Amyloid beta protein gene: cDNA, mRNA distribution, and genetic linkage near the Alzheimer locus

New genes and new insights from old genes: update on Alzheimer disease

PURPOSE OF REVIEW

This article discusses the current status of knowledge regarding the genetic basis of Alzheimer disease (AD) with a focus on clinically relevant aspects.

RECENT FINDINGS

The genetic architecture of AD is complex, as it includes multiple susceptibility genes and likely nongenetic factors. Rare but highly penetrant autosomal dominant mutations explain a small minority of the cases but have allowed tremendous advances in understanding disease pathogenesis. The identification of a strong genetic risk factor, APOE, reshaped the field and introduced the notion of genetic risk for AD. More recently, large-scale genome-wide association studies are adding to the picture a number of common variants with very small effect sizes. Large-scale resequencing studies are expected to identify additional risk factors, including rare susceptibility variants and structural variation.

SUMMARY

Genetic assessment is currently of limited utility in clinical practice because of the low frequency (Mendelian mutations) or small effect size (common risk factors) of the currently known susceptibility genes. However, genetic studies are identifying with confidence a number of novel risk genes, and this will further our understanding of disease biology and possibly the identification of therapeutic targets.

Absence of Nitric Oxide Synthase 3 Increases Amyloid β-Protein Pathology in Tg-5xFAD Mice

AIM

The abnormal accumulation, assembly and deposition of the amyloid β-protein (Aβ) are prominent pathological features of patients with Alzheimer's disease (AD) and related disorders. A number of factors in the brain can influence Aβ accumulation and associated pathologies. The aim of the present study was to determine the consequences of deleting nitric oxide synthase (NOS) 3, the endothelial form of NOS, in Tg-5xFAD mice, a model of parenchymal AD-like amyloid pathology.

METHODS

Tg-5xFAD mice were bred with NOS3^-/- mice. Cohorts of Tg-5xFAD mice and bigenic Tg-5xFAD/NOS3^-/- mice were aged to six months followed by collection of the blood and brain tissues from the mice for biochemical and pathological analyses.

RESULTS

ELISA analyses show that the absence of NOS3 results in elevated levels of cerebral and plasma Aβ peptides in Tg-5xFAD mice. Immunohistochemical analyses show that the absence of NOS3 increased the amount of parenchymal Aβ deposition and fibrillar amyloid accumulation in Tg-5xFAD mice. The elevated levels of Aβ were not due to changes in the expression levels of transgene encoded human amyloid precursor protein (APP), endogenous β-secretase, or increased proteolytic processing of APP.

CONCLUSIONS

The results from this study suggest that the loss of NOS3 activity enhances Aβ pathology in Tg-5xFAD mice. These findings are similar to previous studies of NOS2 deletion suggesting that reduced NOS activity and NO levels enhance amyloid-associated pathologies in human APP transgenic mice.

Apolipoprotein A-I: insights from redox proteomics for its role in neurodegeneration

Proteomics has a wide range of applications, including determination of differences in the proteome in terms of expression and post-translational protein modifications. Redox proteomics allows the identification of specific targets of protein oxidation in a biological sample. Using proteomic techniques, apolipoprotein A-I (ApoA-I) has been found at decreased levels in subjects with a variety of neurodegenerative disorders including in the serum and cerebrospinal fluid (CSF) of Alzheimer disease (AD), Parkinson disease (PD), and Down syndrome (DS) with gout subjects. ApoA-I plays roles in cholesterol transport and regulation of inflammation. Redox proteomics further showed ApoA-I to be highly oxidatively modified and particularly susceptible to modification by 4-hydroxy-2-trans-nonenal (HNE), a lipid peroxidation product. In the current review, we discuss the consequences of oxidation of ApoA-I in terms of neurodegeneration. ROS-associated chemotherapy related ApoA-I oxidation leads to elevation of peripheral levels of tumor necrosis factor-α (TNF-α) that can cross the blood-brain barrier (BBB) causing a signaling cascade that can contribute to neuronal death, likely a contributor to what patients refer to as "chemobrain." Current evidence suggests ApoA-I to be a promising diagnostic marker as well as a potential target for therapeutic strategies in these neurodegenerative disorders.

The genetics of Alzheimer's disease

Alzheimer's disease is a progressive, neurodegenerative disease that represents a growing global health crisis. Two major forms of the disease exist: early onset (familial) and late onset (sporadic). Early onset Alzheimer's is rare, accounting for less than 5% of disease burden. It is inherited in Mendelian dominant fashion and is caused by mutations in three genes (APP, PSEN1, and PSEN2). Late onset Alzheimer's is common among individuals over 65 years of age. Heritability of this form of the disease is high (79%), but the etiology is driven by a combination of genetic and environmental factors. A large number of genes have been implicated in the development of late onset Alzheimer's. Examples that have been confirmed by multiple studies include ABCA7, APOE, BIN1, CD2AP, CD33, CLU, CR1, EPHA1, MS4A4A/MS4A4E/MS4A6E, PICALM, and SORL1. Despite tremendous progress over the past three decades, roughly half of the heritability for the late onset of the disease remains unidentified. Finding the remaining genetic factors that contribute to the development of late onset Alzheimer's disease holds the potential to provide novel targets for treatment and prevention, leading to the development of effective strategies to combat this devastating disease.

Roles of the amyloid precursor protein family in the peripheral nervous system

Compelling evidence from in vivo model systems within the past decade shows that the APP family of proteins is important for synaptic development and function in the central and peripheral nervous systems. The synaptic role promises to be complex and multifaceted for several reasons. The three family members have overlapping and redundant functions in mammals. They have both adhesive and signaling properties and may, in principle, act as both ligands and receptors. Moreover, they bind a multitude of synapse-specific proteins, and we predict that additional interacting protein partners will be discovered. Transgenic mice with modified or abolished expression of APP and APLPs have synaptic defects that are readily apparent. Studies of the neuromuscular junction (NMJ) in these transgenic mice have revealed molecular and functional deficits in neurotransmitter release, in organization of the postsynaptic receptors, and in coordinated intercellular development. The results summarized here from invertebrate and vertebrate systems confirm that the NMJ with its accessibility, large size, and homogeneity provides a model synapse for identifying and analyzing molecular pathways of APP actions.

The genetics of Alzheimer disease

Family history is the second strongest risk factor for Alzheimer disease (AD) following advanced age. Twin and family studies indicate that genetic factors are estimated to play a role in at least 80% of AD cases. The inheritance of AD exhibits a dichotomous pattern. On one hand, rare mutations in APP, PSEN1, and PSEN2 virtually guarantee early-onset (<60 years) familial AD, which represents ∼5% of AD. On the other hand, common gene polymorphisms, such as the ε4 and ε2 variants of the APOE gene, can influence susceptibility for ∼50% of the common late-onset AD. These four genes account for 30%-50% of the inheritability of AD. Genome-wide association studies have recently led to the identification of 11 additional AD candidate genes. This paper reviews the past, present, and future attempts to elucidate the complex and heterogeneous genetic underpinnings of AD.

Synaptic plasticity defect following visual deprivation in Alzheimer's disease model transgenic mice

Amyloid-β (Aβ)-induced changes in synaptic function in experimental models of Alzheimer's disease (AD) suggest that Aβ generation and accumulation may affect fundamental mechanisms of synaptic plasticity. To test this hypothesis, we examined the effect of APP overexpression on a well characterized, in vivo, developmental model of systems-level plasticity, ocular dominance plasticity. Following monocular visual deprivation during the critical period, mice that express mutant alleles of amyloid precursor protein (APPswe) and Presenilin1 (PS1dE9), as well as mice that express APPswe alone, lack ocular dominance plasticity in visual cortex. Defects in the spatial extent and magnitude of the plastic response are evident using two complementary approaches, Arc induction and optical imaging of intrinsic signals in awake mice. This defect in a classic paradigm of systems level synaptic plasticity shows that Aβ overexpression, even early in postnatal life, can perturb plasticity in cerebral cortex, and supports the idea that decreased synaptic plasticity due to elevated Aβ exposure contributes to cognitive impairment in AD.

The genetics of Alzheimer's disease

Genetic factors play a major role in determining a person's risk to develop Alzheimer's disease (AD). Rare mutations transmitted in a Mendelian fashion within affected families, for example, APP, PSEN1, and PSEN2, cause AD. In the absence of mutations in these genes, disease risk is largely determined by common polymorphisms that, in concert with each other and nongenetic risk factors, modestly impact risk for AD (e.g., the ε4-allele in APOE). Recent genome-wide screening approaches have revealed several additional AD susceptibility loci and more are likely to be discovered over the coming years. In this chapter, we review the current state of AD genetics research with a particular focus on loci that now can be considered established disease genes. In addition to reviewing the potential pathogenic relevance of these genes, we provide an outlook into the future of AD genetics research based on recent advances in high-throughput sequencing technologies.

Lead exposure in pheochromocytoma cells induces persistent changes in amyloid precursor protein gene methylation patterns

It has been suggested that lead (Pb) exposure in early life may increase amyloid precursor protein (APP) expression and promote the pathogenesis of Alzheimer's disease in old age. The current study examined whether the DNA methylation patterns of APP gene in rat pheochromocytoma (PC12) cells changed after Pb acetate exposure. Undifferentiated PC12 cells were exposed to three doses of Pb acetate (50, 250, and 500 nM) and one control for 2 days or 1 week. The methylation patterns of APP promoter and global DNA methylation were analyzed. The DNA methyltransferase 1 (DNMT1) expression and the level of amyloid β peptide (Aβ) were also investigated. The results showed that the exposure of the three concentrations of Pb acetate could make the APP promoter hypomethylated. The global DNA methylation level and the expression of DNMT1 were changed in the 500 nM group after 2 days exposure and in the 250 and 500 nM group after 7 days exposure. Thus, Pb may exert neurotoxic effects through mechanisms that alter the global and promoter methylation patterns of APP gene. © 2010 Wiley Periodicals, Inc. Environ Toxicol, 2012.

Amyloid and Alzheimer's disease: inside and out

Alzheimer's disease (AD) is poised to become the most serious healthcare issue of our generation. The leading theory of AD pathophysiology is the Amyloid Cascade Hypothesis, and clinical trials are now proceeding based on this hypothesis. Here, we review the original evidence for the Amyloid Hypothesis, which was originally focused on the extracellular deposition of beta amyloid peptides (Aβ) in large fibrillar aggregates, as well as how this theory has been extended in recent years to focus on highly toxic small soluble amyloid oligomers. We will also examine emerging evidence that Aβ may actually begin to accumulate intracellularly in lysosomes, and the role for intracellular Aβ and lysosomal dysfunction may play in AD pathophysiology. Finally, we will review the clinical implications of these findings.

Open-closed motion of Mint2 regulates APP metabolism

The amyloid-β protein precursor (APP) plays a crucial role in the pathogenesis of Alzheimer's disease (AD). Knock-out and transgenic mouse studies of the adaptor protein Mint2 have revealed that it is a major player in regulating APP metabolism physiologically through the binding of its phosphotyrosine-binding (PTB) domain to the intracellular domain of APP. However, the molecular mechanism of APP dynamically binding to Mint2 remains elusive. Here, we report the structures of APP peptide-free and APP peptide-bound C-terminal Mint2 mutants at resolutions of 2.7 and 3.3 Å, respectively. Our structures reveal that APP peptide-free Mint2 exists in a closed state in which the ARM domain blocks the peptide-binding groove of the PTB domain. In sharp contrast, APP peptide-bound Mint2 exists in an open state in which the ARM domain drastically swings away from the bound peptide. Mutants that control the open-closed motion of Mint2 dynamically regulated APP metabolism both in vitro and in vivo. Our results uncover a novel open-closed mechanism of the PTB domain dynamically binding to its peptide substrate. Moreover, such a conformational switch may represent a general regulation mode of APP family members by Mint proteins, providing useful information for the treatment of AD.

The Role of Insulin and Insulin-Like Growth Factor-1/FoxO-Mediated Transcription for the Pathogenesis of Obesity-Associated Dementia

Epidemiological studies suggest that being obese in midlife is a risk factor for cognitive decline and dementia in later life. Hyperinsulinemia is one of the most frequent endocrine features in overweight people which results in insulin desensitization. Thus, chronically high insulin levels have been identified as risk factor for dementia. Accordingly, chronically high insulin levels might be harmful for brain function. Furthermore, insulin and IGF-1-induced signaling is reduced in the brains of patients suffering from Alzheimer's disease (AD). Interestingly, studies in rodents suggest that reduced insulin receptor (IR) and insulin-like growth factor-1 receptor (IGF-1R) signaling decrease AD pathology, that is, β-amyloid toxicity. Data obtained in C. elegans indicate that the beneficial effect mediated via reduced IR/IGF-1R signaling might partially be induced via the forkhead-box O transcription factors (FoxO). In the mammalian brain, there are FoxO1, FoxO3a, and FoxO6 expressed. Surprisingly, high-fat diet specifically reduces the expression of FoxO3a and FoxO6 suggesting that IR/IGF-1 → FoxO-mediated transcription is involved in the pathogenesis of obesity-associated cognitive impairment. Therefore, the function of FoxO1 and FoxO3a has been investigated in animal models of Alzheimer's disease in detail. The current paper focuses on the role of IR/IGF-1 signaling and IR/IGF-1 → FoxO-mediated transcription for the pathogenesis of obesity-associated dementia.

Alzheimer's disease: redox dysregulation as a common denominator for diverse pathogenic mechanisms

Alzheimer's disease (AD) is the most common cause of dementia and a progressive neurodegeneration that appears to result from multiple pathogenic mechanisms (including protein misfolding/aggregation, involved in both amyloid β-dependent senile plaques and tau-dependent neurofibrillary tangles), metabolic and mitochondrial dysfunction, excitoxicity, calcium handling impairment, glial cell dysfunction, neuroinflammation, and oxidative stress. Oxidative stress, which could be secondary to several of the other pathophysiological mechanisms, appears to be a major determinant of the pathogenesis and progression of AD. The identification of oxidized proteins common for mild cognitive impairment and AD suggests that key oxidation pathways are triggered early and are involved in the initial progression of the neurodegenerative process. Abundant data support that oxidative stress, also considered as a main factor for aging, the major risk factor for AD, can be a common key element capable of articulating the divergent nature of the proposed pathogenic factors. Pathogenic mechanisms influence each other at different levels. Evidence suggests that it will be difficult to define a single-target therapy resulting in the arrest of progression or the improvement of AD deterioration. Since oxidative stress is present from early stages of disease, it appears as one of the main targets to be included in a clinical trial. Exploring the articulation of AD pathogenic mechanisms by oxidative stress will provide clues for better understanding the pathogenesis and progression of this dementing disorder and for the development of effective therapies to treat this disease.

Trisomy for synaptojanin1 in Down syndrome is functionally linked to the enlargement of early endosomes

Enlarged early endosomes have been observed in neurons and fibroblasts in Down syndrome (DS). These endosome abnormalities have been implicated in the early development of Alzheimer's disease (AD) pathology in these subjects. Here, we show the presence of enlarged endosomes in blood mononuclear cells and lymphoblastoid cell lines (LCLs) from individuals with DS using immunofluorescence and confocal microscopy. Genotype-phenotype correlations in LCLs carrying partial trisomies 21 revealed that triplication of a 2.56 Mb locus in 21q22.11 is associated with the endosomal abnormalities. This locus contains the gene encoding the phosphoinositide phosphatase synaptojanin 1 (SYNJ1), a key regulator of the signalling phospholipid phosphatidylinositol-4,5-biphosphate that has been shown to regulate clathrin-mediated endocytosis. We found that SYNJ1 transcripts are increased in LCLs from individuals with DS and that overexpression of SYNJ1 in a neuroblastoma cell line as well as in transgenic mice leads to enlarged endosomes. Moreover, the proportion of enlarged endosomes in fibroblasts from an individual with DS was reduced after silencing SYNJ1 expression with RNA interference. In LCLs carrying amyloid precursor protein (APP) microduplications causing autosomal dominant early-onset AD, enlarged endosomes were absent, suggesting that APP overexpression alone is not involved in the modification of early endosomes in this cell type. These findings provide new insights into the contribution of SYNJ1 overexpression to the endosomal changes observed in DS and suggest an attractive new target for rescuing endocytic dysfunction and lipid metabolism in DS and in AD.

Molecular consequences of amyloid precursor protein and presenilin mutations causing autosomal-dominant Alzheimer's disease

Mutations in both the amyloid precursor protein (APP) and the presenilin (PSEN) genes cause familial Alzheimer's disease (FAD) with autosomal dominant inheritance and early onset of disease. The clinical course and neuropathology of FAD and sporadic Alzheimer's disease are highly similar, and patients with FAD constitute a unique population in which to conduct treatment and, in particular, prevention trials with novel pharmaceutical entities. It is critical, therefore, to exactly defi ne the molecular consequences of APP and PSEN FAD mutations. Both APP and PSEN mutations drive amyloidosis in FAD patients through changes in the brain metabolism of amyloid-β (Aβ) peptides that promote the formation of pathogenic aggregates. APP mutations do not seem to impair the physiological functions of APP. In contrast, it has been proposed that PSEN mutations compromise γ-secretase-dependent and -independent functions of PSEN. However, PSEN mutations have mostly been studied in model systems that do not accurately refl ect the genetic background in FAD patients. In this review, we discuss the reported cellular phenotypes of APP and PSEN mutations, the current understanding of their molecular mechanisms, the need to generate faithful models of PSEN mutations, and the potential bias of APP and PSEN mutations on therapeutic strategies that target Aβ.

Aβ toxicity in Alzheimer's disease

Alzheimer's Disease (AD), the most common age-related neurodegenerative disorder, is characterized by progressive cognitive decline, synaptic loss, the formation of extracellular β-amyloid plaques and intracellular neurofibrillary tangles, and neuronal cell death. Despite the massive neuronal loss in the 'late stage' of disease, dendritic spine loss represents the best pathological correlate to the cognitive impairment in AD patients. The 'amyloid hypothesis' of AD recognizes the Aβ peptide as the principal player in the pathological process. Many lines of evidence point out to the neurotoxicity of Aβ, highlighting the correlation between soluble Aβ oligomer accumulation, rather than insoluble Aβ fibrils and disease progression. Pathological increase of Aβ in AD brains, resulting from an imbalance between its production, aggregation and clearance, might target mitochondrial function promoting a progressive synaptic impairment. The knowledge of the exact mechanisms by which Aβ peptide impairs neuronal function will help us to design new pharmacological tools for preventing AD neurodegeneration.

Tol2 gene trap integrations in the zebrafish amyloid precursor protein genes appa and aplp2 reveal accumulation of secreted APP at the embryonic veins

BACKGROUND

The single spanning transmembrane amyloid precursor protein (APP) and its proteolytic product, amyloid-beta (Ab) peptide, have been intensely studied due to their role in the pathogenesis of Alzheimer's disease. However, the biological role of the secreted ectodomain of APP, which is also generated by proteolytic cleavage, is less well understood. Here, we report Tol2 red fluorescent protein (RFP) transposon gene trap integrations in the zebrafish amyloid precursor protein a (appa) and amyloid precursor-like protein 2 (aplp2) genes. The transposon integrations are predicted to disrupt the appa and aplp2 genes to primarily produce secreted ectodomains of the corresponding proteins that are fused to RFP.

RESULTS

Our results indicate the Appa-RFP and Aplp2 fusion proteins are likely secreted from the central nervous system and accumulate in the embryonic veins independent of blood flow.

CONCLUSIONS

The zebrafish appa and aplp2 transposon insertion alleles will be useful for investigating the biological role of the secreted form of APP.

Rab GTPases, membrane trafficking and diseases

The Rab family of GTPases contains over 60 genes in the human genome and contributes to regulation of intracellular membrane trafficking along endocytic and exocytic pathways as well as specialized pathways in specific cell types. It has become increasingly clear that disruption of the intracellular membrane trafficking system at different stages can cause various diseases. In the past decade, altered expression levels and mutations of Rab GTPases have been associated with such diseases as cancer, Alzheimer's disease, and various genetic disorders. This review discusses the specific Rab GTPases and their involvement in the diseases.

Synaptic dysfunction in Alzheimer's disease

Generation of amyloid peptide (Aβ) is at the beginning of a cascade that leads to Alzheimer's disease (AD). Amyloid precursor protein (APP), as well as β- and γ-secretases, is the principal player involved in Aβ production, while α-secretase cleavage on APP prevents Aβ deposition. Recent studies suggested that soluble assembly states of Aβ peptides can cause cognitive problems by disrupting synaptic function in the absence of significant neurodegeneration. Therefore, current research investigates the relative importance of these various soluble Aβ assemblies in causing synaptic dysfunction and cognitive deficits. Several Aβ oligomers targets and cellular mechanisms responsible of Aβ-induced synaptic failure have been identified. The first and most important mechanism impugns a toxic gain of function for Aβ which results due to self-association and attainment of new structures capable of novel interactions that lead to impaired plasticity. Other scenarios predicate that Aβ has a normal physiological role. On the one hand, insufficient Aβ could lead to a loss of normal function, whereas excess Aβ may precipitate dysfunction. How this occurs and which the main target/s is/are for the synaptic action of Aβ remains to be fully understood and would certainly represent one of the main challenges to future AD research.

Behavioral genetics of neurodegenerative disorders

Alzheimer's disease (AD) is the most common cause of dementia in the elderly, and is typically characterized by memory loss. In addition, during the disease progression, most patients develop behavioural and psychiatric symptoms of dementia (BPSD). Frontotemporal Lobar Degeneration (FTLD) is the most frequent neurodegenerative disorder with a presenile onset. It is characterized mainly by behavioural disturbances, whereas memory is conserved. The two major neuropathologic hallmarks of AD are extracellular Amyloid beta (Ab) plaques and intracellular neurofibrillary tangles (NFTs). Conversely, in FTLD the deposition of tau has been observed in a number of cases, but in several brains there is no deposition of tau but instead a positivity for ubiquitin. In some families these diseases are inherited in an autosomal dominant fashion. Genes responsible for familial AD include the Amyloid Precursor Protein (b-APP), Presenilin 1 (PS1)and Presenilin 2 (PS2). The majority of mutations in these genes are often associated with a very early onset (40–50 years of age). Regarding FTLD, the first mutations described are located in the Microtubule Associated Protein Tau gene(MAPT). Tau is a component of microtubules, which represent the internal support structures for the transport of nutrients, vesicles, mitochondria and chromosomes within the cell. Mutations in MAPT are associated with an early onset of the disease (40–50 years), and the clinical phenotype is consistent with Frontotemporal Dementia (FTD). Recently, mutations in a second gene, named progranulin(GRN), have been identified in some families with FTLD. The pathology associated with these mutations is most frequently characterized by the immunostaining of TAR DNA Binding Protein 43 (TDP-43), which is a transcription factor. The clinical phenotype associated with GRN mutations is highly heterogeneous,including FTD, Progressive Aphasia, Corticobasal Syndrome, and AD. Age at disease onset is variable, ranging from 45 to 85 years of age. The majority of cases of AD and FTLD are however sporadic, and likely several genetic and environmental factors contribute to their development. Concerning AD, it is known that the presence of the e4 allele of the Apolipoprotein E gene is a susceptibility factor,increasing the risk of about 4 fold. A number of additional genetic factors,including cytokines, chemokines, Nitric Oxide Synthases, contribute to the susceptibility for the disease. Some of them also influence the risk to develop FTLD.Variability in serotonin transporter gene could influence the development of BPSD. In this chapter, current knowledge on molecular mechanisms at the basis of AD and FTLD, as well as the role of genetics, will be presented and discussed.

Vitamin A and Alzheimer's disease

The deposition of amyloid β-protein (Aβ) in the brain is an invariant feature of Alzheimer's disease (AD). Vitamin A, which has been traditionally considered an anti-oxidant compound, plays a role in maintaining higher function in the central nervous system. Plasma or cerebrospinal fluid concentrations of vitamin A and β-carotene have been reported to be lower in AD patients, and these vitamins have been clinically shown to slow the progression of dementia. Vitamin A (retinol, retinal and retinoic acid) and β-carotene have been shown in in vitro studies to inhibit the formation, extension and destabilizing effects of β-amyloid fibrils. Recently, the inhibition of the oligomerization of Aβ has been suggested as a possible therapeutic target for the treatment of AD. We have recently shown the inhibitory effects of vitamin A and β-carotene on the oligomerization of Aβ40 and Aβ42 in vitro. In previous in vivo studies, intraperitoneal injections of vitamin A decreased brain Aβ deposition and tau phosphorylation in transgenic mouse models of AD, attenuated neuronal degeneration, and improved spatial learning and memory. Thus, vitamin A and β-carotene could be key molecules for the prevention and therapy of AD.

Alzheimer's - Looking beyond plaques

Mounting evidence shows that inflammation plays a critical role in causing Alzheimer’s disease. Over the last few decades we have gone from a situation where inflammation was generally believed to have no role in the disease to the current picture where chronic activation of IL-1 inflammation has been shown to account for many of the hallmarks of the disease. This review is a personal account of the quest to prove that inflammation plays a critical role in causing Alzheimer’s disease.

Regulation of β-site APP-cleaving enzyme 1 gene expression and its role in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder leading to dementia. Neuritic plaques are the hallmark neuropathology in AD brains. Proteolytic processing of amyloid-β precursor protein at the β site by beta-site amyloid-β precursor protein-cleaving enzyme 1 (BACE1) is essential to generate Aβ, a central component of the neuritic plaques. BACE1 is increased in some sporadic AD brains, and dysregulation of BACE1 gene expression plays an important role in AD pathogenesis. This review will focus on the regulation of BACE1 gene expression at the transcriptional, post-transcriptional, translation initiation, translational and post-translational levels, and its role in AD pathogenesis. Further studies on BACE1 gene expression regulation will greatly contribute to our understanding of AD pathogenesis and reveal potential novel approaches for AD prevention and drug development.

Proteolytic processing of Alzheimer's β-amyloid precursor protein

β-Amyloid precursor protein (APP) is a critical factor in the pathogenesis of Alzheimer's disease (AD). APP undergoes post-translational proteolysis/processing to generate the hydrophobic β-amyloid (Aβ) peptides. Deposition of Aβ in the brain, forming oligomeric Aβ and plaques, is identified as one of the key pathological hallmarks of AD. The processing of APP to generate Aβ is executed by β- and γ-secretase and is highly regulated. Aβ toxicity can lead to synaptic dysfunction, neuronal cell death, impaired learning/memory and abnormal behaviors in AD models in vitro and in vivo. Aside from Aβ, proteolytic cleavages of APP can also give rise to the APP intracellular domain, reportedly involved in multiple types of cellular events such as gene transcription and apoptotic cell death. In addition to amyloidogenic processing, APP can also be cleaved by α-secretase to form a soluble or secreted APP ectodomain (sAPP-α) that has been shown to be mostly neuro-protective. In this review, we describe the mechanisms involved in APP metabolism and the likely functions of its various proteolytic products to give a better understanding of the patho/physiological functions of APP.

Disease-modifying treatments for Alzheimer's disease

The first drugs developed for Alzheimer's disease (AD), acetylcholinesterase inhibitors (AChEI), increase acetylcholine levels, previously demonstrated to be reduced in AD. To date, four AChEI are approved for the treatment of mild-to-moderate AD. A further therapeutic option available for moderate-to-severe AD is memantine. These treatments are symptomatic, whereas drugs under development are intended to modify the pathological steps leading to AD, thus acting on the evolution of the disease. For this reason they are have been termed 'disease-modifying' drugs. To block the progression of the disease they have to interfere with the pathogenic steps responsible for the clinical symptoms, including the deposition of extracellular amyloid beta (Aβ) plaques and of intracellular neurofibrillary tangles, inflammation, oxidative damage, iron deregulation and cholesterol metabolism. In this review, new perspectives will be discussed. In particular, several approaches will be described, including interference with Aβ deposition by anti-Aβ aggregation agents, vaccination, γ-secretase inhibitors or selective Aβ-lowering agents; interference with tau deposition by methylthioninium chloride; and reduction of inflammation and oxidative damage.

Elucidating the role of metals in Alzheimer's disease through the use of Surface-Enhanced Laser Desorption/Ionisation time-of-flight mass spectrometry

Alzheimer's disease (AD) is a highly heterogeneous and progressive dementia which is characterised by a progressive decline in cognitive functioning, selective neuronal atrophy, and loss of cortical volume in areas involved in learning and memory. However, recent research has indicated that the AD-affected brain is also besieged by increases in oxidative stress as well as perturbations to the homeostasis of biometals, such as copper and iron. These metals are known to interact with the neuropathological hallmark of AD, the β-amyloid peptide (Aβ), in a manner which increases Aβ's neurotoxic effects. This knowledge has led to the development of therapeutic measures which act to restore biometal homeostasis within the AD brain. This chapter outlines how Surface-Enhanced Laser Desorption/Ionisation Time-of-Flight Mass Spectrometry can be used to monitor Aβ levels within biological systems as well as describing the use of immobilised metal affinity capture in the observation of synthetic Aβ peptides.

Pyroglutamate amyloid-β (Aβ): a hatchet man in Alzheimer disease

Pyroglutamate-modified amyloid-β (Aβ(pE3)) peptides are gaining considerable attention as potential key participants in the pathology of Alzheimer disease (AD) due to their abundance in AD brain, high aggregation propensity, stability, and cellular toxicity. Transgenic mice that produce high levels of Aβ(pE3-42) show severe neuron loss. Recent in vitro and in vivo experiments have proven that the enzyme glutaminyl cyclase catalyzes the formation of Aβ(pE3). In this minireview, we summarize the current knowledge on Aβ(pE3), discussing its discovery, biochemical properties, molecular events determining formation, prevalence in the brains of AD patients, Alzheimer mouse models, and potential as a target for therapy and as a diagnostic marker.

Analysis of amyloid precursor protein function in Drosophila melanogaster

Amyloid precursor proteins (APPs) are evolutionary conserved from nematodes to man (Jacobsen and Iverfeldt in Cell Mol Life Sci 66:2299-2318, 2009) suggesting an important physiological function of these proteins. Human APP is a key factor in the pathogenesis of Alzheimer's Disease because its proteolytic processing results in the production of the neurotoxic Aβ-peptide, which accumulates in the amyloid plaques characteristic for this disease (Selkoe in Physiol Rev 81(2):741-766, 2001). However, the processing also leads to the production of several other fragments and the role of these products, as well as the function of the full-length protein is so far not well understood. The functional analysis of APP in vertebrates has been hampered by the fact that two close relatives, APLP1 and APLP2, exist and that knock-out mice for APP only show subtle defects. In contrast, invertebrates like Caenorhabditis elegans and Drosophila express only one APP-like protein but whereas a null mutation in the C. elegans APL-1 protein is lethal, flies lacking APPL (Amyloid Precursor Protein-like) are viable but show synaptic defects and behavioral abnormalities. Together with the analyses of flies that express APP proteins ectoptically or xenotopically, these studies show that APP proteins are involved in neuronal differentiation, neuritic outgrowth, and synapse formation. In addition, they play a role in long-term memory formation and maintaining brain integrity in adult flies.

The Role of Striatal-Enriched Protein Tyrosine Phosphatase (STEP) in Cognition

Striatal-enriched protein tyrosine phosphatase (STEP) has recently been implicated in several neuropsychiatric disorders with significant cognitive impairments, including Alzheimer's disease, schizophrenia, and fragile X syndrome. A model has emerged by which STEP normally opposes the development of synaptic strengthening and that disruption in STEP activity leads to aberrant synaptic function. We review the mechanisms by which STEP contributes to the etiology of these and other neuropsychiatric disorders. These findings suggest that disruptions in STEP activity may be a common mechanism for cognitive impairments in diverse illnesses.

APP physiological and pathophysiological functions: insights from animal models

The amyloid precursor protein (APP) has been under intensive study in recent years, mainly due to its critical role in the pathogenesis of Alzheimer's disease (AD). β-Amyloid (Aβ) peptides generated from APP proteolytic cleavage can aggregate, leading to plaque formation in human AD brains. Point mutations of APP affecting Aβ production are found to be causal for hereditary early onset familial AD. It is very likely that elucidating the physiological properties of APP will greatly facilitate the understanding of its role in AD pathogenesis. A number of APP loss- and gain-of-function models have been established in model organisms including Caenorhabditis elegans, Drosophila, zebrafish and mouse. These in vivo models provide us valuable insights into APP physiological functions. In addition, several knock-in mouse models expressing mutant APP at a physiological level are available to allow us to study AD pathogenesis without APP overexpression. This article will review the current physiological and pathophysiological animal models of APP.

Anti-amyloidogenic and anti-apoptotic role of melatonin in Alzheimer disease

Alzheimer disease (AD) is an age-related neurodegenerative disorder characterized by the presence of senile plaques, neurofibrillary tangles and neuronal loss. Amyloid-β protein (Aβ) deposition plays a critical role in the development of AD. It is now generally accepted that massive neuronal death due to apoptosis is a common characteristic in the brains of patients suffering from neurodegenerative diseases, and apoptotic cell death has been found in neurons and glial cells in AD. Melatonin is a secretory product of the pineal gland; melatonin is a potent antioxidant and free radical scavenger and may play an important role in aging and AD. Melatonin decreases during aging and patients with AD have a more profound reduction of this indoleamine. Additionally, the antioxidant properties, the anti-amyloidogenic properties and anti-apoptotic properties of melatonin in AD models have been studied. In this article, we review the anti-amyloidogenic and anti-apoptotic role of melatonin in AD

Addressing the complex etiology of Alzheimer’s disease: the role of p25/Cdk5

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder characterized by the progressive loss of forebrain neurons and the deterioration of learning and memory. Therapies for AD have primarily focused upon either the inhibition of amyloid synthesis or its deposition in the brain, but clinical testing to date has not yet found an effective amelioration of cognitive symptoms. Synaptic loss closely correlates with the degree of dementia in AD patients. However, mouse AD models that target the amyloid-β pathway generally do not exhibit a profound loss of synapses, despite extensive synaptic dysfunction. The increased generation of p25, an activator of the cyclin-dependent kinase 5 (Cdk5) has been found in both human patients and mouse models of neurodegeneration. The current work reviews our knowledge, to date, on the role of p25/Cdk5 in Alzheimer’s disease, with a focus upon the interaction of amyloid-β and p25/Cdk5 in synaptic dysfunction and neuronal loss.

Progress in Alzheimer's disease

After more than one century from Alois Alzheimer and Gaetano Perusini's first report, progress has been made in understanding the pathogenic steps of Alzheimer's disease (AD), as well as in its early diagnosis. This review discusses recent findings leading to the formulation of novel criteria for diagnosis of the disease even in a preclinical phase, by using biological markers. In addition, treatment options will be discussed, with emphasis on new disease-modifying compounds and future trial design suitable to test these drugs in an early phase of the disease.

Suppression of amyloid beta A11 antibody immunoreactivity by vitamin C: possible role of heparan sulfate oligosaccharides derived from glypican-1 by ascorbate-induced, nitric oxide (NO)-catalyzed degradation

Amyloid β (Aβ) is generated from the copper- and heparan sulfate (HS)-binding amyloid precursor protein (APP) by proteolytic processing. APP supports S-nitrosylation of the HS proteoglycan glypican-1 (Gpc-1). In the presence of ascorbate, there is NO-catalyzed release of anhydromannose (anMan)-containing oligosaccharides from Gpc-1-nitrosothiol. We investigated whether these oligosaccharides interact with Aβ during APP processing and plaque formation. anMan immunoreactivity was detected in amyloid plaques of Alzheimer (AD) and APP transgenic (Tg2576) mouse brains by immunofluorescence microscopy. APP/APP degradation products detected by antibodies to the C terminus of APP, but not Aβ oligomers detected by the anti-Aβ A11 antibody, colocalized with anMan immunoreactivity in Tg2576 fibroblasts. A 50-55-kDa anionic, sodium dodecyl sulfate-stable, anMan- and Aβ-immunoreactive species was obtained from Tg2576 fibroblasts using immunoprecipitation with anti-APP (C terminus). anMan-containing HS oligo- and disaccharide preparations modulated or suppressed A11 immunoreactivity and oligomerization of Aβ42 peptide in an in vitro assay. A11 immunoreactivity increased in Tg2576 fibroblasts when Gpc-1 autoprocessing was inhibited by 3-β[2(diethylamino)ethoxy]androst-5-en-17-one (U18666A) and decreased when Gpc-1 autoprocessing was stimulated by ascorbate. Neither overexpression of Gpc-1 in Tg2576 fibroblasts nor addition of copper ion and NO donor to hippocampal slices from 3xTg-AD mice affected A11 immunoreactivity levels. However, A11 immunoreactivity was greatly suppressed by the subsequent addition of ascorbate. We speculate that temporary interaction between the Aβ domain and small, anMan-containing oligosaccharides may preclude formation of toxic Aβ oligomers. A portion of the oligosaccharides are co-secreted with the Aβ peptides and deposited in plaques. These results support the notion that an inadequate supply of vitamin C could contribute to late onset AD in humans.

The use of mass spectrometry to study amyloid-β peptides

Amyloid-β peptide (Aβ) varies in size from 39 to 43 amino acids and arises from sequential β- and γ-secretase processing of the amyloid precursor protein. Whereas the non-pathological role for Aβ is yet to be established, there is no disputing that Aβ is now widely regarded as central to the development of Alzheimer's disease (AD). The so named "amyloid cascade hypothesis" states that disease progression is the result of an increased Aβ burden in affected areas of the brain. To elucidate the Aβ role in AD, many analytical approaches have been proposed as suitable tools to investigate not only the total Aβ load but also many other issues that are considered crucial for AD, such as: (i) the aggregation state in which Aβ is present; (ii) its interaction with other species or metals; (iii) its ability to induce oxidative stress; and (iv) its degradative pathways. This review provides an insight into the use of mass spectrometry (MS) in the field of Aβ investigation aimed to assess its role in AD. In particular, the different MS-based approaches applied in vitro and in vivo that can provide detailed information on the above-mentioned issues are reviewed. Moreover, the advantages offered by the MS methods over all the other techniques are highlighted, together with the recent developments and uses of combined analytical approaches to detect and characterize Aβ.

Biology and pathophysiology of the amyloid precursor protein

The amyloid precursor protein (APP) plays a central role in the pathophysiology of Alzheimer's disease in large part due to the sequential proteolytic cleavages that result in the generation of β-amyloid peptides (Aβ). Not surprisingly, the biological properties of APP have also been the subject of great interest and intense investigations. Since our 2006 review, the body of literature on APP continues to expand, thereby offering further insights into the biochemical, cellular and functional properties of this interesting molecule. Sophisticated mouse models have been created to allow in vivo examination of cell type-specific functions of APP together with the many functional domains. This review provides an overview and update on our current understanding of the pathobiology of APP.

Biomarker-based dissection of neurodegenerative diseases

The diagnosis of neurodegenerative diseases within neurology and psychiatry are hampered by the difficulty in getting biopsies and thereby validating the diagnosis by pathological findings. Biomarkers for other types of disease have been readily adopted into the clinical practice where for instance troponins are standard tests when myocardial infarction is suspected. However, the use of biomarkers for neurodegeneration has not been fully incorporated into the clinical routine. With the development of cerebrospinal fluid (CSF) biomarkers that reflect pathological events within the central nervous system (CNS), important clinical diagnostic tools are becoming available. This review summarizes the most promising biomarker candidates that may be used to monitor different types of neurodegeneration and protein inclusions, as well as different types of metabolic changes, in living patients in relation to the clinical phenotype and disease progression over time. Our aim is to provide the reader with an updated lexicon on currently available biomarker candidates, how far they have come in development and how well they reflect pathogenic processes in different neurodegenerative diseases. Biomarkers for specific pathogenetic processes would also be valuable tools both to study disease pathogenesis directly in patients and to identify and monitor the effect of novel treatment strategies.

Specific Silencing of L392V PSEN1 Mutant Allele by RNA Interference

RNA interference (RNAi) technology provides a powerful molecular tool to reduce an expression of selected genes in eukaryotic cells. Short interfering RNAs (siRNAs) are the effector molecules that trigger RNAi. Here, we describe siRNAs that discriminate between the wild type and mutant (1174 C→G) alleles of human Presenilin1 gene (PSEN1). This mutation, resulting in L392V PSEN1 variant, contributes to early onset familial Alzheimer's disease. Using the dual fluorescence assay, flow cytometry and fluorescent microscopy we identified positions 8th–11th, within the central part of the antisense strand, as the most sensitive to mismatches. 2-Thiouridine chemical modification introduced at the 3′-end of the antisense strand improved the allele discrimination, but wobble base pairing adjacent to the mutation site abolished the siRNA activity. Our data indicate that siRNAs can be designed to discriminate between the wild type and mutant alleles of genes that differ by just a single nucleotide.

Alzheimer's disease: the challenge of the second century

Alzheimer's disease (AD) was first described a little more than 100 years ago. It is the most common cause of dementia with an estimated prevalence of 30 million people worldwide, a number that is expected to quadruple in 40 years. There currently is no effective treatment that delays the onset or slows the progression of AD. However, major scientific advances in the areas of genetics, biochemistry, cell biology, and neuroscience over the past 25 years have changed the way we think about AD. This review discusses some of the challenges to translating these basic molecular and cellular discoveries into clinical therapies. Current information suggests that if the disease is detected before the onset of overt symptoms, it is possible that treatments based on knowledge of underlying pathogenesis can and will be effective.

Power and pitfalls of the genome-wide association study approach to identify genes for Alzheimer's disease

Until recently, the search for genes contributing to Alzheimer's disease (AD) had been slow and disappointing, with the notable exception of the APOE ε4 allele, which increases risk and reduces the age at onset of AD in a dose-dependent fashion. Findings from genome-wide association studies (GWAS) made up of fewer than several thousand cases and controls each have not been replicated. Efforts of several consortia--each assembling much larger datasets with sufficient power to detect loci conferring small changes in AD risk--have resulted in robust associations with many novel genes involved in multiple biological pathways. Complex data mining strategies are being used to identify additional members of these pathways and gene-gene interactions contributing to AD risk. Guided by GWAS results, next-generation sequencing and functional studies are under way with the hope of helping us better understand AD pathology and providing new drug targets.

Mechanisms mediating brain and cognitive reserve: experience-dependent neuroprotection and functional compensation in animal models of neurodegenerative diseases

'Brain and cognitive reserve' (BCR) refers here to the accumulated neuroprotective reserve and capacity for functional compensation induced by the chronic enhancement of mental and physical activity. BCR is thought to protect against, and compensate for, a range of different neurodegenerative diseases, as well as other neurological and psychiatric disorders. In this review we will discuss BCR, and its potential mechanisms, in neurodegenerative disorders, with a focus on Huntington's disease (HD) and Alzheimer's disease (AD). Epidemiological studies of AD, and other forms of dementia, provided early evidence for BCR. The first evidence for the beneficial effects of enhanced mental and physical activity, and associated mechanistic insights, in an animal model of neurodegenerative disease was provided by experiments using HD transgenic mice. More recently, experiments on animal models of HD, AD and various other brain disorders have suggested potential molecular and cellular mechanisms underpinning BCR. We propose that sophisticated insight into the processes underlying BCR, and identification of key molecules mediating these beneficial effects, will pave the way for therapeutic advances targeting these currently incurable neurodegenerative diseases.

Targets for AD treatment: conflicting messages from γ-secretase inhibitors

Current evidence suggests that Alzheimer's disease (AD) is a multi-factorial disease that starts with accumulation of multiple proteins. We have previously proposed that inhibition of γ-secretase may impair membrane recycling causing neurodegeneration starting at synapses (Sambamurti K., Suram A., Venugopal C., Prakasam A., Zhou Y., Lahiri D. K. and Greig N. H. A partial failure of membrane protein turnover may cause Alzheimer's disease: a new hypothesis. Curr. Alzheimer Res., 3, 2006, 81). We also proposed familal AD mutations increase Aβ42 by inhibiting γ-secretase. Herein, we discuss the failure of Eli Lilly's γ-secretase inhibitor, semagacestat, in clinical trials in the light of our hypothesis, which extends the problem beyond toxicity of Aβ aggregates. We elaborate that γ-secretase inhibitors lead to accumulation of amyloid precursor protein C-terminal fragments that can later be processed by γ-secretase to yields bursts of Aβ to facilitate aggregation. Although we do not exclude a role for toxic Aβ aggregates, inhibition of γ-secretase can affect numerous substrates other than amyloid precursor protein to affect multiple pathways and the combined accumulation of multiple peptides in the membrane may impair its function and turnover. Taken together, protein processing and turnover pathways play an important role in maintaining cellular homeostasis and unless we clearly see consistent disease-related increase in their levels or activity, we need to focus on preserving their function rather than inhibiting them for treatment of AD and similar diseases.