Deciphering the genesis and fate of amyloid beta-protein yields novel therapies for Alzheimer disease

Mild cognitive impairment: searching for the prodrome of Alzheimer's disease

The concept of mild cognitive impairment (MCI) identifies persons who are neither cognitively normal nor demented. There is increasing evidence that MCI defines a group of persons who are at near-term risk of developing dementia and particularly Alzheimer's disease (AD). MCI thus constitutes an attractive target population for preventive treatments of AD. MCI is associated with aging and is more prevalent than dementia. There are several clinical and biological markers that are predictive of MCI prognosis, including depressive symptoms, cognitive deficits, brain imaging and neurochemical findings. The clinician needs to be especially alert to depressive and other mood symptoms which are common in MCI and potentially treatable. Trials of current medications for prevention of MCI progression to dementia have been largely negative. There are observational data suggesting that lifestyle modifications including exercise, leisure activities, cognitive stimulation, and social activities may be effective for prevention of MCI progression. There are many novel therapies currently in trials for early AD, and if effective they may prove to be helpful in prevention of MCI progression as well.

5-Lipoxygenase gene disruption reduces amyloid-beta pathology in a mouse model of Alzheimer's disease

5-Lipoxygenase (5LO), by producing leukotrienes, is a proinflammatory enzyme, and there is evidence suggesting that it is up-regulated with aging and may be involved in Alzheimer's disease (AD). In this paper, we studied the effect of 5LO-targeted gene disruption on the amyloid phenotype of a transgenic mouse model of AD, the Tg2576. Amyloid-beta (Abeta) deposition in the brains of Tg2576 mice lacking 5LO was reduced by 64-80% compared with Tg2576 controls. This reduction was associated with a similar significant decrease in Abeta levels measured by sandwich ELISA. Absence of 5LO did not induce any significant change in amyloid-beta precursor protein (APP) levels and processing, or Abeta catabolic pathways. Furthermore, in vitro studies showed that 5LO activation or 5LO metabolites increase, whereas 5LO inhibition decreases, Abeta formation, secondary to correspondent changes in gamma-secretase activity. These data establish for the first time a novel functional role for 5LO in the pathogenesis of AD-like amyloidosis, thereby modulating gamma-secretase activity. Our work suggests that pharmacological inhibition of 5LO could provide a novel therapeutic tool for AD.

Alzheimer's disease: cholesterol, membrane rafts, isoprenoids and statins

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disorder and the most prevalent form of dementia worldwide. AD is characterized pathologically by amyloid-β plaques, neurofibrillary tangles and neuronal loss, and clinically by a progressive loss of cognitive abilities. At present, the fundamental molecular mechanisms underlying the disease are unclear and no treatment for AD is known. Epidemiological evidence continues to mount linking vascular diseases, such as hypertension and diabetes, and hypercholesterolaemia with an increased risk for developing AD. A growing amount of evidence suggests a mechanistic link between cholesterol metabolism in the brain and the formation of amyloid plaques in AD development. Cholesterol and statins clearly modulate β-amyloid precursor protein (βAPP) processing in cell culture and animal models. Statins not only reduce endogenous cholesterol synthesis but also exert other various pleiotrophic effects, such as the reduction in protein isoprenylation. Through these effects statins modulate a variety of cellular functions involving both cholesterol (and membrane rafts) and isoprenylation. Although clearly other factors, such as vascular inflammation, oxidative stress and genetic factors, are intimately linked with the progression of AD, this review focuses on the present research findings describing the effect of cholesterol, membrane rafts and isoprenylation in regulating βAPP processing and in particular γ-secretase complex assembly and function and AD progression, along with consideration for the potential role statins may play in modulating these events.

A statistical-mechanical theory of fibril formation in dilute protein solutions

We outline a theoretical treatment that describes fibril formation in dilute protein solutions. For this, we combine a theory describing self-assembly and conformational transition with a description of the lateral association of linear chains. Our statistical-mechanical model is able to predict the mean degree of polymerization and the length of the fibrils and their precursors, as well as the weight fractions of the different aggregated species in solution. We find that there appear to exist two regimes as a function of concentration, and as a function of the free energies of protein association: one in which low-molecular weight compounds dominate and one in which the fibrils do. The transition between these regimes can be quite sharp, and becomes sharper as more filaments are allowed to associate into a single fibril. The fraction of fibrils consisting of less than the maximum allowed number of filaments turns out to be negligible, in agreement with experimental studies, where the fibril thickness is found to be practically monodisperse. In addition, we find that the description of the fibril ends has a large effect on the predicted fibril length.

GroEL-mediated protein folding: making the impossible, possible

Protein folding is a spontaneous process that is essential for life, yet the concentrated and complex interior of a cell is an inherently hostile environment for the efficient folding of many proteins. Some proteins-constrained by sequence, topology, size, and function-simply cannot fold by themselves and are instead prone to misfolding and aggregation. This problem is so deeply entrenched that a specialized family of proteins, known as molecular chaperones, evolved to assist in protein folding. Here we examine one essential class of molecular chaperones, the large, oligomeric, and energy utilizing chaperonins or Hsp60s. The bacterial chaperonin GroEL, along with its co-chaperonin GroES, is probably the best-studied example of this family of protein-folding machine. In this review, we examine some of the general properties of proteins that do not fold well in the absence of GroEL and then consider how folding of these proteins is enhanced by GroEL and GroES. Recent experimental and theoretical studies suggest that chaperonins like GroEL and GroES employ a combination of protein isolation, unfolding, and conformational restriction to drive protein folding under conditions where it is otherwise not possible.

Modulation of beta-amyloid precursor protein trafficking and processing by the low density lipoprotein receptor family

Amyloid-β peptide (Aβ) accumulation in the brain is an early, toxic event in the pathogenesis of Alzheimer's disease (AD). Aβ is produced by proteolytic processing of a transmembrane protein, β-amyloid precursor protein (APP), by β- and γ-secretases. Mounting evidence has demonstrated that alterations in APP cellular trafficking and localization directly impact its processing to Aβ. Recent studies have shown that members of the low-density lipoprotein receptor family, including LRP, LRP1B, SorLA/LR11, and apolipoprotein E (apoE) receptor 2, interact with APP and regulate its endocytic trafficking. Another common feature of these receptors is their ability to bind apoE, which exists in three isoforms in humans and the presence of the ε4 allele represents a genetic risk factor for AD. In this review, we summarize the current understanding of the function of these apoE receptors with a focus on their role in APP trafficking and processing. Knowledge of the interactions between these distinct low-density lipoprotein receptor family members and APP may ultimately influence future therapies for AD.

Noncovalent interaction between amyloid-beta-peptide (1-40) and oleuropein studied by electrospray ionization mass spectrometry

Beta amyloid peptide (Abeta) is the major proteinaceous component of senile plaques formed in Alzheimer's disease (AD) brain. The aggregation of Abeta is associated with neurodegeneration, loss of cognitive ability, and premature death. It has been suggested that oxidative stress and generation of free radical species have implications in the fibrillation of Abeta and its subsequent neurotoxicity. For this reason, it is proposed that antioxidants may offer a protective or therapeutic alternative against amyloidosis. This study is the first report of the formation of the noncovalent complex between Abeta or its oxidized form and the natural derived antioxidant oleuropein (OE) by electrospray ionization mass spectrometry (ESI MS). ESI MS allowed the real time monitoring of the complex formation between Abeta, OE, and variants thereof. Several experimental conditions, such as elevated orifice potential, low pH values, presence of organic modifier, and ligand concentration were examined, to assess the specificity and the stability of the formed noncovalent complexes.

Cytokine production by a human microglial cell line: effects of beta-amyloid and alpha-melanocyte-stimulating hormone

Senile plaques in the Alzheimer's disease (AD) are formed by aggregation of beta-amyloid (Abeta) peptide. Abeta peptide has been shown to activate microglia and stimulate their production of inflammatory factors, such as cytokines. In the AD brain, the continued presence of amyloid plaques may keep microglia persistently activated, leading to chronic inflammation in the CNS. It is well established that alpha-melanocyte-stimulating hormone (alpha-MSH) gives rise to anti-inflammatory and anti-pyretic effects. The biological activities of alpha-MSH are mediated by one or more of the melanocortin receptor (MCR) subtypes, i.e. MCR1 - MCR5. The aim of the present study was to determine the effect of alpha-MSH alone and on Abeta-activated microglial cells with regard to the secretion of inflammatory cytokines, such as interleukin-6 (IL-6), and to determine which receptor subtype mediates the effects of alpha-MSH. The human microglial cell line, CHME3, was incubated for 24 h with freshly dissolved Abeta(1-40), interferon-gamma (IFN-gamma) and/or alpha-MSH. Freshly dissolved Abeta(1-40) (5-60 microM) resulted in a dose-dependent decrease in cell viability, along with a dose-dependent increase in IL-6 release. Neither IFN-gamma nor alpha-MSH affected the Abeta-induced secretion of IL-6, but resulted in a dose-dependent increase in basal IL-6 release. Agouti, the endogenous antagonist of MCR1 and 4, further increased the alpha-MSH-induced secretion of IL-6. RT-PCR showed the expression of MCR1, MCR3, MCR4 and MCR5 mRNA. The combined data suggest that the effect of alpha-MSH in increasing IL-6 release from the human microglial cell line is mediated by MCR3 or MCR5.

Beta-amyloid protein structure determines the nature of cytokine release from rat microglia

Activated microglia represent a major source of inflammatory factors in Alzheimer's disease and a possible source of cytotoxic factors. beta-Amyloid (Abeta) peptide, the predominant component in amyloid plaques, has been shown to activate microglia and stimulate their production of inflammatory factors. The present study was performed to analyze the responses of microglia to different forms of Abeta, with regard to release of the proinflammatory cytokines interleukin-1alpha (IL-1alpha), IL-1beta, tumor necrosis factor-alpha (TNF-alpha), IL-6, and interferon-gamma (IFN-gamma), as well as the IL-1 receptor antagonist (IL-1ra). Primary cultures of microglia from rat neonatal cerebral cortex were incubated with freshly dissolved Abeta1-40 or Abeta1-42, Abeta1-40 fibrils, Abeta1-40 betaamy balls, or vehicle. Abeta1-40 fibrils did not significantly stimulate any of these cytokines. Freshly dissolved Abeta1-40 resulted in a marked increase in the release of IL-1beta, and freshly dissolved Abeta1-42 significantly stimulated both IL-1alpha and IFN-gamma secretion. The Abeta1-40 betaamy balls stimulated the secretion of IL-1alpha and IL-1beta. Incubation with Abeta peptides did not affect the secretion of IL-1ra, IL-6, or TNF-alpha. In the case of IL-1beta, the response is correlated with the presence of Abeta peptide as monomers and oligomers.

Herpes simplex virus interferes with amyloid precursor protein processing

BACKGROUND

The early events underlying Alzheimer's disease (AD) remain uncertain, although environmental factors may be involved. Work in this laboratory has shown that the combination of herpes simplex virus type 1 (HSV1) in brain and carriage of the APOE-epsilon4 allele of the APOE gene strongly increases the risk of developing AD. The development of AD is thought to involve abnormal aggregation or deposition of a 39-43 amino acid protein--beta amyloid (Abeta)--within the brain. This is cleaved from the much larger transmembranal protein 'amyloid precursor protein' (APP). Any agent able to interfere directly with Abeta or APP metabolism may therefore have the capacity to contribute towards AD. One recent report showed that certain HSV1 glycoprotein peptides may aggregate like Abeta; a second study described a role for APP in transport of virus in squid axons. However to date the effects of acute herpesvirus infection on metabolism of APP in human neuronal-type cells have not been investigated. In order to find if HSV1 directly affects APP and its degradation, we have examined this protein from human neuroblastoma cells (normal and transfected with APP 695) infected with the virus, using Western blotting.

RESULTS

We have found that acute HSV1 (and also HSV2) infection rapidly reduces full length APP levels--as might be expected--yet surprisingly markedly increases levels of a novel C-terminal fragment of APP of about 55 kDa. This band was not increased in cells treated with the protein synthesis inhibitor cycloheximide

CONCLUSION

Herpes virus infection leads to rapid loss of full length APP from cells, yet also causes increased levels of a novel 55 kDa C-terminal APP fragment. These data suggest that infection can directly alter the processing of a transmembranal protein intimately linked to the aetiology of AD.

Cerebral microvascular amyloid beta protein deposition induces vascular degeneration and neuroinflammation in transgenic mice expressing human vasculotropic mutant amyloid beta precursor protein

Cerebral vascular amyloid beta-protein (Abeta) deposition, also known as cerebral amyloid angiopathy, is a common pathological feature of Alzheimer's disease. Additionally, several familial forms of cerebral amyloid angiopathy exist including the Dutch (E22Q) and Iowa (D23N) mutations of Abeta. Increasing evidence has associated cerebral microvascular amyloid deposition with neuroinflammation and dementia in these disorders. We recently established a transgenic mouse model (Tg-SwDI) that expresses human vasculotropic Dutch/Iowa mutant amyloid beta-protein precursor in brain. Tg-SwDI mice were shown to develop early-onset deposition of Abeta exhibiting high association with cerebral microvessels. Here we present quantitative temporal analysis showing robust and progressive accumulation of cerebral microvascular fibrillar Abeta accompanied by decreased cerebral vascular densities, the presence of apoptotic cerebral vascular cells, and cerebral vascular cell loss in Tg-SwDI mice. Abundant neuroinflammatory reactive astrocytes and activated microglia strongly associated with the cerebral microvascular fibrillar Abeta deposits. In addition, Tg-SwDI mouse brain exhibited elevated levels of the inflammatory cytokines interleukin-1beta and -6. Together, these studies identify the Tg-SwDI mouse as a unique model to investigate selective accumulation of cerebral microvascular amyloid and the associated neuroinflammation.

Fas ligand-mediated apoptosis in degenerative disorders of the brain

While defective apoptosis predisposes to neoplasia, inappropriate apoptosis in the brain leads to permanent neurological deficits. Disregulated apoptosis has been implicated in several neurodegenerative disorders including Alzheimer's, Parkinson's, and Huntington's diseases. Recent reports have suggested that the key apoptosis regulator Fas ligand (FasL) may participate in both neuronal and immune cell apoptosis in Alzheimer's disease. FasL has also been implicated as a negative regulator for the inflammatory component of the demyelinating brain disorder multiple sclerosis (MS). Here we discuss how FasL-mediated apoptosis may balance immune cell access to the brain with Alzheimer's disease and MS representing extremes of too little and too much immune access, respectively.

Search strategies used by APP transgenic mice during navigation in the Morris water maze

TgCRND8 mice represent a transgenic mouse model of Alzheimer's disease, with onset of cognitive impairment and increasing amyloid-beta plaques in their brains at 12 weeks of age. In this study, the spatial memory in 25- to 30-week-old TgCRND8 mice was analyzed in two reference and one working memory Morris water maze (MWM) tests. In reference memory tests, the mice were trained to escape to a hidden platform, which in one version of the test was marked by a visual cue. In the working memory test, the hidden platform was moved daily to different locations. The TgCRND8 mice were impaired in reference memory when trained in a hidden platform test. However, the mice developed spatial memory comparable to non-Tg littermates in a cued reference memory test. The mice showed also an impairment in spatial working memory. Analysis of search paths revealed that in contrast to non-Tg littermates, TgCRND8 mice did not use spatial strategies during their navigation. Instead, they learned to locate an escape platform using a nonspatial, chaining strategy. The study showed that (1) the impairment in the reference memory of TgCRND8 mice was reduced when a hidden platform was cued, and that (2) both working and reference memory systems of TgCRND8 mice, but not (3) the plasticity of choice between search strategies, are compromised by the transgene-induced pathology.

Dimeric amyloid beta protein rapidly accumulates in lipid rafts followed by apolipoprotein E and phosphorylated tau accumulation in the Tg2576 mouse model of Alzheimer's disease

To investigate lipid rafts as a site where amyloid beta protein (Abeta) oligomers might accumulate and cause toxicity in Alzheimer's disease (AD), we analyzed Abeta in the Tg2576 transgenic mouse model of AD. Abeta was highly concentrated in lipid rafts, which comprise a small fraction of brain volume but contain 27% of brain Abeta42 and 24% of Abeta40 in young mice. In the Tg2576 model, memory impairment begins at 6 months before amyloid plaques are visible. Here we show that Abeta dimers appear in lipid rafts at 6 months and that raft Abeta, which is primarily dimeric, rapidly accumulates reaching levels >500x those in young mice by 24-28 months. A similar large accumulation of dimeric Abeta was observed in lipid rafts from AD brain. In contrast to extracellular amyloid fibrils, which are SDS-insoluble, virtually all Abeta in lipid rafts is SDS soluble. Coupled with recent studies showing that synthetic and naturally occurring Abeta oligomers can inhibit hippocampal long-term potentiation, the in vivo age-dependent accumulation of SDS-soluble Abeta dimers in lipid rafts at the time when memory impairment begins in Tg2576 mice provides strong evidence linking Abeta oligomers to memory impairment. After dimeric Abeta began to accumulate in lipid rafts of the Tg2576 brain, apolipoprotein E (ApoE) and then phosphorylated tau accumulated. A similar increase in ApoE and a large increase in phosphorylated tau was observed in lipid rafts from AD brain. These findings suggest that lipid rafts may be an important site for interaction between dimeric Abeta, ApoE, and tau.

Fas ligand-mediated apoptosis in degenerative disorders of the brain

While defective apoptosis predisposes to neoplasia, inappropriate apoptosis in the brain leads to permanent neurological deficits. Disregulated apoptosis has been implicated in several neurodegenerative disorders including Alzheimer's, Parkinson's, and Huntington's diseases. Recent reports have suggested that the key apoptosis regulator Fas ligand (FasL) may participate in both neuronal and immune cell apoptosis in Alzheimer's disease. FasL has also been implicated as a negative regulator for the inflammatory component of the demyelinating brain disorder multiple sclerosis (MS). Here, we discuss how FasL-mediated apoptosis may balance immune cell access to the brain with Alzheimer's disease and MS representing extremes of too little and too much immune access, respectively.

Beta-secretase activity increases with aging in human, monkey, and mouse brain

Amyloid beta protein (A beta) accumulates in the brains of aging humans, amyloid precursor protein (APP) transgenic mouse lines, and rhesus monkeys. We tested the hypothesis that aging was associated with increased activity of the beta-site amyloid precursor protein cleaving enzyme (beta-secretase, BACE) in brain. We evaluated BACE activity, BACE protein, and formic acid-extractable A beta levels in cohorts of young (4 months old) and old (14 to 18 months old) nontransgenic mice (n = 16) and Tg2576 APP transgenic mice (n = 17), young (4.4 to 12.7 years old) and old (20.9 to 30.4 years old) rhesus monkeys (n = 17), and a wide age range (18 to 92 years old) of nondemented human brains (n = 25). Aging was associated with increased brain A beta levels in each cohort. Furthermore BACE activity increased significantly with age in mouse, monkey, and human brains, independent of brain region. BACE protein levels, however, were unchanged with age. BACE activity correlated with formic acid-extractable A beta levels in transgenic mouse, nontransgenic mouse, and human cortex, but not in monkey brain. These data suggest that an age-related increase of BACE activity contributes to the increased production and accumulation of brain A beta, and potentially predisposes to Alzheimer's disease in humans.