of transgenic mice expressing wild-type human APP on high fat/cholesterol diet. The findings link hypercholesterolemia with cognitive dysfunction potentially mediated by increased neuroinflammation and APP processing in a non-transgenic mouse model.

Antigen-antibody dissociation in Alzheimer disease: a novel approach to diagnosis.

With the ever-increasing population of aged individuals at risk of developing Alzheimer's disease (AD), there is an urgent need for a sensitive, specific, non-invasive, and diagnostic standard. The majority of efforts have focused on auto-antibodies against amyloid-beta (Abeta) protein, both as a potential treatment, and a reliable biomarker of AD pathology. Naturally occurring antibodies against Abeta are found in the CSF and plasma of patients with AD as well as healthy control subjects. To date, differences between diseased and control subjects have been highly variable. However, some of the antibody will be in preformed antigen-antibody complexes and the extent and nature of such complexes may provide a potential explanation for the variable results reported in human studies. Thus, measuring total amounts of antigen or antibody following unmasking is critical. Here, using a technique for dissociating antibody-antigen complexes, we found significant differences in serum antibodies to Abeta between AD and aged-matched control subjects. While the current study demonstrates the relevance of measuring total antibody, bound and unbound, against Abeta in AD, this technique may be applicable to diseases such as acquired immune deficiency syndrome and hepatitis B where determination of antigen and antibody levels are important for disease diagnosis and assessing disease progression.

An increase in Abeta42 in the prefrontal cortex is associated with a reversal-learning impairment in Alzheimer's disease model Tg2576 APPsw mice.

The medial temporal lobe-dependent memory loss associated with Alzheimer's disease (AD) is often accompanied by a loss of prefrontal cortex-dependent cognitive domains that fall under the broad category of executive function. In this study, we examined the relationship between one type of prefrontal-dependent executive function, discrimination reversal-learning, and levels of the amyloid beta protein (Abeta) of 40 and 42 residues in a transgenic mouse model (Tg2576) of the over-expression of the familial AD mutant form of the amyloid precursor protein (APPsw). Tg2576 and their non-transgenic (NTg) littermates were assessed at 3 and 6 months of age when there is little to no amyloid plaque deposition. After reversal-learning assessment, Abeta40 and Abeta42 were quantified in the prefrontal cortex and hippocampus. Tg2576 mice were impaired in reversal-learning at 6 but not 3 months of age when compared to the NTg group. Coincidently, there was a corresponding approximately 3-fold increase of Abeta42 levels in the prefrontal cortex of 6- compared to 3-month-old Tg2576 mice. In addition, the prefrontal cortex contained higher levels of Abeta42 compared to the hippocampus at both 3 and 6 months of age, regardless of genotype, indicating a high vulnerability of this brain region to Abeta42 accumulation. These data suggest that the early emergence of reversal-learning deficits in the Tg2576 mouse may be due to the localized increase of Abeta42 in the prefrontal cortex.

Genotoxicity in Alzheimer's disease: role of amyloid.

Alzheimer's disease (AD) is a complex neurodegenerative disorder pathologically identified by the presence of extracellular senile plaques (SP) with a proteinaceous core composed of aggregates of the amyloid peptide (Abeta) and intracellular aggregates of the microtubule-associated protein tau (tau) as neurofibrillary tangles (NFTs). These hallmarks consist of abnormally folded proteinaceous components that are believed to be neurotoxic in AD. The mechanisms of toxicity remain unclear although oxidative stress and inflammation are implicated as mediators of the toxicity and these lesions, in turn, are known to damage cellular components including proteins, lipids in the membrane and DNA. However effects on genotoxicity and its role in AD are less clear. The present review discusses various influences, in particular of amyloid, on the genetic material and their possible role in the neurodegeneration in AD. Further, the amalgamation of genomics and proteomics in understanding AD and therapeutic development is suggested.

A partial failure of membrane protein turnover may cause Alzheimer's disease: a new hypothesis.

The amyloid hypothesis has dominated the thinking in our attempts to understand, diagnose and develop drugs for Alzheimer's disease (AD). This article presents a new hypothesis that takes into account the numerous familial AD (FAD) mutations in the amyloid precursor protein (APP) and its processing pathways, but suggests a new perspective beyond toxicity of forms of the amyloid beta-peptide (Abeta). Clearly, amyloid deposits are an invariable feature of AD. Moreover, although APP is normally processed to secreted and membrane-bound fragments, sAPPbeta and CTFbeta, by BACE, and the latter is subsequently processed by gamma-secretase to Abeta and CTFgamma, this pathway mostly yields Abeta of 40 residues, and increases in the levels of the amyloidogenic 42-residue Abeta (Abeta42) are seen in the majority of the mutations linked to the disease. The resulting theory is that the disease is caused by amyloid toxicity, which impairs memory and triggers deposition of the microtubule associated protein, Tau, as neurofibrillary tangles. Nevertheless, a few exceptional FAD mutations and the presence of large amounts of amyloid deposits in a group of cognitively normal elderly patients suggest that the disease process is more complex. Indeed, it has been hard to demonstrate the toxicity of Abeta42 and the actual target has been shifted to small oligomers of the peptide, named Abeta derived diffusible ligands (ADDLs). Our hypothesis is that the disease is more complex and caused by a failure of APP metabolism or clearance, which simultaneously affects several other membrane proteins. Thus, a traffic jam is created by failure of important pathways such as gamma-secretase processing of residual intramembrane domains released from the metabolism of multiple membrane proteins, which ultimately leads to a multiple system failure. In this theory, toxicity of Abeta42 will only contribute partially, if at all, to neurodegeneration in AD. More significantly, this theory would predict that focussing on specific reagents such as gamma-secretase inhibitors that hamper metabolism of APP, may initially show some beneficial effects on cognitive performance by elimination of acutely toxic ADDLs, but over the longer term may exacerbate the disease process by reducing membrane protein turnover.

Neuronutrition and Alzheimer's disease.

Alzheimer's disease (AD) is a complex neurological disorder resulting from both genetic and environmental factors with the latter being particularly important for the sporadic form of the disease. As such, diets rich in saturated fatty acids and alcohol, and deficient in antioxidants and vitamins appear to promote the onset of the disease, while diets rich in unsaturated fatty acids, vitamins, antioxidants, and wine likely suppress its onset. In addition, evidence suggests that diets rich in polyphenols and some spices suppress the onset of AD by scavenging free radicals and preventing oxidative damage. Metal ions are known to catalyze the production of free radicals and induce mental retardation or dementia, and several studies have also identified metals such as Pb, Fe, Al, Cu, and Zn in AD pathogenesis. While specific metal chelators have been tested for therapy, they have not been very successful, probably due to their late administration, i.e., after brain damage has been triggered. Since several dietary polyphenols are known to chelate metals, their routine use may also be protective against the onset of AD. In this review, we summarize beneficial dietary techniques in the fight against AD.

Differential accumulation of secreted AbetaPP metabolites in ocular fluids.

Amyloid-beta (Abeta) accumulates in several types of retinal degeneration and in Alzheimer's disease (AD), but its source has been unclear. We detected the neuronal 695 amino acid form of amyloid-beta protein precursor (AbetaPP) in the normal retina and AbetaPP751 in the retinal pigment epithelium (RPE) and anterior eye tissues. Similar to the brain, alpha- and beta-secretases cleaved AbetaPP to soluble derivatives (sAbetaPP) alpha or beta and membrane-bound C-terminal fragments alpha or beta in the retina and RPE. Levels of sAbetaPP were particularly high in the vitreous and low in aqueous humor revealing a molecular barrier for AbetaPP. In contrast, Abeta40 and Abeta42 levels were only 50% lower in the aqueous than the vitreous humor, indicating relatively barrier-free movement of Abeta. These studies demonstrated a relatively high yield of AbetaPP and Abeta in the ocular fluids, which may serve as a trackable marker for AD. In addition, failure of free clearance from the eye may trigger retina degeneration in a manner similar to Abeta-related neurodegeneration in AD.

Cholinergic degeneration and memory loss delayed by vitamin E in a Down syndrome mouse model.

Down syndrome (DS) individuals develop several neuropathological hallmarks seen in Alzheimer's disease, including cognitive decline and the early loss of cholinergic markers in the basal forebrain. These deficits are replicated in the Ts65Dn mouse, which contains a partial trisomy of murine chromosome 16, the orthologous genetic segment to human chromosome 21. Oxidative stress levels are elevated early in DS, and may contribute to the neurodegeneration seen in these individuals. We evaluated oxidative stress in Ts65Dn mice, and assessed the efficacy of long-term antioxidant supplementation on memory and basal forebrain pathology. We report that oxidative stress was elevated in the adult Ts65Dn brain, and that supplementation with the antioxidant vitamin E effectively reduced these markers. Also, Ts65Dn mice receiving vitamin E exhibited improved performance on a spatial working memory task and showed an attenuation of cholinergic neuron pathology in the basal forebrain. This study provides evidence that vitamin E delays onset of cognitive and morphological abnormalities in a mouse model of DS, and may represent a safe and effective treatment early in the progression of DS neuropathology.

Effect of Casearia esculenta root extract on blood glucose and plasma antioxidant status in streptozotocin diabetic rats.

Our preliminary study shows that an oral administration of an aqueous extract of Casearia esculenta, an indigenous antidiabetic plant popularly used in South India for diabetes mellitus, lowers blood glucose level under normal and glucose load conditions, and in streptozotocin (STZ)-induced diabetes in rats. The study was further undertaken to evaluate the antioxidant potential of C. esculenta in STZ diabetic rats. Oral administration of C. esculenta root extract at doses of 200 and 300 mg/kg for 45 days resulted in significant reduction in plasma thiobarbituric acid reactive substances (TBARS), hydroperoxide and ceruloplasmin and a significant elevation in plasma reduced glutathione (GSH), ascorbic acid (vitamin C) and alpha-tocopherol (vitamin E). The study indicates that C. esculenta root extract at doses of 200 and 300 mg/kg restored all the antioxidant parameters to near normal value.