1 A Brief Introduction to the History of the β

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A Brief Introduction to the History of the β-Amyloid Protein (Aβ) of Alzheimer’s Disease

David H. Small and Colin J. Barrow

1 Alzheimer’s disease (AD) is the most common

cause of dementia in the elderly. Typically, the disease progresses in a prolonged, inexorable manner [1]. Patients initially show symptoms of mild cognitive impairment, which may include some memory loss. As the disease progresses, more severe memory loss occurs (e.g., retrograde amnesia) leading to confusion and lack of orientation. The patient is often institutionalized in this period, as it becomes increasingly difficult for family members to cope with the constant requirements of care. In later stages of the disease, apathy and stupor can occur, and the patient becomes bedridden.

The histopathology of AD is characterized by gliosis and tissue atrophy caused by both synaptic and neuronal loss, which are most pronounced in the frontal and temporal cortices [2]. Proteinaceous deposits are seen in both the intracellular and extracellular compartments of the brain, typically in the hippocampus and neocortex. The intracellular deposits consist of neurofibrillary tangles that are made up of paired helical filaments of a hyper- phosphorylated form of the cytoskeletal protein tau [3]. Extracellular amyloid plaques are found most commonly in the hippocampus and neocortex and may be diffuse or compact in nature [4]. Amyloid is also deposited as cerebral amyloid angiopathy within small- to medium-sized arterioles [5].

Although neurofibrillary tangles are associated with a number of different types of neurodegenera- tive disease, the presence of numerous compact or neuritic amyloid plaques is a hallmark feature of Alzheimer’s disease. For this reason, it may be argued that accumulation of the β-amyloid protein

(Αβ) is a key step in the pathogenic mechanism of Alzheimer’s disease. In contrast, although the den- sity of neurofibrillary tangles correlates more closely with the cognitive symptoms, it is now commonly thought that tangles are a secondary feature or the underlying disease process [6].

1.1 The Role of Aβ in AD

Glenner and Wong [7] first identified the major protein component of vascular amyloid, which was a low-molecular-weight, 4-kDa polypeptide, now referred to as the β-amyloid protein (Αβ). Subse- quent studies established that the same protein was the major component of amyloid plaques [8]. The complete amino acid sequence of Αβ led to the identification of its precursor, the β-amyloid precursor protein (APP) [9].

APP has features of an integral type I transmembrane glycoprotein, with a large ectodomain containing the N-terminus and a small cytoplasmic domain containing the C-terminus (Fig. 1.1).

Multiple mRNA splicing of exons can generate several different isoforms of APP that lack domains homologous to Kunitz-type protease inhibitors (KPI domain) and the OX-2 antigen as well as a domain encoded by an exon that regulates O-linked glycosylation by chondroitin sulfate. The Αβ sequence itself comprises part of the ectodomain of the protein and extends into, but not all the way through, the transmembrane domain [9, 10].

Soon after its identification, APP was shown to undergo ectodomain shedding by an enzyme

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dubbed the α-secretase. The α-secretase cleaves APP within the Αβ sequence, adjacent to lysine-16, thereby destroying the sequence [11, 12]. Recently studies suggest that enzymes of the ADAM family of metalloproteases are responsible for this activity [13, 14]. Other studies have demonstrated that APP can also be cleaved at the N- and C-terminal ends of the Αβ sequence by enzymes dubbed β- and γ-secretase, respectively, to generate the full- lengthΑβ sequence [15]. Amyloidogenic processing by β- and γ-secretase is a normal, albeit minor, pathway of APP processing. The β-secretase has been unequivocally identified as an aspartyl protease termed BACE1 (an acronym for β-site APP cleaving enzyme-1) [16–19]. The γ-secretase comprises a complex of several proteins including presenilin-1, presenilin-2, Aph1, Pen2, and nicas- trin. However, other protein components of this complex may also exist [15].

There is considerable evidence that the accumulation of Αβ in the brain is toxic to neurons and that this toxicity underlies the neurodegeneration that occurs in AD (Fig. 1.1) [20]. Aβ peptides are toxic to cells in culture [21], and this toxicity is

associated with aggregation of the peptide [22].

Recent studies support the view that the most toxic species are the low-molecular-weight, soluble oligomers of Αβ [23].

Despite many studies that have shown that Αβ can disrupt biochemical events within neurons, direct proof that the accumulation of Αβ is the cause of AD has been lacking. Nevertheless, evidence that this is the case has slowly been accumu- lating. Some of the strongest evidence that Αβ accumulation is the cause, rather than an epiphe- nomenon, of AD has come from the finding of familial AD mutations present in the APP gene [24]. All of these mutations have been found to cluster around the Αβ sequence, and all of them have so far been shown to directly or indirectly cause an increase in forms of Αβ that aggregate [25]. For example, although the most commonly produced form of Αβ contains 40-amino-acid residues (Αβ40), a minor form containing 42 residues is also formed. This minor form aggregates into amyloid fibrils much more readily than Αβ40 [26].

The first mutation to be identified in the APP gene, the London mutation, involves a single base change at codon 717, which encodes a form of APP that is more readily cleaved to produce Αβ42. To date, at least 10 familial AD mutations are known to occur in APP [27].

The direct involvement of APP and Αβ in the pathogenesis of AD is also strongly supported by studies on transgenic mice. A number of transgenic lines have been developed in which human APP is expressed [28]. Many of these mice develop amyloid plaques. In addition, other features of AD pathology such as neuritic dystrophy, abnormal tau phosphorylation, gliosis, synaptic loss, and behav- ioral abnormalities have been observed. Although human APP mice do not develop neurofibrillary tangles, this is probably due to differences between mouse tau and human tau isoforms. Indeed, in double transgenic mice expressing both mutant human tau and APP, Αβ is seen to increase tau deposition [29].

Mutations in the APP gene account for only a very small percentage of all familial Alzheimer’s disease (FAD) cases. Shortly after the identification of the first familial AD mutation in the APP gene, mutations were identified in two other genes, PS1 encoding presenilin-1 and PS2 encoding

2 D.H. Small and C.J. Barrow

c APP

C99 N

α-cleavage β-cleavage

Aβ

γ-cleavage

Aβ oligomer γ-cleavage

C83

p3

Neurotoxicity AD aggregation

FIGURE1.1. Proteolytic processing of APP and the role of Aβ in AD. APP can be proteolytically cleaved via two different processing pathways. Cleavage by α-secretase and γ-secretase generates C-terminal fragments known as C83 and p3, whereas cleavage by β-secretase (BACE1) and γ-secretase generates C99 and Aβ.

According to the amyloid hypothesis, Aβ aggregates into amyloid fibrils or low-molecular-weight oligomers that are neurotoxic. The resulting neurotoxicity causes neurodegeneration and leads to dementia.

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presenilin-2, located on chromosomes 14 and 1, respectively [30, 31]. Both presenilin proteins are components of the γ-secretase complex, and familial AD mutations within the PS1 and PS2 genes alterγ-secretase processing in a way that leads to the production of more Αβ42 [32].

In general, mutations in the APP, PS1, and PS2 genes lead to early-onset forms of AD. In contrast, the apolipoprotein E (apoE) gene located on chromosome 19 is a risk factor for late-onset AD [33]. There are three forms of apoE, termed E2, E3, and E4, encoded by three allelic variants ε2, ε3, and ε4. The ε4 variant is a risk factor for late-onset AD, whereas the ε2 may be protective. Although the reason for this is still unknown, it is undoubtedly related to Aβ production, aggregation, or clearance from the brain. Individuals with the ε4 allele have more Aβ deposition within the brain [34]. In addition, APP x apoE knockout transgenic mice develop little amyloid deposition in their brains, unlike normal APP mice [35]. Thus, studies on the role of apoE in AD provide strong support for the Aβ hypothesis.

1.2 Anti-Αβ Therapies for AD

The idea that Aβ is a primary causative agent in AD leads inevitably to the view that an effective therapy based on inhibiting the production, aggregation, clearance, or toxicity of Aβ may be achiev- able. One of the most promising but controversial approaches in recent years has been Αβ immunization. Studies show that in transgenic mice, immunization with Αβ42 leads to the generation of an immune response [36]. Anti-amyloid antibodies bind to amyloid plaques and appear to facilitate their removal from the brain, leading to an improvement in cognitive performance compared with nonimmunized control animals. Unfortu- nately, clinical trials of this approach in humans have been halted because a small percentage of individuals immunized with Αβ have developed a severe meningoencephalitis [37]. Nevertheless, there is some evidence that patients who develop a strong immune response to Αβ without the associated brain inflammation may benefit from this approach [38].

1.3 Current Status of the Aβ Hypothesis of AD

There is now very strong evidence that accumulation of oligomeric or fibrillar Αβ in the brain is a key event in the pathogenesis of AD. Perhaps the most important unresolved question is the mechanism by which Αβ causes its neurotoxic effect. It is also unclear what form of aggregated Αβ is the most neurotoxic. Another major question is how many unidentified genetic risk factors there are and how these risk factors affect Αβ production, aggregation, or clearance. If anti-Αβ therapies can be used successfully for the treatment of AD, then the remaining concerns about the role of Αβ in the pathogenesis of AD will have been answered.

References

1. Storey E, Kinsella GJ, Slavin MJ. The neuropsycho- logical diagnosis of Alzheimer’s disease. J Alzheimers Dis 2001; 3:261-285.

2. Probst A, Langui D, Ulrich J. Alzheimer’s disease: a description of the structural lesions. Brain Pathol 1991; 1:229-239.

3. Iqbal K, Alonso Adel C, Chen S, et al. Tau pathology in Alzheimer’s disease and other tauopathies.

Biochim Biophys Acta 2005; 1739:198-210.

4. Wisniewski HM, Wegiel J, Kotula L. Review. David Oppenheimer Memorial Lecture 1995: Some neu- ropathological aspects of Alzheimer’s disease and its relevance to other disciplines. Neuropathol Appl Neurobiol 1996; 22:3-11.

5. Castellani RJ, Smith MA, Perry G, Friedland RP.

Cerebral amyloid angiopathy: major contributor or decorative response to Alzheimer’s disease pathogenesis. Neurobiol Aging 2004; 25:599-602.

6. Small DH, McLean CA. Alzheimer’s disease and the amyloid beta protein: What is the role of amyloid?

J Neurochem 1999; 73:443-449.

7. Glenner GG, Wong CW. Alzheimer’s disease: initial report of the purification and characterization of a novel cerebrovascular amyloid protein. Biochem Biophys Res Commun 1984; 120:885-890.

8. Masters CL, Simms G, Weinman NA, et al. Amyloid plaque core protein in Alzheimer’s disease and Down syndrome. Proc Natl Acad Sci U S A 1985; 82:4245- 4249.

9. Kang J, Lemaire HG, Unterbeck A, et al. The precursor of Alzheimer’s disease amyloid A4 protein resem- bles a cell-surface receptor. Nature 1987; 325:733-736.

1. Brief Introduction to the History of the β-Amyloid Protein 3

(4)

10. Wilquet V, De Strooper B. Amyloid-beta precursor protein processing in neurodegeneration. Curr Opin Neurobiol 2004; 14:582-588.

11. Weidemann A, Konig G, Bunke D, et al.

Identification, biogenesis, and localization of precur- sors of Alzheimer’s disease A4 amyloid protein. Cell 1989; 57:115-126.

12. Esch FS, Keim PS, Beattie EC, et al. Cleavage of amyloid beta peptide during constitutive processing of its precursor. Science 1990; 248:1122-1124.

13. Buxbaum JD, Liu KN, Luo Y, et al. Evidence that tumor necrosis factor alpha converting enzyme is involved in regulated alpha-secretase cleavage of the Alzheimer amyloid protein precursor. J Biol Chem 1998; 273:27765-27767.

14. Lammich S, Kojro E, Postina R, et al. Constitutive and regulated alpha-secretase cleavage of Alzheimer’s amyloid precursor protein by a disintegrin metal- loprotease. Proc Natl Acad Sci U S A 1999; 96:

3922-3927.

15. Nunan J, Small DH. Regulation of APP cleavage by alpha-, beta- and gamma-secretases. FEBS Lett 2000; 483:6-10.

16. Vassar R, Bennett BD, Babu-Khan S, et al. Beta-secretase cleavage of Alzheimer’s amyloid precursor protein by the transmembrane aspartic protease BACE. Science 1999; 286:735-741.

17. Lin X, Koelsch G, Wu S, et al. Human aspartic protease memapsin 2 cleaves the beta-secretase site of beta-amyloid precursor protein. Proc Natl Acad Sci U S A 2000; 97:1456-1460.

18. Sinha S, Anderson JP, Barbour R, et al. Purification and cloning of amyloid precursor protein beta-secretase from human brain. Nature 1999; 402:537-540.

19. Yan R, Bienkowski MJ, Shuck ME, et al. Membrane- anchored aspartyl protease with Alzheimer’s disease beta-secretase activity. Nature 1999; 402:533-537.

20. Small DH, Mok SS, Bornstein JC. Alzheimer’s disease and Abeta toxicity: from top to bottom. Nat Rev Neurosci 2001; 2:595-598.

21. Yankner BA, Dawes LR, Fisher S, et al. Neurotox- icity of a fragment of the amyloid precursor associated with Alzheimer’s disease. Science 1989;

245:417-420.

22. Pike CJ, Walencewicz-Wasserman AJ, Kosmoski J, et al. Structure-activity analyses of beta-amyloid peptides: contributions of the beta 25-35 region to aggregation and neurotoxicity. J Neurochem 1995;

64:253-265.

23. Walsh DM, Selkoe DJ. Deciphering the molecular basis of memory failure in Alzheimer’s disease.

Neuron 2004; 44:181-193.

24. Hardy JA, Higgins GA. Alzheimer’s disease: the amyloid cascade hypothesis. Science 1992; 256:184- 185.

25. Scheuner D, Eckman C, Jensen M, et al. Secreted amyloid beta-protein similar to that in the senile plaques of Alzheimer’s disease is increased in vivo by the presenilin 1 and 2 and APP mutations linked to familial Alzheimer’s disease. Nat Med 1996;

2:864-870.

26. Jarrett JT, Lansbury PT, Jr. Seeding “one-dimen- sional crystallization” of amyloid: a pathogenic mechanism in Alzheimer’s disease and scrapie? Cell 1993; 73:1055-1058.

27. Bertram L, Tanzi RE. The current status of Alzheimer’s disease genetics: what do we tell the patients? Pharmacol Res 2004; 50:385-396.

28. Hock BJ Jr., Lamb BT. Transgenic mouse models of Alzheimer’s disease. Trends Genet 2001; 17:S7-12.

29. Lewis J, Dickson DW, Lin WL, et al. Enhanced neurofibrillary degeneration in transgenic mice expressing mutant tau and APP. Science 2001;

293:1487-1491.

30. Sherrington R, Rogaev EI, Liang Y, et al. Cloning of a gene bearing missense mutations in early-onset familial Alzheimer’s disease. Nature 1995; 375:754- 760.

31. Levy-Lahad E, Wasco W, Poorkaj P, et al. Candidate gene for the chromosome 1 familial Alzheimer’s disease locus. Science 1995; 269:973-977.

32. Saunders AM, Strittmatter WJ, Schmechel D, et al.

Association of apolipoprotein E allele epsilon 4 with late-onset familial and sporadic Alzheimer’s disease.

Neurology 1993; 43:1467-1472.

33. Schmechel DE, Saunders AM, Strittmatter WJ, et al.

Increased amyloid beta-peptide deposition in cerebral cortex as a consequence of apolipoprotein E genotype in late-onset Alzheimer’s disease. Proc Natl Acad Sci U S A 1993; 90:9649-9653.

34. Bales KR, Verina T, Dodel RC, et al. Lack of apolipoprotein E dramatically reduces amyloid beta- peptide deposition. Nat Genet 1997; 17:263-264.

35. Schenk D, Barbour R, Dunn W, et al. Immunization with amyloid-beta attenuates Alzheimer’s-disease- like pathology in the PDAPP mouse. Nature 1999;

400:173-177.

36. Nicoll JA, Wilkinson D, Holmes C, et al.

Neuropathology of human Alzheimer’s disease after immunization with amyloid-beta peptide: a case report. Nat Med 2003; 9:448-452.

37. Hock C, Konietzko U, Streffer JR, et al. Antibodies against beta-amyloid slow cognitive decline in Alzheimer’s disease. Neuron 2003; 38:547-554.

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