Understanding the Amyloid Hypothesis in Alzheimer’s Disease

Abstract

The amyloid hypothesis (AH) is still the most accepted model to explain the pathogenesis of inherited Alzheimer’s disease (IAD). However, despite the neuropathological overlapping with the non-inherited form (NIAD), AH waver in explaining NIAD. Thus, 30 years after its first statement several questions are still open, mainly regarding the role of amyloid plaques (AP) and apolipoprotein E (APOE). Accordingly, a pathogenetic model including the role of AP and APOE unifying IAD and NIAD pathogenesis is still missing. In the present understanding of the AH, we suggested that amyloid-β (Aβ) peptides production and AP formation is a physiological aging process resulting from a systemic age-related decrease in the efficiency of the proteins catabolism/clearance machinery. In this pathogenetic model Aβ peptides act as neurotoxic molecules, but only above a critical concentration [Aβ]_c. A threshold mechanism triggers IAD/NIAD onset only when [Aβ]≥[Aβ]_c. In this process, APOE modifies [Aβ]_c threshold in an isoform-specific way. Consequently, all factors influencing Aβ anabolism, such as amyloid beta precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) gene mutations, and/or Aβ catabolism/clearance could contribute to exceed the threshold [Aβ]_c, being characteristic of each individual. In this model, AP formation does not depend on [Aβ]_c. The present interpretation of the AH, unifying the pathogenetic theories for IAD and NIAD, will explain why AP and APOE4 may be observed in healthy aging and why they are not the cause of AD. It is clear that further studies are needed to confirm our pathogenetic model. Nevertheless, our suggestion may be useful to better understand the pathogenesis of AD.

Keywords

Alpha-secretase Alzheimer’s disease amyloid hypothesis amyloid plaque amyloid-alpha peptide amyloid-beta peptide amyloid precursor protein apolipoprotein E beta-secretase inherited Alzheimer’s disease non-inherited Alzheimer’s disease

INTRODUCTION

The non-inherited form of Alzheimer’s disease (NIAD) accounts for 50–70% of the 47 million people with dementia worldwide [1, 2] and for over 99% of all AD cases [2]. With a global aging estimation of 30 million people, as aging is a primary risk factor for NIAD, the ratio of NIAD to all AD cases continues to increase. According to current statistics, the global prevalence of NIAD is expected to double in a decade, making NIAD a critical economic concern for society [2, 3]. However, despite a number of studies investigating NIAD pathogenesis, no cause has been identified and accepted so far. Thus, as underlined by the therapeutic failure of over 400 candidate drugs in the last two decades [4, 5], effective treatments are also missing.

For over 100 years, scientists have recognized a strong correlation between the clinical signs of dementia and the abnormal deposition of β-sheet protein in central nervous system. This evidence has led in subsequent years to the amyloid school of thought, i.e., to the concept that amyloid protein deposition in the brain is the cause of dementia. Accordingly, it is currently widely accepted that both the inherited form of AD (IAD), and NIAD are caused by the same pathologically-defined condition due to the accumulation of amyloid-β (Aβ) peptides in the brain cortex. However, whereas the triggering cause of Aβ accumulation in IAD is well known [6 –9], in NIAD, it is still missing [1 , 10]. Moreover, despite in vitro and in vivo evidence supporting Aβ deposition as the cause of IAD/NIAD, recent data have shown that amyloid plaques (AP) resulting from the Aβ deposition are also present in brain cortex of healthy elderly [11 –18], suggesting that Aβ overproduction and AP formation is a physiological process of aging. Therefore, the main question still is: Is Aβ deposition responsible for IAD/NIAD onset? The search of an answer to this question has stimulated the formulation of a number of alternative pathogenetic hypotheses. However, despite the profusion of mechanisms that have been proposed in the recent years [19], the amyloid hypothesis (AH) [20] remain the most widely accepted pathogenetic mechanism for IAD, still leaving the role of Aβ deposition in NIAD pathogenesis an open question.

More questions arise from the genetic role of the APOE. The missing genetic causes of Aβ deposition in NIAD have stimulated the scientific community to search for alternative genetic components underlying NIAD pathogenesis. The gap was filled in the course of 20 years when the allelic variants ɛ4 and ɛ2 were discovered to be the main genetic factor increasing [21 –25] or decreasing [21 , 26–28] the incidence of NIAD. However, despite the amount of experimental data, no relevant conclusions have been formulated [29 –32]. Therefore, their role remains undefined, with several questions still waiting for answers: why is the ɛ4 allelic variant the major genetic risk factor of NIAD? Is the allelic variant ɛ4 responsible for NIAD onset? Why is the allelic variant ɛ2 a genetic protective factor for NIAD? And also, in the hypothesis of a common pathogenetic pathway underlying IAD and NIAD, what is the role of the ɛ4 and ɛ2 allelic variants in IAD?

Until today no conclusive interpretation has been formulated, and a pathogenetic model including the role of Aβ deposition and APOE unifying IAD and NIAD pathogenesis is still missing. Based on the experimental evidence, in the present paper we propose our interpretation of the AH in an attempt to answer these pending issues. It is important to note that communication in the field of AD clinics and genetics is sometimes unclear because even though widely accepted definitions are used, they still lend themselves to misinterpretstion. Accordingly, an attempt to clarify terms, definitions, and misunderstandings is reported in Table 1.

Table 1

Definitions, acronyms, and symbols used in the text

APP:	The amyloid beta precursor protein gene at locus 21q21.3. Despite its name, reported according to the HGNC HUGO Gene Nomenclature Committee [183], the APP gene encodes the precursor protein of both amyloid α- and β-peptides.
AβPP:	The amyloid protein precursor of both α and β peptides, despite its name reported according to the officially accepted nomenclature.
Amyloid-α peptides (Aα):	The peptides family product by the AβPP proteolysis of α-secretase. This is the so called “non-amyloidogenic pathway”. In fact, the α-secretase cleavage occurs in the middle of the β domain [54]. These peptides did not aggregate in oligomers, and are correctly metabolized by neurons.
Amyloid-β peptides (Aβ):	The peptides family produced by the AβPP proteolysis of β-secretase. This is the so called “amyloidogenic pathway” since the great part of these peptides are not metabolized by neurons. They are also hydro soluble, and sticky, and tends to aggregate in oligomers and to precipitate to form the so-called amyloid plaques (AP).
Soluble Aβ:	Those rare Aβ oligomers intermediate of the AP formation that resulted to be hydro soluble. Sometimes this term is also used incorrectly to indicates Aα.
Non-soluble Aβ:	Those oligomers intermediate of the AP formation that resulted to be not hydro soluble.
Neurotoxic Aβ:	Those Aβ peptides and oligomers poison for the neuron implying that, before plaque formation, they may be the first to cause of neuronal death in AD. According to the present interpretation of the AH, the neurotoxic action is manifested only when Aβ exceed a critical concentration threshold.
Amyloid plaques (AP):	Also known as “senile plaques” or “neuritic plaques”, are extraneuronal deposition of Aβ.
Inherited AD (IAD):	The Alzheimer form caused by one mutation in the APP gene, or in presenilin 1 (PSEN1) gene, or in presenilin 2 (PSEN2) gene.
Familial AD (FAD):	The adjective “familial” indicates the presence in a family of more than one AD case, condition needed but not sufficient to identify IAD.
Early-onset AD (EOAD):	This is the clinical definition of IAD because to the age at onset before 60–65 years.
Monogenic AD:	Synonymous of IAD since caused by a mutation in one gene.
Non-inherited AD (NIAD):	The non-inherited AD form. This form has no recognized cause/s.
Sporadic AD (SAD):	The adjective “sporadic” indicates an occasionally, singly, irregular or random AD occurrence in time. Indicative of NIAD.
Late-onset AD (LOAD):	This is the clinical definition of NIAD because to the age at onset after 60–65 years.
[ ]:	In the text we used square brackets to indicate protein concentration.
[Aβ]_c:	Critical Aβ concentration, above of which Aβ become neurotoxic.

THE AMYLOID HYPOTHESIS

The concept that both IAD and NIAD are a pathologically-defined condition due to the accumulation of Aβ was first proposed in 1984, when Glenner and Wong isolated Aβ from AD brain cortex. They suggested for the first time that the accumulation of Aβ in cerebral cortex could be the cause of AD. This is probable the first statement of the AH [33]. In the same year, they also found that the amino acid sequence of Aβ was the same as that isolated from the brain cortex of individuals with Down’s syndrome [34], always developing AD [35]. In 1985, the gene encoding the amyloid beta precursor protein (APP) was located on chromosome 21 [36]. Two years later, in 1987, the gene APP was cloned at locus 21q21.3 [37 –39].

Since the official statement in 1992 [20], AH was improved in several aspects [40 –43] up to a more recent reinterpretation [44 –46], in which Aβ deposition resulted from an imbalance between Aβ peptides production and/or clearance. This imbalance “ ... leads to gradual accumulation and aggregation of the peptide in the brain, initiating a neurodegenerative cascade ... ” [47], giving a central role to the protein homeostasis as a result of a balance between anabolism and catabolism/clearance. This and other statements of AH, however, did not change the basis of the initial hypothesis: a linear pathway that begins with Aβ peptides production, continues with Aβ deposition and AP formation, and ends with IAD/NIAD onset. Therefore, the AH has become one of the most accepted models for IAD/NIAD pathogenesis, so that Aβ deposits are recognized as a histological hallmark of postmortem brain tissues [40, 48], and considered a fundamental feature of the histopathological diagnosis [49]. In fact, until today, a clinical diagnosis of IAD/NIAD can only be “possible” or “probable” [50 –52] and confirmed only after death with the identification of Aβ deposits in the brain cortex [53, 54].

Thus, since both IAD and NIAD share common clinical and neuropathological characteristics and assuming the “lex parsimoniae” stated by the Franciscan friar William of Ockam (1287–1347) that “frustra fit per plura quod fieri potest per pauciora”, the simplest explanation of the causes suggests that both IAD and NIAD also share the same pathogenetic mechanism proposed by the AH, that implies a common genetic background. Thus, the “lex parsimoniae” becomes “the genetic approximation” of the AH. In fact, whereas mutations in the genes APP, PSEN1, or PSEN2 are recognized as causal factors in the Aβ deposition in IAD, in NIAD, if any, are still missing [1 , 10]. Moreover, the recent data indicating a similar amyloid burden in cognitively healthy elderly brain [11 –18] also suggested that not even Aβ deposits are the cause of AD. Finally, in recent years, it has been well-recognized that the major genetic risk factor for NIAD, the ɛ4 allelic variant of the APOE gene, is not a cause of the disease [55 –57].

Aβ ANABOLISM

It is well known that proteolytic processing of the amyloid beta protein precursor (AβPP) leading to Aβ production follows two parallel biochemical pathways that are in equilibrium with each other [58]. These pathways are commonly known as the “non-amyloidogenic” and the “amyloidogenic” pathway, since the latter produces the family of Aβ, that are non-soluble and sticky, prone to aggregation to form oligomers, while the first produces a family of peptides, the amyloid α-peptides (Aα) that are soluble, and correctly metabolized by the neurons. In fact, despite its historical name, the AβPP is the precursor of both Aα and Aβ peptides (Table 1).

In detail, the AβPP processing pathway is a two-step mechanism in which the first step is catalyzed by the α-secretase or the β-secretase enzymes, in the non-amyloidogenic or the amyloidogenic pathway respectively, followed by a second common step catalyzed by the γ-secretase, producing Aα first and Aβ second. Notably, α - and β -secretases have opposite effects in generating Aα and Aβ due to their competitive cleavages of AβPP. In fact, increased α-secretase activity through protein kinase C stimulation decreases Aβ generation [59 –61], while BACE1 knockout mice lack cerebral Aβ [62, 63,]. Therefore, the equilibrium between the two biochemical pathways is fundamental for Aβ anabolism. However, the most important difference is that Aβ peptides, and most of its oligomers, seem to be neurotoxic [64 –69], while Aα seems to be neuroprotective [70 –73]. Thus, a physiological age-related unbalance of this mechanism may be the first responsible for an age-related overproduction of Aβ, such as those observed in aging, and for a first addressing of the organism towards neurodegeneration or neuroprotection.

To schematize this process, assuming that Aβ overproduction and deposition is a physiological process of aging, in the weight scale of the anabolism of amyloid peptides, given as $\frac{[A α]}{[A β]}$ the ratio between amyloid peptide concentrations, during physiological aging $\frac{[A α]}{[A β]} < 1$ . In this context all factors affecting AβPP processing may influence the $\frac{[A α]}{[A β]}$ ratio, increasing [Aβ] and leading the organism toward possible neurodegeneration $(\frac{[A α]}{[A β]} ⪡ 1)$ or increasing [Aα] and leading the organism toward neuroprotection and possibly a successful aging $(\frac{[A α]}{[A β]} ⩾ 1)$ . Thus, a ratio < <1 is a necessary but not a sufficient condition for IAD/NIAD onset.

However, since the balance between protein anabolism and catabolism/clearance determines the quantity of a protein in a biological system [44, 46], the final [Aβ] in brain cortex is determined by the balance between Aβ anabolism and catabolism/clearance [74].

Aβ CATABOLISM/CLEARANCE

The concept that the balance between protein anabolism and catabolism/clearance determines the quantity of a protein in a biological system is well known [44, 46]. Thus, the final [Aβ] in the brain cortex is determined by the balance between Aβ production and Aβ degradation/clearance [74]. This means that all factors influencing Aβ anabolism and/or Aβ catabolism/clearance decide the final [Aβ] in the brain cortex.

A summary of the possible alterations in Aβ homeostasis is summarized in Fig. 1. Assuming a physiological (Phy) age-related unbalance of this mechanism as the first cause of an age-related overproduction and deposition of Aβ [11 –17], during aging, $Phy A β_{catabolism / clearance} < Phy A β_{anabolism}$

Fig.1

The relationship between Aβ anabolism and β catabolism/ clearance in the pathogenesis of IAD/NIAD. A) The overproduction of Aβ is a physiological process of aging resulting from an age-related reduction of protein catabolism/clearance in the organism, increasing Aβ concentration [Aβ]. This is a very slow process leading to Aβ deposition and amyloid plaque (AP) formation. B) In the non-inherited AD form (NIAD), a series of environmental and physiological factors further decrease Aβ catabolism/clearance, increasing [Aβ]. This is a slow process leading to Aβ deposition and AP formation. C) In the inherited AD form (IAD), mutations in the amyloid-precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) causes the production of at least more 50% (in the heterozygotes) of Aβ, increasing [Aβ]. This is a very fast process leading to Aβ deposition and AP formation.

and [Aβ] slowly increases (Fig. 1A). However, during aging, Aβ catabolism/clearance may decrease in several ways, such as a decrease in the enzymatic activity of the major Aβ-degrading enzymes, neprilysin [45 , 76], leading to an age-related reduced clearance [77], or due to the series of AD risk factors such as immune system dysregulation [78 –80], mitochondrial dysfunction and oxidative stress [81, 82], metabolism abnormalities and diabetes [83 –86], cholesterol homeostasis failure [87, 88], and deregulation of cell cycle pacing mechanisms [89, 90], all affecting Aβ catabolism. Thus, during aging $A β_{catabolism / clearance} ⪡ Phy A β_{catabolism / clearance}$

As a consequence, $A β_{catabolism / clearance} ⪡ Phy A β_{anabolism}$

and [Aβ] increases faster than the normal physiological conditions, leading the organism toward a slow neurodegeneration (Fig. 1B). This may be the condition in which NIAD onset. Conversely, mutations of APP, PSEN1, and PSEN2 genes may instead affect Aβ anabolism. Thus, in presence of APP, PSEN1, and PSEN2 gene mutations $A β_{anabolism} ⪢ phy A β_{anabolism}$ (1)

As a consequence, $A β_{anabolism} ⪢ phy A β_{catabolism / clearance}$ (2)

and [Aβ] rapidly increases, leading the organism toward a fast neurodegeneration (Fig. 1C). This may be the condition in which IAD onset.

THE APOLIPOPROTEIN E

The pivotal role of APOE in the neuropathology of the central nervous system is widely documented [91], and the genetics of the APOE polymorphism has been well investigated [92 –94]. Briefly, the three common genetic variants known as ɛ2, ɛ3, and ɛ4 of the APOE gene are determined by three of the four haplotypes resulting from the combination of the alleles of the T-to-C and C-to-T single-nucleotide polymorphisms (SNPs) rs429358 and rs7412 at the APOE gene locus. Among the three genetic variants, ɛ3 (haplotype T-C) has been observed as the most common in the population worldwide, and therefore considered as the “normal variant”, followed by ɛ4 (haplotype C-C), and ɛ2 (haplotype T-T), being the fourth haplotype C-T (variant ɛ3_r) the rarest [94, 96]. Interestingly, both SNPs change the first base of two CGC codons encoding arginine (Arg) resulting in TGC codons encoding cysteine (Cys). Thus, at codons 112 and 158, the APOE encoded by the three genetic variants carries the different amino acid combinations Cys-Arg (APOE3), Arg-Arg (APOE4), and Cys/Cys (APOE2), with the Arg-Cys (APOE3r) the rarest [94, 96].

Since 1991, involvement of the E4 isoform in the clinic and neuropathology of AD has been well documented as the main genetic risk factor for NIAD [21 –24] compared with the normal E3 isoform. Until now, APOE is still confirmed as major risk factor for NIAD, namely the main gene with a semi-dominant inheritance [25]. Indeed, a protective role of the E2 isoform in NIAD has also been proposed [21 , 26–28]. However, despite this evidence, it has been well demonstrated that, from a genetic point of view, the presence of the ɛ4 allele is neither necessary nor sufficient to cause AD [55 –57]. Accordingly, the clinicopathologic series in which the role of APOE in the diagnosis of NIAD was estimated [55 , 96] did not support a rationale for its genotyping in the clinical setting [97]. Conversely, it has been well documented that in the presence of a clinical diagnosis of possible/probable NIAD, APOE genotyping may be a useful diagnostic tool [98 –102].

To place APOE in the AH frame, its interaction with Aβ must be considered. It is well known that APOE binds and transports Aβ [103 –106]. Interestingly, the different APOE isoform shows a different binding affinity for Aβ, according to the following order: $E_{4} > E_{3} > E_{2}$ , indicating that the different APOE isoforms may differently influence Aβ deposition and clearance but not its production [110 –115], and thus have different protective [74] and risk [106, 110] role in the development of AD, depending on the Aβ titration by APOE [113 , 117].

In the present interpretation of the AH (Fig. 2), we assume that the neurotoxic action of Aβ follows a threshold model in which the AD onset is a consequence of a critical Aβ concentration ([Aβ]_c), i.e., when $[A β] \geq [A β]_{c} .$

Fig.2

A critical concentration of Aβ ([Aβ]_c) determined the onset of IAD/NIAD. Until [Aβ] < [Aβ]_c, healthy aging progress. When [Aβ]≥[Aβ]_c, IAD/NIAD onset. Around [Aβ]_c probably exists a shaded area in which a clinical prodromal condition may be identified.

This is the necessary and sufficient condition to IAD/NIAD onset. Moreover according to the reported different affinities of the APOE isoforms for Aβ, [Aβ]_c increases in the following order: ${[A β]}_{c}^{E 4} < {[A β]}_{c}^{E 3} < {[A β]}_{c}^{E 2}$

thus, predisposing one to AD onset at a lower [Aβ]_c in ɛ4 and at higher [Aβ]_c in ɛ2 carriers (Fig. 3). Thus, a higher amyloid burden is expected in the presence of the ɛ4 allele in a dose-dependent fashion [113, 118]. In this context, all factors affecting Aβ metabolism, and therefore [Aβ]_c, may be involved in IAD and NIAD onset. Indeed, in a study investigating erythrocyte associated Aβ and its potential role as a biomarker to diagnose dementia [119], we observed increased concentrations of erythrocyte associated Aβ in the presence of APOE4 and a decreased concentration in APOE2 as compared with APOE3 carriers (data not showed).

Fig.3

Apolipoprotein E (APOE) titrate [Aβ]. In respect to E3, which is the most common isoform, E4 have an increased Aβ affinity, lowering [Aβ]_c threshold and the age at onset. Conversely, E2 have a decreased affinity for Aβ, increasing [Aβ]_c threshold and age at onset. Notably in IAD, since the rapid increase in [Aβ], the effect of the APOE isoform on [Aβ]_c and age at onset is negligible.. It is, conversely, significant in NIAD. The first consequence of this model is the increased amyloid burden associated with APOE4 and the decreased amyloid burden associated with APOE2.

DISCUSSION

The amyloid hypothesis

In the present study, we proposed an interpretation of the AH aimed to address some missing answers, mainly regarding the pathogenetic roles of Aβ deposition and the APOE polymorphism. An overall summary of the proposed model, starting from AβPP processing and ending with IAD/NIAD onset or healthy aging is summarized in Fig. 4.

Fig.4

A summary of the present interpretation of the AH. The original formulation suggested the amyloid plaques (AP) as the cause of the onset of inherited and non-inherited Alzheimer’s disease (IAD, NIAD). However, it did not explain why AP were observed in healthy brain, and the role of the apolipoprotein E (APOE) isoforms APOE2 and APOE4. (1) Aβ anabolism. The competition between α- and β-secretase to cleave the amyloid-beta protein precursor (AβPP) is the first determinant of the physiological Aβ concentration ([Aβ]). (2) Aβ catabolism/clearance. The equilibrium between Aβ anabolism and Aβ catabolism/clearance is the second determinant of the physiological [Aβ]. (3) Mutations in the amyloid-beta precursor protein (APP), presenilin-1 (PSEN1), or presenilin-2 (PSEN2) genes strongly affect Aβ anabolism, increasing [Aβ] and unbalancing $\frac{[A α]}{[A β]}$ . In this condition [Aβ] rapidly exceed the critical neurotoxicity threshold [Aβ]_c, underlying the onset of severe Alzheimer’s disease (AD), the inherited form of AD (IAD). Otherwise, factor with a moderate effect in decreasing Aβ catabolism/clearance contribute to the slow Aβ deposition process underlying the onset of NIAD. (4) Our model hypothesized the existence of a critical threshold of Aβ concentration ([Aβ]_c) determining the Aβ neurotoxic action only when [Aβ]≥[Aβ]_c. (5) The apolipoprotein E (APOE). The APOE isoforms E2, E3, and E4 showed different biding affinity for Aβ in the order E4 > E3 > E2. In a condition in which [Aβ] slowly rise, such as those of non-inherited AD (NIAD) the different isoform strongly affect the onset and the severity of the disease. Conversely, their effects are negligible when [Aβ] rapidly rise, such as in IAD. (6) Amyloid plaques (AP). The AP formation is a direct consequence of Aβ production, but is independent from [Aβ]_c. The present interpretation of the AH well explained the role of AP and APOE in AD pathogenesis. Accordingly, 1) the presence of AP is necessary but not sufficient to IAD/NIAD onset; 2) APOE4 is the major genetic risk factor for NIAD, negligible for IAD; 3) The presence of APOE4 is not necessary neither sufficient for NIAD onset, i.e., is not the cause of NIAD; 4) The presence of APOE2 is protective for NIAD, negligible for IAD.

In the present understanding we assume that exists a critical concentration of Aβ peptides and oligomers that is determined by an unbalance of the Aβ homeostasis, the APOE genotype, and individual physiological conditions do exist, and that AD arises only when the concentration of Aβ peptides and oligomers exceed this threshold. This means that the neurotoxic effect of Aβ is manifested only beyond this threshold. In this context, AP formation is a direct consequence of Aβ production, but independent of the neurotoxic action, thus it becomes a necessary but not sufficient condition for AD onset. In fact, it is well established that AP are observed both in AD and in healthy brain, since Aβ deposition is a normal physiological process of aging [11 –18]. Therefore, considering that the AβPP non-amyloidogenic pathway did not form AP, we can observe healthy brains with and without AP. Conversely, we can observe AD brains only with AP. This mechanism of action also explains why Aβ deposition begins years before the AD clinical onset and why AP are observed years before AD clinical onset. Notably, since the topography of Aβ deposits in AD brain follows a well-defined geographic progression [120, 121], it may be possible that AP distribution may be different between IAD, NIAD, and a normal aged brain.

The second question concerns the role of the APOE polymorphism, and in particular of the ɛ4 allele variant in NIAD, because despite a well-documented role as a major genetic risk factor increasing NIAD risk 4-fold in ɛ3/ɛ4 heterozygous and 8-fold in ɛ4/ɛ4 homozygous, its presence is neither necessary nor sufficient to cause the disease. This means that a genetic test identifying the ɛ4 allele variant in asymptomatic patients might be prognostic, since it increases the risk of NIAD, but not diagnostic because it is not the cause of the disease. In the present model, we suggest that APOE4 is associated to a higher amyloid burden, lowering the age at onset. This mechanism identifies the ɛ4 allele variant as a genetic risk factor directly involved in the NIAD pathogenesis but not responsible for its onset. This hypothesis also answers the other two questions. Is the ɛ2 allelic variant a genetic protective factor for AD? Is Aβ deposition prodromal to AD? The first answer is yes, since APOE2, with a lower affinity for Aβ, is associated with a lower amyloid burden increasing the age at onset. This explains the role of APOE2 as a protective factor regardless of NIAD causes of onset. Conversely, the second answer is no, because when [Aβ] < [Aβ]_c, the disease does not occur. It is clear that a prodromal role of [Aβ] near [Aβ]_c cannot be excluded. However similarly to APOE4, Aβ deposition is a necessary but not sufficient condition for disease onset. Notably this model implies that the higher/lower amyloid burden associated with APOE4 the first, and APOE2 the second observed in the AP, are similar in AD and healthy brain. Therefore, AD onset is an event resulting from a physiological process of Aβ production that can be accelerated by a series of environmental and non-environmental factors and much more rarely by mutations in AD genes. In this context, both AP and APOE4 are necessary but not sufficient to cause AD. It is notewhorthy that, the AβO hypothesis [64–66 , 69] (see also Table 2), that places the Aβ peptides and oligomers, and not the Aβ fibrils, first for their neurotoxic role [68] may well fit with our AH interpretation. Indeed, the importance of the AβO hypothesis has grown so much that recently, it has been reported that “ ... it has all but supplanted the amyloid cascade ... ” [67].

Table 2

AD pathogenesis: The AH and the first event schools of thought

1st event/s	2nd event/s	Note	Reference
The amyloid hypothesis (AH)¹
Aβ deposition	All the others	Causative role for Aβ (deposits). APOE and age not included in the hypothesis.	1992, Hardy [20]^*
			1995, Younkin [122]
			1997, Hardy [184]
			1998, Small [185]
			2001, Selkoe [48]
			2002, Hardy [41]
			2006, Selkoe [40]
			2009, Hardy [129]
			2017, Hardy [43]
The β -amyloid oligomer hypothesis (AβOH)¹
Aβ deposition	All the others	Causative role for Aβ (oligomers). APOE and age not included in the hypothesis.	1998, Lambert [64]
			2005, Cleary [65]
			2012, Bao [66]
			2013, Teplow [67]
			2017, Ono [68]
			2018, Cline [69]*
The amyloid hypothesis - Revision 2018 (AH-R2018)¹
Aβ deposition	All the others	Causative role for Aβ exceeding a critical concentration. APOE and age included in the hypothesis.	Present study
The τ hypothesis²
AβPP impaired metabolism	τ aggregation	Aβ deposition as one of the consequences. APOE is included in the hypothesis. Age is not included in the hypothesis.	2018, Kametani [172]^*
Mitochondrial dysfunction³	Increased oxidative stress (microglia activation and τ hyperphosphorylation)	Aβ deposition as one of the consequences. Age is included in the hypothesis. APOE not included in the hypothesis.	2010 Maccioni [186]
			2011 Bonda [187]
The mitochondrial hypothesis (MH)²
Mitochondrial dysfunction³	Increased oxidative stress	Aβ deposition as one of the consequences. Age is included in the hypothesis. APOE not included in the hypothesis.	1996, Harman [143]^*
			2000, Harman [145]
			2001, Joseph [138]
			2002, Harman [188]
			2004, Swerdlow [189]
			2005, Moreira [82]
			2006, Castellani [140]
			2006, Harman [190]
			2006, Lee [141]
			2006, Moreira [83]
			2009, Castellani [191]
			2012, Moreira [85]
			2014, Swerdlow [143]
	Increased oxidative stress (and Aβ production)⁴	Aβ deposition as one of the consequences. Age is included in the hypothesis. APOE not included in the hypothesis.	2004, Lee [128, 162]
			2005, Lee [163]
			2009, Castellani [142]
	Increased oxidative stress (and/or protein glycation)	Aβ deposition as one of the consequences. Age is included in the hypothesis. APOE is not included in the hypothesis.	1995, Smith [146]^*
The bioflocculant hypothesis²
Mitochondrial dysfunction³	Increased oxidative stress (and Aβ production)⁵	Aβ deposition as one of the consequences. Age is included in the hypothesis. APOE not included in the hypothesis.	2002, Bishop [192]^*
The two-hits hypothesis²
Mitochondrial dysfunction³	Increased oxidative stress and/or mitotic abnormalities	Aβ deposition as one of the consequences. Age is included in the hypothesis. APOE not included in the hypothesis.	2004, Zhu [139]^*
			2005, Webber [147]
			2007, Zhu [148]
The Dr. Jekyll and Mr. Hyde hypothesis²
Mitotic abnormalities	Multiple interacting	Aβ deposition as one of the consequences. APOE and age not included in the hypothesis.	2003, Arendt [90]^*
			2007, Arendt [91, 92]
The presenilin hypothesis (PSH) ⁶
Presenilins mutations⁷	Multiple interacting	Aβ deposition as one of the consequences. APOE and age not included in the hypothesis.	2007, Shen [193]^*
			2018, Hunter [156]
The infection hypothesis²
Microbial/viral infection	Aβ production⁸	Aβ deposition as one of the consequences. APOE and age not included in the hypothesis.	2016, Bourgade [151]
			2016, Itzhaki [149]
			2016, Kumar [150]
			2017, Bolos [156]
			2017, Sochocka [155]
			2018, Fulop [152]^*
Inflammatory hypothesis²
Aβ deposition⁹	Microglia activation¹⁰	Aβ deposition as one of the consequences. APOE and age not included in the hypothesis.	1996, Pasinetti [153]^*
			2014, Cai [154]
The vascular hypothesis (VH)²
Cerebrovascular impairment	Multiple interacting	Aβ deposition as one of the consequences. APOE and age are included in the hypothesis.	2002, de la Torre [158]
			2013, Bu [159]^*
			2014, Drachman [157]
Senescence hypothesis^1, ¹¹
Cellular senescence	Multiple interacting	Aβ deposition as one of the consequences. APOE is not included in the hypothesis (?). Age is included in the hypothesis.	2013, Hunter [126]^*
The multifactorial/complex disease hypothesis
Multiple interacting	Multiple interacting	Aβ deposition as one of the consequences. Age is included in the hypothesis (?). APOE is not included in the hypothesis (?).	1998, Neve [127]
			2000, Khachaturian [161]
			2007, Castellani [194]
			2009, Pimplikar [130]
			2010, Gustaw-Rothenberg [187]
			2010, Mondragón-Rodríguez [131]
The AβPP matrix approach (AMA) hypothesis⁶
APP mutations¹²	Multiple interacting	Aβ deposition as one of the consequences. APOE and age are not included in the hypothesis.	2018, Hunter [156*, 160]

^*Articles in which the hypothesis was proposed for the first time. ¹Pathogenetic hypothesis valid for both IAD and NIAD. ²Pathogenetic hypothesis valid only for NIAD. ³Mitochondrial dysfunction resulting by an age-related accumulation of mtDNA mutations. ⁴Aβ production as a protective adaptation to the disease. ⁵Soluble Aβ acting as neuroprotective factor. ⁶Pathogenetic hypothesis valid only for IAD. ⁷Causing presenilin loss of function. ⁸The initial Aβ production as antimicrobial peptide will be beneficial on first microbial challenge but will become progressively detrimental as the infection becomes chronic and reactivates from time to time. ⁹Age related Aβ-deposition. ¹⁰A vicious cycle of inflammation has been formed between Aβ accumulation, activated microglia, and microglial inflammatory mediators, which enhance Aβ deposition and neuroinflammation. ¹¹Factors associated with AD can be seen as partial stressors within the matrix of signaling pathways that underlie cell survival and function. Senescence pathways are triggered when stressors exceed the cells ability to compensate for them. ¹²Causing impaired AβPP metabolism.

In conclusion, assuming a threshold model in which [Aβ] is titrated by APOE and AD occurs only beyond [Aβ]_c, the most common questions about the role of APOE in the AH can be considered answered.

Other hypotheses

In the present paper, by means of an interpretation of the AH including the role of aging and APOE, the major risk factors for AD, we try to offer a complete overview of IAD and NIAD pathogenesis [122]. Clearly, this interpretation takes for granted Aβ deposition as a pivotal event in the pathogenetic process. However, since the AH is still criticized [47 , 123–133], all the hypotheses considering Aβ deposition as a secondary event, if any, cannot be excluded (Table 2). These hypotheses include: 1) the mitochondrial hypothesis suggesting an age-related increase of oxidative stress [82 , 134–138], also resulting from mitochondrial mutations [139 –141], sometimes in synergistic effect with protein glycation [142] or abnormalities in mitotic signaling [90 , 144], either individually [147] or in a synergistic manner [90 , 148]; 2) the infection hypothesis suggesting a bacterial/virus infection as the first event [145 –148], also considering inflammation as a secondary event [149 –152]; and 3) the vascular hypothesis suggesting vascular deficits as first event [153 –155]. The final hypothesis in which AD is the result of an interaction among these causes suggesting that it may be a complex disease [126 , 157] must be also considered.

Notably, a protective role of Aβ deposition which acts as protective adaptation to the neuronal stress resulting from the disease, have been also suggested [128 , 159]. It is interesting to note that the confirmation or confutation of the amyloid hypothesis is crucial for the treatment to be followed, i.e., the anti-amyloid or the non-anti-amyloid treatment [160 –167]. Paradoxically, in the acceptance of a protective role of Aβ in AD, all therapeutic efforts aimed at lowering the Aβ production or removing Aβ deposit will only serve to exacerbate the disease process [128].

Aβ and τ

Of interest is the integration of the role of τ in the AH, asking a further question. What is the connection between AH and τ pathology? Indeed, some studies consider τ deposition as the first event in AD pathogenesis, thus formulating a further non-amyloid hypothesis, the “τ hypothesis” [172]. However, from the “amyloid point of view”, currently the AH has no answer to this question [135]. It is clear that if τ deposition is considered the first pathological event in AD, the question is up-side down [168], i.e., what is the connection between the τ hypothesis and Aβ deposition? In fact, it has recently been reported that τ lesions occurred before Aβ accumulation [169, 170], suggesting that AD progression is strongly associated with τ pathology, rather than Aβ pathology. Although the transmission mechanism of τ cell-to-cell aggregates is still not clear, τ pathology spreads in the brain in a well-defined way, and it has been demonstrated that its distribution can be correlated with the clinical stages of the disease [171]. In fact, it is reported that τ pathology correlates better than Aβ pathology with clinical features of AD [172]. As a result, it has been recently reported that the increase of AβPP with or without familial AD mutations, not Aβ, can function as an abnormal τ fibrils receptor and promote the intracellular τ aggregation [172], suggesting that AβPP, rather than Aβ, can accelerate τ accumulation and propagation [172]. In animal models, it has already been reported that the suppression or deletion of τ has a profound protective effect against brain damage and neurological deficit [173 –178]. Therefore, it may be possible that AD is a disorder triggered by impairment of AβPP metabolism and progresses through τ pathology. However, despite these suggestions, a complete connection to add the role of τ in the AH is still missing. Accordingly, the AT(N) marker grouping [179] of the National Institute on Aging and Alzheimer’s Association (NIAA) Research Framework [180] place amyloid biomarkers at the apex of the preclinical biomarker hierarchy, with τ biomarkers as second.

Aβ and biomarkers

In addition to treatment, to accept or not of Aβ deposition as the first pathological event in AD also defines its role as early biomarker of the disease. In particular, although according to the present interpretation of the AH Aβ deposition alone is not sufficient to cause AD, we cannot exclude that, it may be sufficient to cause downstream pathologic changes (i.e., tauopathy and neurodegeneration) that ultimately lead to AD [52]. This suggests doubts that Aβ alone could serve as an early biomarker or as the defining signature of AD. However, both Aβ and τ deposits are necessary to fulfill neuropathologic criteria for AD diagnosis, which suggests that evidence of abnormalities in both Aβ and pathologic τ biomarkers should be present to apply the label “Alzheimer’s disease” in a living person [54, 181]. Thus, although it is possible that AP and neurofibrillary τ deposits are not causal in AD pathogenesis, these abnormal protein deposits define AD as a unique neurodegenerative disease among several disorders that can lead to dementia [185]. In this context, the definition of AD as a biological construct will allow a more accurate characterization and understanding of the sequence of events leading to cognitive impairment associated with AD, as well as the multifactorial etiology of dementia. This approach will also allow a more precise approach to interventional trials in which specific pathways can be targeted in the disease process and in the appropriate patients [185, 182].

CONCLUSIONS

In the present paper, we propose an interpretation of the AH that unifies the pathogenetic models for IAD/NIAD and explains why AP and APOE4 are also observed in healthy aging and why they are not the cause of AD.

Of interest may be the implications of the proposed AH interpretation in influencing future clinical and research directions. The key point that we propose as central in the model, namely the neurotoxic action of Aβ only above a critical concentration, suggests that raising [Aβ]_c may be a correct action to delay IAD/NIAD onset, and therefore the way to setup a pharmacological treatment against IAD/NIAD. In this context, an action mechanism that mimics those of the APOE interaction with Aβ, in which the first titrates the second, may be the correct model to follow to develop new drugs for AD. It is clear that the APOE cannot be a therapeutic target itself, just as non-physiological Aβ deposition as those observed in the use of antibodies against Aβ must be avoided. Moreover, since Aβ is needed for a certain number of physiological functions, in addition to its role in the pathogenesis of IAD/NIAD, a failure or adverse effect resulting from actions against Aβ production may be hypothesized. It is also clear that is not possible to exclude a role toward IAD/NIAD of the treatment that modulates the actions of the risk factors that revolve around IAD/NIAD pathogenesis. Thus, the correct understanding of the IAD/NIAD pathogenesis can open new previously missing therapeutic pathways.

Clearly, the proposed interpretation of the AH did not exclude the validity of other model considering Aβ deposition as a secondary event. Undoubtedly, among these non-amyloid hypotheses, the most reliable is to put mitochondria and oxidative stress as the first event. This hypothesis clearly includes age as a primary risk factor for AD. Aging is the accumulation of changes that increases the risk of death. Accordingly, neurons involved in the lesions associated with NIAD are apparently aging at a faster rate than normal since the same lesions are observed at later ages in normal individuals. There is a growing consensus that these deleterious changes are produced by free radical reactions, initiated largely by mitochondria during normal metabolism at a rate that progressively increases with age, while the life span of an individual is determined by the rate of such damage to the mitochondria. Accordingly, Aβ deposition will follow as a consequence of an increase in reactive oxygen species concentration as a secondary event. This hypothesis has been well investigated and confirmed by a series of papers [82 , 138–143]. However, despite this pathogenetic hypothesis including age as a major risk factor for AD, the role of APOE as a main genetic risk factor for AD is far from being explained. Similarly, while this hypothesis fits well with NIAD onset, there are some difficulties in justifying it as the cause of the mutation responsible for IAD onset.

In conclusion, more studies are needed to better understand the molecular mechanisms underlying the behavior of Aβ and APOE in the AH context to verify the correctness of our understanding.

Footnotes

ACKNOWLEDGMENTS

This work was fully supported by “Ministero della Salute”, I.R.C.C.S. Research Program, Ricerca Corrente 2018–2020, Linea n. 2 “Meccanismi genetici, predizione e terapie innovative delle malattie complesse” and by the “5×1000” voluntary contribution to the Fondazione I.R.C.C.S. Ospedale “Casa Sollievo della Sofferenza”.

Authors’ disclosures available online ().

References

Winblad

, Amouyel

, Andrieu

, Ballard

, Brayne

, Brodaty

, Cedazo-Minguez

, Dubois

, Edvardsson

, Feldman

, Fratiglioni

, Frisoni

, Gauthier

, Georges

, Graff

, Iqbal

, Jessen

, Johansson

, Jönsson

, Kivipelto

, Knapp

, Mangialasche

, Melis

, Nordberg

, Rikkert

, Qiu

, Sakmar

, Scheltens

, Schneider

, Sperling

, Tjernberg

, Waldemar

, Wimo

, Zetterberg

(2016) Defeating Alzheimer’s disease and other dementias: A priority for European science and society. Lancet Neurol 15, 455–532.

Prince

, Comas-Herrera

, Knapp

, Guerchet

, Karagiannidou

(2016) World Alzheimer Report 2016. Improving healthcare for people living with dementia: Coverage, quality and costs now and in the future. Alzheimer Disease International, London, UK, https://www.alz.co.uk/research/WorldAlzheimerReport2016.pdf.

Holtzman

, Morris

, Goate

(2011) Alzheimer’s disease: The challenge of the second century. Sci Transl Med 3, 77sr1.

Mangialasche

, Solomon

, Winblad

, Mecocci

, Kivipelto

(2010) Alzheimer’s disease: Clinical trials and drug development. Lancet Neurol 9, 702–716.

Cummings

, Morstorf

, Zhong

(2014) Alzheimer’s disease drug-development pipeline: Few candidates, frequent failures. Alzheimers Res Ther 6, 37.

Goate

, Chartier-Harlin

, Mullan

, Brown

, Crawford

, Fidani

, Giuffra

, Haynes

, Irving

, James

, Mant

, Newton

, Rooke

, Roques

, Talbot

, Pericak-Vance

, Roses

, Williamson

, Rossor

, Owen

, Hardy

(1991) Segregation of a missense mutation in the amyloid precursor protein gene with familial Alzheimer’s disease. Nature 349, 704–706.

Levy-Lahad

, Wasco

, Poorkaj

, Romano

, Oshima

, Pettingell

, Yu

, Jondro

, Schmidt

, Wang

, Crowley

, Fu

Y-H

, Guenette

, Galas

, Nemens

, Wijsman

, Bird

, Schellenberg

, Tanzi

(1995) Candidate gene for the chromosome 1 familial Alzheimer’s disease locus. Science 269, 973–977.

Rogaev

, Sherrington

, Rogaeva

, Levesque

, Ikeda

, Liang

, Chi

, Lin

, Holman

, Tsuda

, Mar

, Sorbi

, Nacmias

, Placentini

, Amaducci

, Chumakov

, Cohen

, Lannfelt

, Fraser

, Rommens

, St George-Hyslop

(1995) Familial Alzheimer’s disease in kindreds with missense mutations in a gene on chromosome 1 related to the Alzheimer’s disease type 3 gene. Nature 376, 775–778.

Sherrington

, Rogaev

, Liang

, Rogaeva

, Levesque

, Ikeda

, Chi

, Lin

, Li

, Holman

, Tsuda

, Mar

, Foncin

, Bruni

, Montesi

, Sorbi

, Rainero

, Pinessi

, Nee

, Chumakov

, Pollen

, Brookes

, Sanseau

, Polinsky

, Wasco

, Da Silva

, Haines

, Perkicak-Vance

, Tanzi

, Roses

, Fraser

, Rommens

, St George-Hyslop

(1995) Cloning of a gene bearing missense mutations in early-onset familial Alzheimer’s disease. Nature 375, 754–760.

10.

Pistollato

, Ohayon

, Lam

, Langley

, Novak

, Pamies

, Perry

, Trushina

, Williams

, Roher

, Hartung

, Harnad

, Barnard

, Morris

, Lai

, Merkley

, Chandrasekera

(2016) Alzheimer disease research in the 21st century: Past and current failures, new perspectives and funding priorities. Oncotarget 7, 38999–39016.

11.

Neuropathology Group. Medical Research Council Cognitive Function and Aging Study (2001) Pathological correlates of late-onset dementia in a multicentre, community-based population in England and Wales. Neuropathology Group of the Medical Research Council Cognitive Function and Ageing Study (MRC CFAS). Lancet 357, 169–175.

12.

Nunomura

, Castellani

, Lee

, Moreira

, Zhu

, Perry

, Smith

(2006) Neuro pathology in Alzheimer’s disease: Awaking from a hundred-year-old dream. Sci Aging Knowledge Environ 2006, pe10.

13.

Selkoe

(2006) Amyloid beta-peptide is produced by cultured cells during normal metabolism: A reprise. J Alzheimers Dis 9, 163–168.

14.

Schmitt

, Davis

, Wekstein

, Smith

, Ashford

, Markesbery

(2000) “Preclinical” AD revisited: Neuropathology of cognitively normal older adults. Neurology 55, 370–376.

15.

Savva

, Wharton

, Ince

, Forster

, Matthews

, Brayne

; Medical Research Council Cognitive Function and Ageing Study (2009) Age, neuropathology, and dementia. N Engl J Med 360, 2302–2309.

16.

Brayne

, Richardson

, Matthews

, Fleming

, Hunter

, Xuereb

, Paykel

, Mukaetova-Ladinska

, Huppert

, O’Sullivan

, Dening

; Cambridge City Over-75s Cohort Cc75c Study Neuropathology Collaboration (2009) Neuropathological correlates of dementia in over-80-year-old brain donors from the population-based Cambridge city over-75s cohort (CC75C) study. J Alzheimers Dis 18, 645–658.

17.

Brayne

(2007) The elephant in the room - healthy brains in later life, epidemiology and public health. Nat Rev Neurosci 8, 233–239.

18.

Carvalho

, St Louis

, Knopman

, Boeve

, Lowe

, Roberts

, Mielke

, Przybelski

, Machulda

, Petersen

, Jack

Jr , Vemuri

(2018) Association of excessive daytime sleepiness with longitudinal β-amyloid accumulation in elderly persons without dementia. JAMA Neurol 75, 672–680.

19.

Magalingam

, Radhakrishnan

, Ping

, Haleagrahara

(2018) Current concepts of neurodegenerative mechanisms in Alzheimer’s disease. Biomed Res Int 2018, 3740461.

20.

Hardy

, Higgins

(1992) Alzheimer’s disease: The amyloid cascade hypothesis. Science 256, 184–185.

21.

Corder

, Saunders

, Strittmatter

, Schmechel

, Gaskell

, Small

, Roses

, Haines

, Pericak-Vance

(1993) Gene dose of apolipoprotein E type 4 allele and the risk of Alzheimer’s disease in late onset families. Science 261, 921–923.

22.

Chartier-Harlin

, Parfitt

, Legrain

, Pérez-Tur

, Brousseau

, Evans

, Berr

, Vidal

, Roques

, Gourlet

, Fruchart

J-C

, Delacourte

, Rossor

, Amouyel

(1994) Apolipoprotein E, epsilon 4 allele as a major risk factor for sporadic early and late-onset forms of Alzheimer’s disease: Analysis of the 19q13,2 chromosomal region. Hum Mol Genet 3, 569–574.

23.

Farrer

, Cupples

, Haines

, Hyman

, Kukull

, Mayeux

, Myers

, Pericak-Vance

, Risch

, Van Duijn

(1997) Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease. A meta-analysis. APOE and Alzheimer Disease Meta Analysis Consortium. J Am Med Assoc 278, 1349–1356.

24.

Masullo

, Daniele

, Seripa

, Filippini

, Gravina

, Carbone

, Gainotti

, Fazio

(1998) Apolipoprotein E genotype in sporadic early- and late-onset Alzheimer’s disease. Dement Geriatr Cogn Disord 9, 121–125.

25.

Genin

, Hannequin

, Wallon

, Sleegers

, Hiltunen

, Combarros

, Bullido

, Engelborghs

, De Deyn

, Berr

, Pasquier

, Dubois

, Tognoni

, Fiévet

, Brouwers

, Bettens

, Arosio

, Coto

, Del Zompo

, Mateo

, Epelbaum

, Frank-Garcia

, Helisalmi

, Porcellini

, Pilotto

, Forti

, Ferri

, Scarpini

, Siciliano

, Solfrizzi

, Sorbi

, Spalletta

, Valdivieso

, Vepsäläinen

, Alvarez

, Bosco

, Mancuso

, Panza

, Nacmias

, Bossù

, Hanon

, Piccardi

, Annoni

, Seripa

, Galimberti

, Licastro

, Soininen

, Dartigues

, Kamboh

, Van Broeckhoven

, Lambert

, Amouyel

, Campion

(2011) APOE and Alzheimer disease: A major gene with semi-dominant inheritance. Mol Psychiatry 16, 903–907.

26.

Corder

, Saunders

, Risch

, Strittmatter

, Schmechel

, Gaskell

Jr , Rimmler

, Locke

, Conneally

, Schmader

, Small

, Roses

, Haines

, Pericak-Vance

(1994) Protective effect of apolipoprotein E type 2 allele for late onset Alzheimer disease. Nat Genet 7, 180–184.

27.

Panza

, Solfrizzi

, Torres

, Mastroianni

, Colacicco

, Basile

, Capurso

, D’Introno

, Del Parigi

, Capurso

(2000) Apolipoprotein E in Southern Italy: Protective effect of epsilon 2 allele in early- and late-onset sporadic Alzheimer’s disease. Neurosci Lett 292, 79–82.

28.

Chiang

, Insel

, Tosun

, Schuff

, Truran-Sacrey

, Raptentsetsang

, Jack

Jr , Aisen

, Petersen

, Weiner

; Alzheimer’s Disease Neuroimaging Initiative (2010) Hippocampal atrophy rates and CSF biomarkers in elderly APOE2 normal subjects. Neurology 75, 1976–1981.

29.

Kim

, Youn

, Jhoo

, Lee

, Woo

(2002) Apolipoprotein E epsilon 4 allele is not associated with the cognitive impairment in community-dwelling normal elderly individuals. Int J Geriatr Psychiatry 17, 635–640.

30.

Seripa

, Franceschi

, Matera

, Panza

, Kehoe

, Gravina

, Orsitto

, Solfrizzi

, Di Minno

, Dallapiccola

, Pilotto

(2006) Sex differences in the association of apolipoprotein E and angiotensin-converting enzyme gene polymorphisms with healthy aging and longevity: A population-based study from Southern Italy. J Gerontol A Biol Sci Med Sci 61, 918–923.

31.

Panza

, Frisardi

, Capurso

, D’Introno

, Colacicco

, Seripa

, Pilotto

, Vendemiale

, Capurso

, Solfrizzi

(2009) Apolipoprotein E, dementia, and human longevity. J Am Geriatr Soc 57, 740–742.

32.

Heise

, Filippini

, Ebmeier

, Mackay

(2011) The APOE ɛ4 allele modulates brain white matter integrity in healthy adults. Mol Psychiatry 16, 908–916.

33.

Glenner

, Wong

(1984) Alzheimer’s disease: Initial report of the purification and characterization of a novel cerebrovascular amyloid protein. Biochem Biophys Res Commun 120, 885–890.

34.

Glenner

, Wong

(1984) Alzheimer’s disease and Down’s syndrome: Sharing of a unique cerebrovascular amyloid fibril protein. Biochem Biophys Res Commun 122, 1131–1135.

35.

Olson

, Shaw

(1969) Presenile dementia and Alzheimer’s disease in mongolism. Brain 92, 147–156.

36.

Masters

, Simms

, Weinman

, Multhaup

, McDonald

, Beyreuther

(1985) Amyloid plaque core protein in Alzheimer disease and Down syndrome. Proc Natl Acad Sci U S A 82, 4245–4249.

37.

Goldgaber

, Lerman

, McBride

, Saffiotti

, Gajdusek

(1987) Characterization and chromosomal localization of a cDNA encoding brain amyloid of Alzheimer’s disease. Science 235, 877–880.

38.

Robakis

, Ramakrishna

, Wolfe

, Wisniewski

(1987) Molecular cloning and characterization of a cDNA encoding the cerebrovascular and the neuritic plaque amyloid peptides. Proc Natl Acad Sci U S A 1984, 4190–4194.

39.

Tanzi

, Gusella

, Watkins

, Bruns

, St George-Hyslop

, Van Keuren

, Patterson

, Pagan

, Kurnit

, Neve

(1987) Amyloid beta protein gene: cDNA, mRNA distribution, and genetic linkage near the Alzheimer locus. Science 235, 880–884.

40.

Selkoe

(2006) Toward a comprehensive theory for Alzheimer’s disease. Hypothesis: Alzheimer’s disease is caused by the cerebral accumulation and cytotoxicity of amyloid beta-protein. Ann NY Acad Sci 924, 17–25.

41.

Hardy

, Selkoe

(2002) The amyloid hypothesis of Alzheimer’s disease: Progress and problems on the road to therapeutics. Science 297, 353–356.

42.

Selkoe

, Hardy

(2016) The amyloid hypothesis of Alzheimer’s disease at 25 years. EMBO Mol Med 8, 595–608.

43.

Hardy

(2017) The discovery of Alzheimer-causing mutations in the APP gene and the formulation of the “amyloid cascade hypothesis”. FEBS J 284, 1040–1044.

44.

Saido

, Iwata

(2006) Metabolism of amyloid beta peptide and pathogenesis of Alzheimer’s disease: Towards presymptomatic diagnosis, prevention and therapy. Neurosci Res 54, 235–253.

45.

Hellström-Lindahl

, Ravid

, Nordberg

(2008) Age-dependent decline of neprilysin in Alzheimer’s disease and normal brain: Inverse correlation with A beta levels. Neurobiol Aging 29, 210–221.

46.

Sasaguri

, Nilsson

, Hashimoto

, Nagata

, Saito

, De Strooper

, Hardy

, Vassar

, Winblad

, Saido

(2017) APP mouse models for Alzheimer’s disease preclinical studies. EMBO J 36, 2473–2487.

47.

Herrup

(2015) The case for rejecting the amyloid cascade hypothesis. Nat Neurosci 18, 794–799.

48.

Selkoe

(2001) Alzheimer’s disease results from the cerebral accumulation and cytotoxicity of amyloid beta-protein. J Alzheimers Dis 3, 75–80.

49.

McKhann

, Knopman

, Chertkow

, Hyman

, Jack

Jr , Kawas

, Klunk

, Koroshetz

, Manly

, Mayeux

, Mohs

, Morris

, Rossor

, Scheltens

, Carrillo

, Thies

, Weintraub

, Phelps

(2011) The diagnosis of dementia due to Alzheimer’s disease: Recommendations from the National Institute on Aging-Alzheimer’s Association workgroups on diagnostic guidelines for Alzheimer’s disease. Alzheimers Dement 7, 263–269.

50.

Morris

, Heyman

, Mohs

, Hughes

, van Belle

, Fillenbaum

, Mellits

, Clark

(1989) The consortium to establish a registry for Alzheimer’s disease (CERAD). Part I. Clinical and neuropsychological assessment of Alzheimer’s disease. Neurology 39, 1159–1165.

51.

Jack

Jr , Albert

, Knopman

, McKhann

, Sperling

, Carrillo

, THies

, Phelps

(2011) Introduction to the recommendations from the National Institute on Aging - Alzheimer’s Association workgroups on diagnostic guidelines for Alzheimer’s disease. Alzheimers Dement 7, 257–262.

52.

Jack

Jr , Bennett

, Blennow

, Carrillo

, Dunn

, Haeberlein

, Holtzman

, Jagust

, Jessen

, Karlawish

, Liu

, Molinuevo

, Montine

, Phelps

, Rankin

, Rowe

, Scheltens

, Siemers

, Snyder

, Sperling

; Contributors (2018) NIA-AA Research Framework: Toward a biological definition of Alzheimer’s disease. Alzheimers Dement 14, 535–562.

53.

Mirra

, Heyman

, McKeel

, Sumi

, Crain

, Brownlee

, Vogel

, Hughes

, van Belle

, Berg

(1991) The consortium to establish a registry for Alzheimer’s disease (CERAD). Part II. Standardization of the neuropathologic assessment of Alzheimer’s disease. Neurology 41, 479–486.

54.

Hyman

, Phelps

, Beach

, Bigio

, Cairns

, Carrillo

, Dickson

, Duyckaerts

, Frosch

, Masliah

, Mirra

, Nelson

, Schneider

, Thal

, Thies

, Trojanowski

, Vinters

, Montine

(2012) National Institute on Aging - Alzheimer’s Association guidelines for the neuropathologic assessment of Alzheimer’s disease. Alzheimers Dement 8, 1–13.

55.

Mayeux

, Saunders

, Shea

, Mirra

, Evans

, Roses

, Hyman

, Crain

, Tang

, Phelps

(1998) Utility of the apolipoprotein E genotype in the diagnosis of Alzheimer’s disease. Alzheimer’s Disease Centers Consortium on Apolipoprotein E and Alzheimer’s Disease. N Engl J Med 338, 506–511.

56.

Richard

, Amouyel

(2001) Genetic susceptibility factors for Alzheimer’s disease. Eur J Pharmacol 412, 1–12.

57.

Zick

, Mathews

, Roberts

, Cook-Deegan

, Pokorski

, Green

(2005) Genetic testing for Alzheimer’s disease and its impact on insurance purchasing behavior. Health Aff (Millwood) 24, 483–490.

58.

Jeong

(2017) Molecular and cellular basis of neurodegeneration in Alzheimer’s disease. Mol Cells 40, 613–620.

59.

Hung

, Haass

, Nitsch

, Qiu

, Citron

, Wurtman

, Growdon

, Selkoe

(1993) Activation of protein kinase C inhibits cellular production of the amyloid beta-protein. J Biol Chem 268, 22959–22962.

60.

Lin

, Georgievska

, Mattsson

, Isacson

(1999) Cognitive changes and modified processing of amyloid precursor protein in the cortical and hippocampal system after cholinergic synapse loss and muscarinic receptor activation. Proc Natl Acad Sci U S A 96, 12108–12113.

61.

Skovronsky

, Moore

, Milla

, Doms

, Lee

(2000) Protein kinase C-dependent alfa-secretase competes with gamma-secretase for cleavage of amyloid-beta precursor protein in the trans-golgi network. J Biol Chem 275, 2568–2575.

62.

Cai

, Wang

, McCarthy

, Wen

, Borchelt

, Price

, Wong

(2001) BACE1 is the major beta-secretase for generation of Abeta peptides by neurons. Nat Neurosci 4, 233–234.

63.

Luo

, Bolon

, Kahn

, Bennett

, Babu-Khan

, Denis

, Fan

, Kha

, Zhang

, Gong

, Martin

, Louis

, Yan

, Richards

, Citron

, Vassar

(2001) Mice deficient in BACE1, the Alzheimer’s beta-secretase, have normal phenotype and abolished beta-amyloid generation. Nat Neurosci 4, 231–232.

64.

Lambert

, Barlow

, Chromy

, Edwards

, Freed

, Liosatos

, Morgan

, Rozovsky

, Trom- mer

, Viola

, Wals

, Zhang

, Finch

, Krafft

, Klein

(1998) Diffusible, nonfibrillar ligands derived from Abeta1-42 are potent central nervous system neurotoxins. Proc Natl Acad Sci U S A 95, 6448–6453.

65.

Cleary

, Walsh

, Hofmeister

, Shankar

, Kuskowski

, Selkoe

, Ashe

(2005) Natural oligomers of the amyloid-beta protein specifically disrupt cognitive function. Nat Neurosci 8, 79–84.

66.

Bao

, Wicklund

, Lacor

, Klein

, Nordberg

, Marutle

(2012) Different β-amyloid oligomer assemblies in Alzheimer brains correlate with age of disease onset and impaired cholinergic activity. Neurobiol Aging 33, 825.e1–825.e13.

67.

Teplow

(2013) On the subject of rigor in the study of amyloid β-protein assembly. Alzheimers Res Ther 5, 39.

68.

Ono

(2018) Alzheimer’s disease as oligomeropathy. Neurochem Int 119, 57–70.

69.

Cline

, Bicca

, Viola

, Klein

(2018) The amyloid-β oligomer hypothesis: Beginning of the third decade. J Alzheimers Dis 64, S567–S610.

70.

Kojro

, Fahrenholz

(2005) The non-amyloidogenic pathway: Structure and function of alpha-secretases. Subcell Biochem 38, 105–127.

71.

Fahrenholz

, Postina

(2006) Alpha-secretase activation - an approach to Alzheimer’s disease therapy. Neurodegener Dis 3, 255–261.

72.

Kuruva

, Reddy

(2017) Amyloid beta modulators and neuroprotection in Alzheimer’s disease: A critical appraisal. Drug Discov Today 22, 223–233.

73.

Habib

, Sawmiller

, Tan

(2017) Restoring soluble amyloid precursor protein α functions as a potential treatment for Alzheimer’s disease. J Neurosci Res 95, 973–991.

74.

Bruinsma

, Wilhelmus

, Kox

, Veerhuis

, de Waal

, Verbeek

(2010) Apolipoprotein E protects cultured pericytes and astrocytes from D-Abeta (1-40)-mediated cell death. Brain Res 1315, 169–180.

75.

Iwata

, Tsubuki

, Takaki

, Shirotani

, Lu

, Gerard

, Hama

, Lee

, Saido

(2001) Metabolic regulation of brain Abeta by neprilysin. Science 292, 1550–1552.

76.

Iwata

, Takaki

, Fukami

, Tsubuki

, Saido

(2002) Region-specific reduction of A beta-degrading endopeptidase, neprilysin, in mouse hippocampus upon aging. J Neurosci Res 70, 493–500.

77.

Mawuenyega

, Sigurdson

, Ovod

, Munsell

, Kasten

, Morris

, Yarasheski

, Bateman

(2010) Decreased clearance of CNS beta-amyloid in Alzheimer’s disease. Science 330, 1774.

78.

Bettcher

, Kramer

(2014) Longitudinal inflammation, cognitive decline, and Alzheimer’s disease: A mini-review. Clin Pharmacol Ther 96, 464–469.

79.

Estes

, McAllister

(2014) Alterations in immune cells and mediators in the brain: It’s not always neuroinflammation! Brain Pathol 24, 623–630.

80.

Heneka

, Carson

, El Khoury

, Landreth

, Brosseron

, Feinstein

, Jacobs

, Wyss-Coray

, Vitorica

, Ransohoff

, Herrup

, Frautschy

, Finsen

, Brown

, Verkhratsky

, Yamanaka

, Koistinaho

, Latz

, Halle

, Petzold

, Town

, Morgan

, Shinohara

, Perry

, Holmes

, Bazan

, Brooks

, Hunot

, Joseph

, Deigendesch

, Garaschuk

, Boddeke

, Dinarello

, Breitner

, Cole

, Golenbock

, Kummer

(2015) Neuroinflammation in Alzheimer’s disease. Lancet Neurol 14, 388–405.

81.

Moreira

, Honda

, Liu

, Santos

, Oliveira

, Aliev

, Nunomura

, Zhu

, Smith

, Perry

(2005) Oxidative stress: The old enemy in Alzheimer’s disease pathophysiology. Curr Alzheimer Res 2, 403–408.

82.

Moreira

, Cardoso

, Santos

, Oliveira

(2006) The key role of mitochondria in Alzheimer’s disease. J Alzheimers Dis 9, 101–110.

83.

Accardi

, Caruso

, Colonna-Romano

, Camarda

, Monastero

, Candore

(2012) Can Alzheimer disease be a form of type 3 diabetes? Rejuvenation Res 15, 217–221.

84.

Moreira

(2012) Alzheimer’s disease and diabetes: An integrative view of the role of mitochondria, oxidative stress, and insulin. J Alzheimers Dis 30, S199–S215.

85.

Hoyer

(2002) The aging brain. Changes in the neuronal insulin/insulin receptor signal transduction cascade trigger late-onset sporadic Alzheimer disease (SAD). A mini-review. J Neural Transm 109, 991–1002.

86.

Gasparini

, Netzer

, Greengard

, Xu

(2002) Does insulin dysfunction play a role in Alzheimer’s disease? Trends Pharmacol Sci 23, 288–293.

87.

Koudinov

, Koudinova

(2003) Cholesterol, synaptic function and Alzheimer’s disease. Pharmacopsychiatry 36, S107–S112.

88.

Koudinov

, Koudinova

(2005) Cholesterol homeostasis failure as a unifying cause of synaptic degeneration. J Neurol Sci 229-230, 233–240.

89.

Arendt

(2003) Synaptic plasticity and cell cycle activation in neurons are alternative effector pathways: The ‘Dr. Jekyll and Mr. Hyde concept’ of Alzheimer’s disease or the yin and yang of neuroplasticity. Prog Neurobiol 71, 83–248.

90.

Arendt

, Brückner

(2007) Linking cell-cycle dysfunction in Alzheimer’s disease to a failure of synaptic plasticity. Biochim Biophys Acta 1772, 413–421.

91.

Mahley

, Rall

Jr (2000) Apolipoprotein E: Far more than a lipid transport protein. Annu Rev Genomics Hum Genet 1, 507–537.

92.

Seripa

, Matera

, Daniele

, Bizzarro

, Rinaldi

, Gravina

, Bisceglia

, Corbo

, Panza

, Solfrizzi

, Fazio

, Forno

, Masullo

, Dallapiccola

, Pilotto

(2007) The missing ApoE allele. Ann Hum Genet 71, 496–500.

93.

Seripa

, Panza

, Franceschi

, D’Onofrio

, Solfrizzi

, Dallapiccola

, Pilotto

(2009) Non-apolipoprotein E and apolipoprotein E genetics of sporadic Alzheimer’s disease. Ageing Res Rev 8, 214–236.

94.

Seripa

, D’Onofrio

, Panza

, Cascavilla

, Masullo

, Pilotto

(2011) The genetics of the human APOE polymorphism. Rejuvenation Res 14, 491–500.

95.

Tsuang

, Larson

, Bowen

, McCormick

, Teri

, Nochlin

, Leverenz

, Peskind

, Lim

, Raskind

, Thompson

, Mirra

, Gearing

, Schellenberg

, Kukull

(1999) The utility of apolipoprotein E genotyping in the diagnosis of Alzheimer disease in a community-based case series. Arch Neurol 56, 1489–1495.

96.

Waldemar

, Dubois

, Emre

, Georges

, McKeith

, Rossor

, Scheltens

, Tariska

, Winblad B;

EFNS

(2007) Recommendations for the diagnosis and management of Alzheimer’s disease and other disorders associated with dementia: EFNS guideline. Eur J Neurol 14, E1–E26.

97.

Ertekin-Taner

(2007) Genetics of Alzheimer’s disease: A centennial review. Neurol Clin 25, 611–667.

98.

Roses

, Strittmatter

, Pericak-Vance

, Corder

, Saunders

, Schmechel

(1994) Clinical application of apolipoprotein E genotyping to Alzheimer’s disease. Lancet 343, 1564–1565.

99.

American College of Medical Genetics/American Society of Human Genetics Working Group on ApoE and Alzheimer disease (1995) Statement on use of apolipoprotein E testing for Alzheimer disease. JAMA 274, 1627–1629.

100.

Post

, Whitehouse

, Binstock

, Bird

, Eckert

, Farrer

, Fleck

, Gaines

, Juengst

, Karlinsky

, Miles

, Murray

, Quaid

, Relkin

, Roses

, St George-Hyslop

, Sachs

, Steinbock

, Truschke

, Zinn

(1997) The clinical introduction of genetic testing for Alzheimer disease. An ethical perspective. JAMA 277, 832–836.

101.

Roses

(1997) Genetic testing for Alzheimer’s disease. Arch Neurol 54, 1226–1229.

102.

Bird

(2008) Genetic aspects of Alzheimer’s disease. Genet Med 10, 231–239.

103.

Strittmatter

, Saunders

, Schmechel

, Pericak-Vance

, Enghild

, Salvesen

, Roses

(1993) Apolipoprotein E: High-avidity binding to beta-amyloid and increased frequency of type 4 allele in late-onset familial Alzheimer disease. Proc Natl Acad Sci U S A 90, 1977–1981.

104.

Fagan

, Holtzman

(2000) Astrocyte lipoproteins, effects of apoE on neuronal function, and role of apoE in amyloid-beta deposition in vivo. Microsc Res Tech 50, 297–304.

105.

Bentley

, Ladu

, Rajan

, Getz

, Reardon

(2002) Apolipoprotein E structural requirements for the formation of SDS-stable complexes with beta-amyloid-(1–40): The role of salt bridges. Biochem J 366, 273–279.

106.

Arélin

, Kinoshita

, Whelan

, Irizarry

, Rebeck

, Strickland

, Hyman

(2002) LRP and senile plaques in Alzheimer’s disease: Colocalization with apolipoprotein E and with activated astrocytes. Brain Res Mol Brain Res 104, 38–46.

107.

Tokuda

, Calero

, Matsubara

, Vidal

, Kumar

, Permanne

, Zlokovic

, Smith

, Ladu

, Rostagno

, Frangione

, Ghiso

(2002) Lipidation of apolipoprotein E influences its isoform-specific interaction with Alzheimer’s amyloid beta peptides. Biochem J 348, 359–365.

108.

Deane

, Sagare

, Hamm

, Parisi

, Lane

, Finn

, Holtzman

, Zlokovic

(2008) ApoE isoform-specific disruption of amyloid beta peptide clearance from mouse brain. J Clin Invest 118, 4002–4013.

109.

Petrlova

, Hong

, Bricarello

, Harishchandra

, Lorigan

, Jin

, Voss

(2011) A differential association of Apolipoprotein E isoforms with the amyloid-β oligomer in solution. Proteins 79, 402–416.

110.

Dafnis

, Argyri

, Sagnou

, Tzinia

, Tsilibary

, Stratikos

, Chroni

(2016) The ability of apolipoprotein E fragments to promote intraneuronal accumulation of amyloid beta peptide 42 is both isoform and size-specific. Sci Rep 6, 30654.

111.

Koistinaho

, Lin

, Wu

, Esterman

, Koger

, Hanson

, Higgs

, Liu

, Malkani

, Bales

, Paul

(2004) Apolipoprotein E promotes astrocyte colocalization and degradation of deposited amyloid-β peptides. Nat Genet 10, 719–726.

112.

Castellano

, Kim

, Stewart

, Jiang

, DeMattos

, Patterson

, Fagan

, Morris

, Mawuenyega

, Cruchaga

, Goate

, Bales

, Paul

, Bateman

, Holtzman

(2011) Human apoE isoforms differentially regulate brain amyloid-β peptide clearance. Sci Transl Med 3, 89ra57.

113.

, Yee

, Brewer

, Das

, Potter

(1994) The amyloid-associated proteins a1-antichymotrypsin and apolipoprotein E promote the assembly of the Alzheimer β-protein into filaments. Nature 372, 92–94.

114.

Wisniewski

, Castano

, Golabek

, Vogel

, Frangione

(1994) Acceleration of Alzheimer’s fibril formation by apolipoprotein E in vitro. Am J Pathol 145, 1030–1035.

115.

Evans

, Berger

, Cho

, Weisgraber

, Lansbury

(1995) Apolipoprotein E is a kinetic but not a thermodynamic inhibitor of amyloid formation: Implications for the pathogenesis and treatment of Alzheimer disease. Proc Natl Acad Sci U S A 92, 763–767.

116.

Huynh

, Davis

, Ulrich

, Holtzman

(2017) Apolipoprotein E and Alzheimer’s disease: The influence of apolipoprotein E on amyloid-β and other amyloidogenic proteins. J Lipid Res 58, 824–836.

117.

Robert

, Button

, Yuen

, Gilmour

, Kang

, Bahrabadi

, Stukas

, Zhao

, Kulic

, Wellington

(2017) Clearance of beta-amyloid is facilitated by apolipoprotein E and circulating high-density lipoproteins in bioengineered human vessels. Elife 6, e29595.

118.

Younkin

(1995) Evidence that A beta 42 is the real culprit in Alzheimer’s disease. Ann Neurol 37, 287–288.

119.

Lauriola

, Paroni

, Ciccone

, D Onofrio

, Cascavilla

, Paris

, Gravina

, Urbano

, Seripa

, Greco

(2018) Erythrocyte associated amyloid-β as potential biomarker to diagnose dementia. Curr Alzheimer Res 15, 381–385.

120.

Thal

, Rüb

, Orantes

, Braak

(2002) Phases of A beta-deposition in the human brain and its relevance for the development of AD. Neurology 58, 1791–1800.

121.

Duyckaerts

, Potier

, Delatour

(2008) Alzheimer disease models and human neuropathology: Similarities and differences. Acta Neuropathol 115, 5–38.

122.

Hunter

, Arendt

, Brayne

(2013) The senescence hypothesis of disease progression in Alzheimer disease: An integrated matrix of disease pathways for FAD and SAD. Mol Neurobiol 48, 556–570.

123.

Neve

, Robakis

(1998) Alzheimer’s disease: A re-examination of the amyloid hypothesis. Trends Neurosci 21, 15–19.

124.

Lee

, Casadesus

, Zhu

, Joseph

, Perry

, Smith

(2004) Perspectives on the amyloid-beta cascade hypothesis. J Alzheimers Dis 6, 137–145.

125.

Hardy

(2009) The amyloid hypothesis for Alzheimer’s disease: A critical reappraisal. J Neurochem 110, 1129–1134.

126.

Pimplikar

(2009) Reassessing the amyloid cascade hypothesis of Alzheimer’s disease. Int J Biochem Cell Biol 41, 1261–1268.

127.

Mondragón-Rodríguez

, Basurto-Islas

, Lee

, Perry

, Zhu

, Castellani

, Smith

(2010) Causes versus effects: The increasing complexities of Alzheimer’s disease pathogenesis. Expert Rev Neurother 10, 683–691.

128.

Armstrong

(2014) A critical analysis of the “amyloid cascade hypothesis”. Folia Neuropathol 52, 211–225.

129.

Castello

, Soriano

(2014) On the origin of Alzheimer’s disease. Trials and tribulations of the amyloid hypothesis. Ageing Res Rev 13, 10–12.

130.

Morris

, Clark

, Vissel

(2014) Inconsistencies and controversies surrounding the amyloid hypothesis of Alzheimer’s disease. Acta Neuropathol Commun 2, 135.

131.

Karran

, De Strooper

(2016) The amyloid cascade hypothesis: Are we poised for success or failure? J Neurochem 139, 237–252.

132.

Ricciarelli

, Fedele

(2017) The amyloid cascade hypothesis in Alzheimer’s disease: It’s time to change our mind. Curr Neuropharmacol 15, 926–935.

133.

Kepp

(2017) Ten challenges of the amyloid hypothesis of Alzheimer’s disease. J Alzheimers Dis 55, 447–457.

134.

Joseph

, Shukitt-Hale

, Denisova

, Martin

, Perry

, Smith

(2001) Copernicus revisited: Amyloid beta in Alzheimer’s disease. Neurobiol Aging 22, 131–146.

135.

Zhu

, Raina

, Perry

, Smith

(2004) Alzheimer’s disease: The two-hit hypothesis. Lancet Neurol 3, 219–226.

136.

Castellani

, Lee

, Zhu

, Nunomura

, Perry

, Smith

(2006) Neuropathology of Alzheimer disease: Pathognomonic but not pathogenic. Acta Neuropathol 111, 503–509.

137.

Lee

, Zhu

, Nunomura

, Perry

, Smith

(2006) Amyloid beta: The alternate hypothesis. Curr Alzheimer Res 3, 75–80.

138.

Castellani

, Lee

, Siedlak

, Nunomura

, Hayashi

, Nakamura

, Zhu

, Perry

, Smith

(2009) Reexamining Alzheimer’s disease: Evidence for a protective role for amyloid-beta protein precursor and amyloid-beta. J Alzheimers Dis 18, 447–452.

139.

Harman

(1996) A hypothesis on the pathogenesis of Alzheimer’s disease. Ann N Y Acad Sci 786, 152–168.

140.

Swerdlow

, Burns

, Khan

(2014) The Alzheimer’s disease mitochondrial cascade hypothesis: Progress and perspectives. Biochim Biophys Acta 1842, 1219–1231.

141.

Harman

(2000) Alzheimer’s disease: A hypothesis on pathogenesis. J Am Aging Assoc 23, 147–161.

142.

Smith

, Sayre

, Monnier

, Perry

(1995) Radical AGEing in Alzheimer’s disease. Trends Neurosci 18, 172–176.

143.

Webber

, Raina

, Marlatt

, Zhu

, Prat

, Morelli

, Casadesus

, Perry

, Smith

(2005) The cell cycle in Alzheimer disease: A unique target for neuropharmacology. Mech Ageing Dev 126, 1019–1025.

144.

Zhu

, Lee

, Perry

, Smith

(2007) Alzheimer disease, the two-hit hypothesis: An update. Biochim Biophys Acta 1772, 494–502.

145.

Itzhaki

, Lathe

, Balin

, Ball

, Bearer

, Braak

, Bullido

, Carter

, Clerici

, Cosby

, Del Tredici

, Field

, Fulop

, Grassi

, Griffin

, Haas

, Hudson

, Kamer

, Kell

, Licastro

, Letenneur

, Lövheim

, Mancuso

, Miklossy

, Otth

, Palamara

, Perry

, Preston

, Pretorius

, Strandberg

, Tabet

, Taylor-Robinson

, Whittum-Hudson

(2016) Microbes and Alzheimer’s disease. J Alzheimers Dis 51, 979–84.

146.

Kumar

, Choi

, Washicosky

, Eimer

, Tucker

, Ghofrani

, Lefkowitz

, McColl

, Goldstein

, Tanzi

, Moir

(2016) Amyloid-β peptide protects against microbial infection in mouse and worm models of Alzheimer’s disease. Sci Transl Med 8, 340ra72.

147.

Bourgade

, Dupuis

, Frost

, Fülöp

(2016) Anti-viral properties of amyloid- β peptides. J Alzheimers Dis 54, 859–878.

148.

Fulop

, Witkowski

, Bourgade

, Khalil

, Zerif

, Larbi

, Hirokawa

, Pawelec

, Bocti

, Lacombe

, Dupuis

, Frost

(2018) Can an infection hypothesis explain the beta amyloid hypothesis of Alzheimer’s disease? Front Aging Neurosci 10, 224.

149.

Pasinetti

(1996) Inflammatory mechanisms in neurodegeneration and Alzheimer’s disease: The role of the complement system. Neurobiol Aging 17, 707–716.

150.

Cai

, Hussain

, Yan

(2014) Microglia, neuroinflammation, and beta-amyloid protein in Alzheimer’s disease. Int J Neurosci 124, 307–321.

151.

Sochocka

, Zwolińska

, Leszek

(2017) The infectious etiology of Alzheimer’s disease. Curr Neuropharmacol 15, 996–1009.

152.

Bolós

, Perea

, Avila

(2017) Alzheimer’s disease as an inflammatory disease. Biomol Concepts 8, 37–43.

153.

Drachman

(2014) The amyloid hypothesis, time to move on: Amyloid is the downstream result, not cause, of Alzheimer’s disease. Alzheimers Dement 10, 372–80.

154.

de la Torre

(2002) Alzheimer disease as a vascular disorder: Nosological evidence. Stroke 33, 1152–1162.

155.

, Liu

, Kanekiyo

(2013) Vascular hypothesis of Alzheimer’s disease: Role of apoE and apoE receptors. Mol Neurodegener 8, O20.

156.

Hunter

, Smailagic

, Brayne

(2018) Aβ and the dementia syndrome: Simple versus complex perspectives. Eur J Clin Invest 48, e13025.

157.

Khachaturian

(2000) Toward a comprehensive theory of Alzheimer’s disease-challenges, caveats, and parameters. Ann N Y Acad Sci 924, 184–193.

158.

Lee

, Casadesus

, Zhu

, Takeda

, Perry

, Smith

(2004) Challenging the amyloid cascade hypothesis: Senile plaques and amyloid-beta as protective adaptations to Alzheimer disease. Ann N Y Acad Sci 1019, 1–4.

159.

Sommer

(2002) Alzheimer’s disease and the amyloid cascade hypothesis: Ten years on. Curr Opin Pharmacol 2, 87–92.

160.

Lee

, Castellani

, Zhu

, Perry

, Smith

(2005) Amyloid-beta in Alzheimer’s disease: The horse or the cart? Pathogenic or protective? Int J Exp Pathol 86, 133–138.

161.

Lemere

, Masliah

(2010) Can Alzheimer disease be prevented by amyloid-beta immunotherapy? Nat Rev Neurol 6, 108–119.

162.

Anand

, Gill

, Mahdi

(2014) Therapeutics of Alzheimer’s disease: Past, present and future. Neuropharmacology 76 Pt A, 27–50.

163.

Karran

, Hardy

(2014) A critique of the drug discovery and phase 3 clinical programs targeting the amyloid hypothesis for Alzheimer disease. Ann Neurol 76, 185–205.

164.

Hyman

, Sorger

(2014) Failure analysis of clinical trials to test the amyloid hypothesis. Ann Neurol 76, 159–61.

165.

Harrison

, Owen

(2016) Alzheimer’s disease: The amyloid hypothesis on trial. Br J Psychiatry 208, 1–3.

166.

Amanatkar

, Papagiannopoulos

, Grossberg

(2017) Analysis of recent failures of disease modifying therapies in Alzheimer’s disease suggesting a new methodology for future studies. Expert Rev Neurother 17, 7–16.

167.

Makin

(2018) The amyloid hypothesis on trial. Nature 559, S4–S7.

168.

Kametani

, Hasegawa

(2018) Reconsideration of amyloid hypothesis and tau hypothesis in Alzheimer’s disease. Front Neurosci 12, 25.

169.

Braak

, Del Tredici

(2014) Are cases with tau pathology occurring in the absence of Abeta deposits part of the AD-related pathological process? Acta Neuropathol 128, 767–772.

170.

Johnson

, Schultz

, Betensky

, Becker

, Sepulcre

, Rentz

, Mormino

, Chhatwal

, Amariglio

, Papp

, Marshall

, Albers

, Mauro

, Pepin

, Alverio

, Judge

, Philiossaint

, Shoup

, Yokell

, Dickerson

, Gomez-Isla

, Hyman

, Vasdev

, Sperling

(2015) Tau PET imaging in aging and early Alzheimer’s disease. Ann Neurol 79, 110–119.

171.

Braak

, Braak

(1991) Neuropathological stageing of Alzheimer-related changes. Acta Neuropathol 82, 239–259.

172.

Takahashi

, Miyata

, Kametani

, Nonaka

, Akiyama

, Hisanaga

, Hasegawa

(2015) Extracellular association of APP and tau fibrils induces intracellular aggregate formation of tau. Acta Neuropathol 129, 895–907.

173.

Rapoport

, Dawson

, Binder

, Vitek

, Ferreira

(2002) Tau is essential to β-amyloid-induced neurotoxicity. Proc Natl Acad Sci U S A 99, 6364–6369.

174.

SantaCruz

, Lewis

, Spires

, Paulson

, Kotilinek

, Ingelsson

, Guimaraes

, DeTure

, Ramsden

, McGowan

, Forster

, Yue

, Orne

, Janus

, Mariash

, Kuskowski

, Hyman

, Hutton

, Ashe

(2005) Tau suppression in a neurodegenerative mouse model improves memory function. Science 309, 476–481.

175.

Roberson

, Scearce-Levie

, Palop

, Yan

, Cheng

, Wu

, Gerstein

, Yu

, Mucke

(2007) Reducing endogenous tau ameliorates amyloid beta-induced deficits in an Alzheimer’s disease mouse model. Science 316, 750–754.

176.

Miao

, Chen

, Zhang

, Sun

(2010) Deletion of tau attenuates heat shock-induced injury in cultured cortical neurons. J Neurosci Res 88, 102–110.

177.

Shipton

, Leitz

, Dworzak

, Acton

CEJ

, Tunbridge

, Denk

, Dawson

, Vitek

, Wade-Martins

, Paulsen

, Vargas-Caballero

(2011) Tau protein is required for amyloid β-induced impairment of hippocampal long-term potentiation. J Neurosci 31, 1688–1692.

178.

, Gladbach

, Van Eersel

, Ittner

, Przybyla

, Van Hummel

, Chua

, van der Hoven

, Lee

, Müller

, Parmar

, Jonquieres

, Stefen

, Guccione

, Fath

, Housley

, Klugmann

, Ke

, Ittner

(2017) Tau exacerbates excitotoxic brain damage in an animal model of stroke. Nat Commun 8, 473.

179.

Jack

Jr , Bennett

, Blennow

, Carrillo

, Feldman

, Frisoni

, Hampel

, Jagust

, Johnson

, Knopman

, Petersen

, Scheltens

, Sperling

, Dubois

(2016) A/T/N: An unbiased descriptive classification scheme for Alzheimer disease biomarkers. Neurology 87, 539–547.

180.

Sperling

, Aisen

, Beckett

, Bennett

, Craft

, Fagan

, Iwatsubo

, Jack CR

, Kaye

, Montine

, Park

, Reiman

, Rowe

, Siemers

, Stern

, Yaffe

, Carrillo

, Thies

, Morrison-Bogorad

, Wagster

, Phelps

(2011) Toward defining the preclinical stages of Alzheimer’s disease: Recommendations from the National Institute on Aging-Alzheimer’s Association workgroups on diagnostic guidelines for Alzheimer’s disease. Alzheimers Dement 7, 280–292.

181.

Montine

, Phelps

, Beach

, Bigio

, Cairns

, Dickson

, Duyckaerts

, Frosch

, Masliah

, Mirra

, Nelson

, Schneider

, Thal

, Trojanowski

, Vinters

, Hyman

; National Institute on Aging; Alzheimer’s Association (2012) National Institute on Aging-Alzheimer’s Association guidelines for the neuropathologic assessment of Alzheimer’s disease: A practical approach. Acta Neuropathol 123, 1–11.

182.

Gustaw-Rothenberg

, Lerner

, Bonda

, Lee

, Zhu

, Perry

, Smith

(2010) Biomarkers in Alzheimer’s disease: Past, present and future. Biomark Med 4, 15–26.

183.

Human Genome Organisation (HUGO) Gene Nomenclature Committee (HGNC). Available at URL https://www.genenames.org

184.

Hardy

(1997) The Alzheimer family of diseases: Many etiologies, one pathogenesis? Proc Natl Acad Sci U S A 94, 2095–2097.

185.

Small

(1998) The Sixth International Conference on Alzheimer’s disease, Amsterdam. The Netherlands, July 1998. The amyloid cascade hypothesis debate: Emerging consensus on the role of A beta and amyloid in Alzheimer’s disease. Amyloid 5, 301–304.

186.

Maccioni

, Farías

, Morales

, Navarrete

(2010) The revitalized tau hypothesis on Alzheimer’s disease. Arch Med Res 41, 226–231.

187.

Bonda

, Castellani

, Zhu

, Nunomura

, Lee

, Perry

, Smith

(2011) A novel perspective on tau in Alzheimer’s disease. Curr Alzheimer Res 8, 639–642.

188.

Harman

(2002) Alzheimer’s disease: Role of aging in pathogenesis. it Ann N Y Acad Sci, 959, 384–395; discussion 463-465.

189.

Swerdlow

, Khan

(2004) A “mitochondrial cascade hypothesis” for sporadic Alzheimer’s disease. Med Hypotheses 63, 8–20.

190.

Harman

(2006) Alzheimer’s disease pathogenesis: Role of aging. Ann N Y Acad Sci 1067, 454–460.

191.

Castellani

, Zhu

, Lee

, Smith

, Perry

(2009) Molecular pathogenesis of Alzheimer’s disease: Reductionist versus expansionist approaches. Int J Mol Sci 10, 1386–1406.

192.

Bishop

, Robinson

(2002) The amyloid hypothesis: Let sleeping dogmas lie? Neurobiol Aging 23, 1101–1105.

193.

Shen

, Kelleher

3rd (2007) The presenilin hypothesis of Alzheimer’s disease: Evidence for a loss-of-function pathogenic mechanism. Proc Natl Acad Sci U S A 104, 403–409.

194.

Castellani

, Zhu

, Lee

, Moreira

, Perry

, Smith

(2007) Neuropathology and treatment of Alzheimer disease: Did we lose the forest for the trees? Expert Rev Neurother 7, 473–485.