7

Abstract: The accumulation and aggregation of amyloid-β (Aβ) peptides in the brain is believed to be the initial trigger in the molecular pathology of Alzheimer’s disease (AD). In addition to the widely studied full-length Aβ peptides (mainly Aβ_1–40 and Aβ_1–42), a variety of amino-terminally truncated (N-truncated) peptides, such as Aβ_pE3-x and Aβ_4-x, have been detected in high abundance in autopsy samples from sporadic and familial AD patients. N-truncated Aβ species adopt specific physicochemical properties resulting in a higher aggregation propensity and increased peptide stability, which likely account for their neurotoxic potential. The presence of N-truncated Aβ peptides in transgenic mouse models of AD and the selective overexpression of specific N-truncated variants in the murine brain have facilitated their investigation in relevant in vivo settings. In this chapter, we address the pathological relevance of N-truncated Aβ peptide species and summarize the current knowledge about the enzymatic activities that might be involved in their generation.

Keywords: ADAMTS4; Alzheimer’s disease; amyloid; N-truncation; protease

INTRODUCTION

The deposition of extracellular plaques consisting of amyloid-β (Aβ) peptides in the brain parenchyma is one of the neuropathological hallmarks of Alzheimer’s disease (AD). Although these deposits have also been found in non-demented control individuals, they are believed to play an important role in the disease process, and their presence and abundance is an obligatory criterion for a diagnosis of AD. Full-length Aβ peptides composed of 40 (Aβ_1–40) or 42 (Aβ_1–42) amino acids constitute the main components of extracellular amyloid plaques, together with other proteins such as ubiquitin and different proteoglycans. These peptides are generated by sequential proteolytic cleavage of the amyloid precursor protein (APP), a large type-I transmembrane protein that in rare families was found to carry mutations causative of inherited cases of AD. After an initial cleavage by either α- or β-secretase, which facilitates shedding of the APP ectodomain, the remaining membrane-bound β- or α-C-terminal fragments (CTFs) are cleaved by γ-secretase within their transmembrane domains. In the latter case, a small peptide fragment named p3 is released, while the cleavage of β-CTFs results in the generation of Aβ peptides (1) (Figure 1).

Figure 1 APP processing pathways. A) The non-amyloidogenic processing pathway (depicted on the right) is initiated through cleavage by α-secretase, which cleaves within the Aβ domain and generates the soluble ectodomain sAPPα. Subsequent cleavage of the membrane-bound C-terminal APP fragment C83 by the γ-secretase complex releases the soluble fragments p3 and the APP intracellular domain (AICD). Amyloidogenic APP processing (left panel) is initiated by β-secretase cleavage with the liberation of the soluble sAPPβ fragment. The remaining C-terminal fragment C99 is then cleaved by γ-secretase generating Aβ peptides as well as AICD. B) APP is a large transmembrane protein containing up to 770 amino acids. The Aβ peptide sequence (in red) starts within the ectodomain and ends within the transmembrane (TM) domain.

The analysis of brain samples from non-demented control cases, pathological aging (which is being regarded as a prodromal phase of AD), and AD revealed that, apart from full-length Aβ_1–40 and Aβ_1–42, N-truncated Aβ_x-42 species were the most abundant in AD with considerable overlap in pathological aging samples (2). This is interesting from a pathological point of view as full-length Aβ peptides are normal metabolites generated under physiological conditions. The exact physiological function of these peptides remains unresolved; however, it has been hypothesized that modulation of endogenous Aβ production might play an important role in the regulation of neuronal activity via a feedback loop mechanism (3). Other possible physiological functions include promoting recovery from traumatic brain injury, sealing leaks in the blood–brain barrier, or antimicrobial activities (4). While full-length Aβ peptides starting with an aspartic acid (Asp) residue at position 1 of the Aβ sequence are generated by an enzymatic activity called β-site APP cleaving enzyme 1 (BACE1) (5, 6), much less is known about the proteases responsible for the production of N-truncated Aβ peptides.

Aβ peptides with varying N-termini were described more than 30 years ago. In 1984, the identification of full-length Aβ peptides starting with an Asp residue in position 1 purified from cerebrovascular amyloid deposits was reported (7). The following year, N-terminal sequencing of Aβ peptides purified from amyloid plaque cores from AD cases demonstrated the presence of peptides starting with phenylalanine (Phe) in position four (Aβ_4-x), as well as with serine (Ser) or glycine (Gly) in position eight (Aβ_8-x) or nine (Aβ_9-x) (8, 9). By means of immunohistochemistry, N-truncated Aβ species with post-translational modifications such as pyroglutamylation at position 3 (Aβ_pE3-x) and 11(Aβ_pE11-x) were subsequently described in human AD brains (10, 11). The loss of charged amino acids at the N-terminus changes the biophysical properties of the Aβ peptides, thus influencing their aggregation propensity and toxicity. As a consequence, efforts to understand the relevance of N-truncated Aβ species in the pathogenesis of AD, as well as the mechanisms responsible for their generation, have recently increased.

HETEROGENEITY OF N-TRUNCATED Aβ SPECIES IN AD BRAIN

Several studies employing mass spectrometry (MS) that intended to analyze the full spectrum of Aβ peptides in postmortem brain samples of AD patients have been published. In the earliest of these studies, purified amyloid core and cerebrovascular amyloid peptides were sequenced using matrix-assisted laser-desorption-time-of-flight (MALDI-TOF) mass spectrometry. While the amino acid composition of cerebrovascular Aβ peptides consisted mainly of species starting with residues 1 or 2, the preparations from amyloid cores were more heterogeneous, corresponding to peptides beginning with every residue between Asp-1 and Glu-11(Figure 2), with major signals for peptides starting with Phe-4, Ser-8, and Glu-11 (12). In good agreement, using surface-enhanced laser desorption/ionization time-of-flight (SELDI-TOF) mass spectrometry, Aβ_4-42 was also identified as the major N-truncated species in postmortem brain samples from aged controls, patients with vascular dementia, and AD patients (13). This suggested that N-truncated species account for a substantial proportion of total Aβ in the aged human brain, a finding that was corroborated in subsequent studies. The entire spectrum of Aβ peptides ranging from Aβ_1-x to Aβ_pE11-xwas detected in frontal cortex samples of a sporadic AD case and of an individual affected by the FAD-associated presenilin (PSEN1) V261I mutation. This mutation is associated with the deposition of the so-called cotton wool plaques, which are lesions lacking a central amyloid core (14). By investigating non-demented individuals with incipient amyloid pathology as well as AD patients, it was further demonstrated that initial insoluble Aβ aggregates are largely composed of N-truncated Aβ42 variants such as peptides starting at positions 4-, 5-, 8-, or 9–42 (15). Portelius et al. also studied the Aβ isoform pattern in the hippocampus, cortex, and cerebellum of non-demented controls, sporadic AD cases, and patients suffering from familial AD (FAD). In all groups, Aβ_1–42, Aβ_1–40, Aβ_pE3–42, and Aβ_4–42 were identified as the dominant isoforms (16), which is in good agreement with the most recent studies from other investigators (2, 17, 18).

Figure 2 Sequence of the Aβ N-terminus with indicated cleavage sites and enzymes involved in the generation of N-truncated Aβ species. Amino acids (AA) are color-coded according to their properties (red: charged AA; grey: uncharged AA; blue: nonpolar hydrophobic AA).

N-TERMINALLY TRUNCATED Aβ SPECIES IN TRANSGENIC MOUSE MODELS OF AD

Transgenic mouse models overexpressing mutant forms of human APP, either alone or in combination with mutant forms of PSEN1 or PSEN2, are valuable and widely used model systems to study AD-associated pathological alterations such as extracellular amyloid deposition, inflammatory responses, and cognitive deficits (19–21). The analysis of Aβ peptide species in brain samples using mass spectrometry revealed that most transgenic AD mouse models only partially reflect the Aβ spectrum in human sporadic AD. While the overall heterogeneity of N-terminal truncated Aβ species could be reproduced in mouse models such as APP/PS1KI (22) or 5XFAD (23, 24), the ratio of full-length Aβ peptides to N-truncated variants is much different in human brain samples. While N-truncated variants such as Aβ_pE3-x or Aβ_4-x might be present in comparable quantities compared to full-length Aβ_1–40 or Aβ_1–42 species in human samples (16), full-length peptides comprise by far the majority of all Aβ peptides in transgenic AD models (23, 25, 26). This is likely explained by the fact that most of these models (e.g., Tg2576 (27), APP23 (28), APP/PS1KI (22), 5XFAD (29), or APPswe/PSEN1dE9 (30)) utilize the Swedish APP mutation. Cell lines transfected with the Swedish APP670/671 mutation have been shown to release three to six times more Aβ peptides than wild-type cells (31, 32). Due to the location of the double mutation in the immediate vicinity of the β-secretase cleavage site (Figure 1), the Swedish mutation increases the affinity of the substrate APP for BACE1, thus favoring the generation of full-length Aβ peptides starting with Asp in position 1 (33).

Using two-dimensional gel electrophoresis with subsequent mass spectrometry analysis, a variety of N-truncated Aβ species have been detected in APP/PS1KI mice. While full-length Aβ_1–42 peptides were already detectable in young mice at 2.5 months of age, other Aβ variants such as Aβ_2/3–42, Aβ_pE3–42, and Aβ_4/5–42 became apparent only at later time points (22). Mass spectrometry analyses have also supported that N-truncated species represent only a small percentage of the total Aβ peptide amount in mouse models such as 5XFAD or APP23, although variable ionization efficiencies for the different Aβ species might contribute to a distorted image of the Aβ peptide composition in both mouse and human brains (23, 34). In conclusion, N-truncated Aβ species are substantially underrepresented in transgenic mouse models compared to human AD brain samples (34, 35).

MAJOR N-TERMINAL TRUNCATED Aβ SPECIES DETECTED IN HUMAN BRAIN

As pointed out above, a huge variety of different N-terminal truncated Aβ species has been identified by either MS or immunohistochemical staining methods in brain samples from human AD patients. In this section, we discuss the current knowledge on the most important variants in more detail.

Aβ_2-x

In AD patients, a consistent elevation of Aβ peptides lacking the N-terminal Asp residue have been observed in the detergent-soluble pool of brain extracts, as well as in cerebrospinal fluid (CSF) samples (36). Using SELDI-MS, several Aβ peptides including those starting with Ala-2 were found in extractions from senile plaques (13). Immunohistochemical analysis of postmortem brain samples using an Aβ_2-x-specific polyclonal antibody confirmed the presence of Aβ_2-x peptides in both parenchymal and vascular deposits of sporadic AD cases as well as transgenic mouse models such as APP/PS1KI or 5XFAD (37). As the sequence of full-length Aβ starts with an Asp residue in position 1, it has been suggested that proteolysis of Aβ_1-x peptides by the exopeptidase aminopeptidase A, which releases Glu and Asp residues from the N-termini of proteins, could result in the generation of Aβ_2-x species (38). However, the evidence in this study was limited to showing that Western blot immunoreactivity with an Aβ_1-x-specific antibody was reduced after the co-incubation of purified aminopeptidase A with recombinant full-length Aβ_1–40 peptides (Table 1). The identity of specific degradation products and, in particular, the generation of Aβ_2-x species, was not confirmed by mass spectrometry or other methodology (38). In contrast, it has been convincingly demonstrated in cell-free and cell-based assays that cleavage of APP or Aβ by the metalloprotease meprin-β can result in the generation of Aβ_2-x species (39, 40). In both HEK293T and CHO cells, co-expression of human APP and meprin-β facilitated the secretion of Aβ_2–40 peptides, whose identity was confirmed by mass spectrometry, and this was blocked by treatment with a γ-secretase but not a β-secretase inhibitor, indicating that Aβ_2–40 peptides were produced through a BACE1-independent mechanism. Later, these results were partially confirmed by another group (41). Still missing is in vivo proof that meprin-β is responsible for the brain production of Aβ_2-x peptides in AD mouse models. However, this experiment is complicated by the fact that meprin-β does not generate Aβ_2-x peptides with Swedish mutant APP as a substrate, which excludes most of the commonly used APP-transgenic strains as in vivo model systems (40).

TABLE 1 List of proteases involved in the generation of N-truncated Aβ species

Protease	Levels/activity in human AD brain versus control	Cleavage site	Potential Aβ peptides	References
BACE1	Increased (42)	Met(-1) ↓ Asp(1) Tyr(10) ↓ Glu(11)	Aβ1-x Aβ11-x, AβpE11-x	(6)
Aminopeptidase A	Reduced (43)	Asp(1) ↓ Ala(2)	Aβ2-x	(38)
Meprin-β	Unknown	Asp(1) ↓ Ala(2)	Aβ2-x	(39, 40)
Neprilysin (NEP)	Increased (44) Reduced activity (45)	Asp(2) ↓ Ala(3) Ala(3) ↓ Phe(4) Arg(5) ↓ His(6) Gly(9) ↓ Tyr(10)	Aβ3-x, AβpE3-x Aβ4-x Aβ6-x Aβ10-x	(46, 47) (48, 49)
ADAMTS4	Unknown	Ala(3) ↓ Phe(4)	Aβ4-x	(24)

Aβ_pE3-x

Pyroglutamate-modified Aβ_pE3-x represents a major Aβ species identified in human AD brains (16, 50). In 1995, Saido et al. reported the identification of these post-translationally modified peptides in which the glutamate at position three becomes converted to pyroglutamate through intramolecular dehydration (51). This cyclization alters the physicochemical properties of Aβ and results in increased hydrophobicity due to the loss of a negative charge, faster aggregation kinetics compared to full-length Aβ peptides in in vitro assays (52–54), and increased insolubility and stability (55). Importantly, higher abundance of these peptides in AD as compared to age-matched non-demented control patients has been demonstrated (56–58). With regard to their toxic properties, increased neurotoxicity compared to full-length Aβ peptides (59) has been reported; however, some studies found full-length and Aβ_pE3-x peptides to be equally toxic (60, 61), while others suggested that Aβ/Aβ_pE hetero-oligomers constitute the main neurotoxic Aβ fraction (62). Interestingly, related properties have also been reported for pyroglutamylated ABri and ADan peptides, representing the major peptide species accumulating in the neurodegenerative disorders familial British dementia and familial Danish dementia (63, 64).

The formation of Aβ_pE3-x peptides appears to be at least a two-step process, with removal of the first two amino acid residues from full-length Aβ followed by cyclization. Recently, it has been suggested that meprin-β might not only generate Aβ_2-x but also Aβ_3-x peptides as substrates for cyclization (41). However, Aβ_3-x peptides were not detected in an earlier study by mass spectrometry (39), and whether genetic deletion of meprin-β would reduce Aβ_pE3-x peptide formation in vivo is unknown. In contrast, solid evidence supports that glutaminyl cyclase (QC) is at least one of the enzymes capable of catalyzing the second step of Aβ_pE3-x formation (65). Treatment using an orally available QC inhibitor resulted in a reduction of the Aβ_pE3–42 burden in transgenic mouse models of AD (66). The same was also seen in 5XFAD mice on a QC knock-out background and was accompanied by a rescue of behavioral deficits (67). The observation of a significant age-dependent increase of the Aβ_pE3-x parenchymal plaque burden at the expense of Aβ_1-x full-length peptides suggested that Aβ_pE3-x formation might occur late in the process of amyloidosis and could involve the remodeling of existing extracellular amyloid deposits (68). On the other hand, the presence of Aβ_pE3-x peptides has been also described within neurons both in mouse models (22, 69) and human AD samples (53, 70), raising the question of whether the localization is important for toxicity. In order to address such questions, transgenic mouse models have been developed with constructs that only encode the Aβ_3-x peptide, with a glutamate to glutamine substitution at the initial position to facilitate cyclization (71–73). This construct is expressed under the control of the murine neuron-specific Thy1-promotor and contains the thyrothropin-releasing hormone (TRH) signal peptide sequence to ensure liberation of the peptide preferentially in the secretory pathway (74). In contrast to other models, these mice do not express human full-length APP or any FAD-associated mutations, but impress with a rapid onset of behavioral deficits, neuron loss, and microgliosis (71, 72).

Aβ_4-x

Aβ_4–42 was one of the first Aβ peptide species that was detected in the amyloid plaque cores of human AD brains (9). More recently, novel Aβ_4-x specific antibodies have been described, and the localization of Aβ_4-x to amyloid plaque cores has been confirmed in immunohistochemical studies in both human AD and transgenic AD mouse models (75, 76). In addition, Aβ_4-x peptides were also found within blood vessels in the majority of the analyzed AD cases (76). Similar to Aβ_pE3-x peptides, Aβ_4–42 peptides lacking another charged amino acid residue have also been described to quickly aggregate into soluble oligomers and fibrillar, high-molecular weight aggregates (61, 75, 76). Quantitatively, Aβ_4–42 peptides seem to be among the most abundant Aβ species in human AD brain with equal or even higher amounts compared to Aβ_1–42. It should be noted again, however, that in studies using mass spectrometry to assess Aβ peptide patterns, the ratios between the respective peptide variants cannot be regarded as a direct reflection of their abundance (16, 77). With regard to their neurotoxicity, Aβ_4–42 and Aβ_4–40 demonstrated equal toxicity as Aβ_1–42 or Aβ_pE3–42 using in vitro assays with primary neuronal cultures. This was also observed in an in vivo setting in which freshly prepared Aβ peptides were applied by intraventricular injection followed by an analysis of working memory using a Y-maze task after 5 days (61).

The metalloprotease neprilysin (NEP) has been proposed as a candidate enzyme responsible for the generation of Aβ_4-x peptides by cleaving between Glu-3 and Phe-4 among other sites, with full-length Aβ_1-x peptides acting as the immediate substrate. This has been shown by high-performance liquid chromatography analysis yielding several product peaks after incubation of Aβ_1–40 with either recombinant soluble NEP produced in Sf9 cells or NEP purified from rabbit kidney cortex (46). More recent studies using synthetic Aβ peptides and recombinant human NEP confirmed the generation of Aβ peptide fragments starting with Phe-4 (such as Aβ_4–9 or Aβ_4–16 but also the existence of several other cleavage sites, at least under the given in vitro conditions (48, 49). Therefore, it is currently unclear whether NEP might contribute to the generation of longer Aβ peptides such as Aβ_4–40 and Aβ_4–42. However, we regard this possibility as unlikely as in vivo studies have demonstrated that the rate-limiting step in the proteolysis of Aβ by NEP is cleavage of the Gly-9–Tyr-10 bond, which would rule out the generation of full-length Aβ_4–40 and Aβ_4–42 peptides (78).

Most recently, it was shown that APP contains a cleavage site for the metalloprotease ADAMTS4 (a disintegrin-like and metalloprotease with thrombospondin type 1 motif) between Glu-3 and Phe-4 of the Aβ peptide sequence (24). ADAMTS proteases constitute a family of secreted Zn²⁺-metalloproteases that degrade or modify major components of the extracellular matrix (79). ADAMTS4 participates in the proteolytic degradation of proteoglycans like aggrecan, brevican, and versican (80). Aggrecan is a hyaluronan-binding proteoglycan, which is present in large amounts in the articular cartilage. In an important pathological process leading to osteoarthritis and rheumatoid arthritis, aggrecan is degraded by ADAMTS4 and the homologous family member ADAMTS5, leading to the exposure and subsequent degradation of collagen fibrils by collagenases (81). Co-expression of ADAMTS4 and APP in HEK293 cells resulted in the secretion of Aβ_4–40 peptides as measured by mass spectrometry and ELISA, while several species of Aβ_1-x peptides were not affected (24). Aβ_4–40 secretion was not blocked by treatment of the cells with a potent β-secretase inhibitor indicating that Aβ_4-x peptides were generated in a BACE1-independent fashion. IHC analysis of ADAMTS4 reporter mice showed that ADAMTS4 was exclusively expressed in oligodendrocytes in the adult murine brain. Consistently, the culture of murine oligodendrocytes demonstrated that these primary cells secrete Aβ_4–40 peptides among a spectrum of other Aβ species very similar to established cell lines. However, Aβ_4–40 peptides were undetectable in primary oligodendrocytes derived from ADAMTS4 knockout (KO) mice, providing genetic proof that ADAMTS4 is responsible for Aβ_4–40 peptide generation in this cell type. In vivo, the crossing of 5XFAD mice to ADAMTS4 knockout mice reduced Aβ_4–40 levels by 50%, but the overall amyloid plaque load and the distribution of Aβ_4-x peptides in amyloid plaque cores appeared to be unchanged, clearly suggesting that other mechanisms for Aβ_4-x generation beside ADAMTS4 must exist. Compellingly, abundant Aβ_4-x immunoreactivity was observed in white matter structures of 5XFAD mice, and this signal was entirely abolished in the ADAMTS4 knockout background (24). This could be of pathological relevance as numerous neuropathological, biochemical, and imaging studies have reported white matter abnormalities and oligodendrocyte dysfunction in AD patients (82, 83). However, further studies are required to define a potential detrimental role of Aβ_4-x peptides in white matter structures. In any case, the recent link of ADAMTS4 to AD risk as well as single-cell transcriptomic data supporting that many oligodendroglia-specific and myelination-associated genes are dysregulated in human AD brains should provide new urgency to consider the role of oligodendrocytes in AD (84, 85).

As a tool to investigate the in vivo role of Aβ_4-x peptides, a transgenic mouse model has been generated that only expresses Aβ_4–42 peptides under the control of the murine Thy1-promotor. These mice develop age-dependent behavioral deficits with spatial or working memory impairments, which are detectable in paradigms such as Morris water maze or novel object recognition task, as well as motor deficits. These mice do not develop amyloid plaque pathology but show a robust hippocampal CA1 neuron loss correlating with the transgene expression pattern in a gene-dose dependent manner (61, 86). Interestingly, altered basal excitatory synaptic transmission with Aβ_4–42-dependent neuronal hyperexcitability is already obvious in young Tg4–42 mice preceding neuron loss and behavioral deficits (87).

Aβ_5-x

Aβ peptides starting with an Arg residue at position 5 have been detected in brains of transgenic mice such as APP/PS1KI (22) or 5XFAD (23), as well as in human AD brains (15–17) by mass spectrometry. Conditions of BACE1 inhibition resulted in strongly increased levels of Aβ_5-x species in cellular models (88–90). This clearly suggests that Aβ_5-x peptides are produced through a BACE1-independent pathway, with some evidence supporting α-secretase-like proteases (e.g., ADAM family proteases such as TACE or ADAM10) as potential candidate enzymes (88).

In vivo studies with several BACE1 inhibitors in beagle dogs confirmed the absolute signal reduction of all Aβ isoforms in the CSF except for Aβ_5–40 peptides, and an analysis of relative levels demonstrated a clear increase of Aβ_5–40 (90). This was further corroborated in a placebo-controlled study in healthy human subjects in which dose-dependent increases in Aβ_5-x levels were measured in the CSF upon treatment with the BACE1 inhibitor LY2811376 (91). Immunohistochemical analyses using Aβ_5-x selective antibodies confirmed the presence of Aβ_5-x peptides in brain tissues samples from sporadic AD patients showing immunoreactivity primarily in vascular deposits (88, 92). In cases from individuals harboring FAD-associated APP or PSEN1 mutations, both vascular and parenchymal deposits were detected, while in mouse models such as 5XFAD, APP/PS1KI, or 3xTg Aβ_5-x, immunoreactivity was confined to extracellular plaques (92).

Aβ_11-x/Aβ_pE11-x

In addition to the cleavage site between methionine and aspartate in position 1 (Asp-1) generating β-CTFs, BACE1 has also been shown to cleave APP between tyrosine and glutamate in position 11 (Glu-11) of the Aβ sequence resulting in N-truncated Aβ_11-x species (β′-cleavage) (6). There is evidence that the BACE1 cleavage preference depends on the intracellular localization, with β′-cleavage being favored in the trans-Golgi network (93). Aβ_11-x peptides have been detected in brains from AD and Down’s syndrome patients (94) and have been shown to accumulate within neurons in cellular models upon BACE1 overexpression (95). Similar to Glu-3, the free Glu residue in position 11 can also undergo cyclization and modification to an N-terminal pyroglutamate (Aβ_pE11-x). In contrast to Aβ_pE3–42, which is mainly confined to mature plaque cores in AD patients, unmodified Aβ_11–40 and Aβ_pE11–40 peptides have been detected in the vasculature using selective antibodies (95). Within amyloid plaques cores, Aβ_pE11-x has been found to co-localize with full-length Aβ peptides but also with Aβ_pE3-x (96).

CONCLUSION

There is substantial evidence that N-truncated Aβ species, in addition to the extensively studied full-length Aβ peptides, might play an important role in the molecular pathology of AD. In recent years, new candidate proteases and nonneuronal cell types have been linked to the generation of N-truncated Aβ species. Novel antibodies specific for some N-truncated Aβ peptides have been developed, and this should allow the development of quantitative detection assays to better define their abundance in relation to full-length Aβ peptides. To advance the functional analysis of N-truncated Aβ peptides, novel animal models might be needed as N-truncated Aβ species are underrepresented in the available AD models. These efforts should improve our understanding of the pathological role of N-truncated Aβ peptides. They could provide novel insights into currently unexplained aspects of AD pathology, and they might be crucial to develop novel therapeutic approaches.

Acknowledgments: This work has been supported by the Alzheimer Forschung Initiative (grant #16103 to OW), Gerhard Hunsmann Stiftung and Alzheimer Stiftung Göttingen (to OW).

Conflict of interest: The authors declare no potential conflicts of interest with respect to research, authorship, and/or publication of this chapter.

Copyright and Permission Statement: To the best of our knowledge, the materials included in this chapter do not violate copyright laws. All original sources have been appropriately acknowledged and/or referenced. Where relevant, appropriate permissions have been obtained from the original copyright holder(s).

REFERENCES

1. Thinakaran G, Koo EH. Amyloid precursor protein trafficking, processing, and function. J Biol Chem. 2008;283(44):29615–19. http://dx.doi.org/10.1074/jbc.R800019200
2. Moore BD, Chakrabarty P, Levites Y, Kukar TL, Baine AM, Moroni T, et al. Overlapping profiles of abeta peptides in the Alzheimer’s disease and pathological aging brains. Alzheimers Res Ther. 2012;4(3):18. http://dx.doi.org/10.1186/alzrt121
3. Kamenetz F, Tomita T, Hsieh H, Seabrook G, Borchelt D, Iwatsubo T, et al. APP processing and synaptic function. Neuron. 2003;37(6):925–37. http://dx.doi.org/10.1016/S0896-6273(03)00124-7
4. Brothers HM, Gosztyla ML, Robinson SR. The physiological roles of amyloid-β peptide hint at new ways to treat Alzheimer’s disease. Front Aging Neurosci. 2018;10:118. http://dx.doi.org/10.3389/fnagi.2018.00118
5. Sinha S, Anderson JP, Barbour R, Basi GS, Caccavello R, Davis D, et al. Purification and cloning of amyloid precursor protein beta-secretase from human brain. Nature. 1999;402(6761):537–40. http://dx.doi.org/10.1038/990114
6. Vassar R, Bennett BD, Babu-Khan S, Kahn S, Mendiaz EA, Denis P, et al. Beta-secretase cleavage of Alzheimer’s amyloid precursor protein by the transmembrane aspartic protease BACE. Science. 1999;286(5440):735–41. http://dx.doi.org/10.1126/science.286.5440.735
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;120:885–90. http://dx.doi.org/10.1016/S0006-291X(84)80190-4
8. Masters CL, Multhaup G, Simms G, Pottgiesser J, Martins RN, Beyreuther K. Neuronal origin of a cerebral amyloid: Neurofibrillary tangles of Alzheimer’s disease contain the same protein as the amyloid of plaque cores and blood vessels. EMBO J. 1985;4(11):2757–63. http://dx.doi.org/10.1002/j.1460-2075.1985.tb04000.x
9. Masters CL, Simms G, Weinman NA, Multhaup G, McDonald BL, Beyreuther K. Amyloid plaque core protein in Alzheimer disease and Down syndrome. Proc Natl Acad Sci U S A. 1985;82(12):4245–9. http://dx.doi.org/10.1073/pnas.82.12.4245
10. Saido TC, Yamao-Harigaya W, Iwatsubo T, Kawashima S. Amino- and carboxyl-terminal heterogeneity of beta-amyloid peptides deposited in human brain. Neurosci Lett. 1996;215(3):173–6. http://dx.doi.org/10.1016/0304-3940(96)12970-0
11. Iwatsubo T, Saido TC, Mann DM, Lee VM, Trojanowski JQ. Full-length amyloid-beta (1-42(43)) and amino-terminally modified and truncated amyloid-beta 42(43) deposit in diffuse plaques. Am J Pathol. 1996;149(6):1823–30.
12. Miller DL, Papayannopoulos IA, Styles J, Bobin SA, Lin YY, Biemann K, et al. Peptide compositions of the cerebrovascular and senile plaque core amyloid deposits of Alzheimer’s disease. Arch Biochem Biophys. 1993;301(1):41–52. http://dx.doi.org/10.1006/abbi.1993.1112
13. Lewis H, Beher D, Cookson N, Oakley A, Piggott M, Morris CM, et al. Quantification of Alzheimer pathology in ageing and dementia: Age-related accumulation of amyloid-beta(42) peptide in vascular dementia. Neuropathol Appl Neurobiol. 2006;32(2):103–18. http://dx.doi.org/10.1111/j.1365-2990.2006.00696.x
14. Miravalle L, Calero M, Takao M, Roher AE, Ghetti B, Vidal R. Amino-terminally truncated Abeta peptide species are the main component of cotton wool plaques. Biochemistry. 2005;44(32):10810–21. http://dx.doi.org/10.1021/bi0508237
15. Sergeant N, Bombois S, Ghestem A, Drobecq H, Kostanjevecki V, Missiaen C, et al. Truncated beta-amyloid peptide species in pre-clinical Alzheimer’s disease as new targets for the vaccination approach. J Neurochem. 2003;85(6):1581–91. http://dx.doi.org/10.1046/j.1471-4159.2003.01818.x
16. Portelius E, Bogdanovic N, Gustavsson MK, Volkmann I, Brinkmalm G, Zetterberg H, et al. Mass spectrometric characterization of brain amyloid beta isoform signatures in familial and sporadic Alzheimer’s disease. Acta Neuropathol. 2010;120(2):185–93. http://dx.doi.org/10.1007/s00401-010-0690-1
17. Gkanatsiou E, Portelius E, Toomey CE, Blennow K, Zetterberg H, Lashley T, et al. A distinct brain beta amyloid signature in cerebral amyloid angiopathy compared to Alzheimer’s disease. Neurosci Lett. 2019;701:125–31. http://dx.doi.org/10.1016/j.neulet.2019.02.033
18. Wildburger NC, Esparza TJ, LeDuc RD, Fellers RT, Thomas PM, Cairns NJ, et al. Diversity of amyloid-beta proteoforms in the Alzheimer’s disease brain. Sci Rep. 2017;7(1):9520. http://dx.doi.org/10.1038/s41598-017-10422-x
19. LaFerla FM, Green KN. Animal models of Alzheimer disease. Cold Spring Harb Perspect Med. 2012;2(11):a006320. http://dx.doi.org/10.1101/cshperspect.a006320
20. Morrissette DA, Parachikova A, Green KN, LaFerla FM. Relevance of transgenic mouse models to human Alzheimer disease. J Biol Chem. 2009 Mar 6;284(10):6033–7. http://dx.doi.org/10.1074/jbc.R800030200
21. Duyckaerts C, Potier MC, Delatour B. Alzheimer disease models and human neuropathology: Similarities and differences. Acta Neuropathol. 2008;115(1):5–38. http://dx.doi.org/10.1007/s00401-007-0312-8
22. Casas C, Sergeant N, Itier JM, Blanchard V, Wirths O, van der Kolk N, et al. Massive CA1/2 neuronal loss with intraneuronal and N-terminal truncated Abeta42 accumulation in a novel Alzheimer transgenic model. Am J Pathol. 2004;165(4):1289–300. http://dx.doi.org/10.1016/S0002-9440(10)63388-3
23. Reinert J, Richard BC, Klafki HW, Friedrich B, Bayer TA, Wiltfang J, et al. Deposition of C-terminally truncated Aβ species Aβ37 and Aβ39 in Alzheimer’s disease and transgenic mouse models. Acta Neuropathol Comm. 2016;4:24. http://dx.doi.org/10.1186/s40478-016-0294-7
24. Walter S, Jumpertz T, Hüttenrauch M, Ogorek I, Gerber H, Storck SE, et al. The metalloprotease ADAMTS4 generates N-truncated Aβ4–x species and marks oligodendrocytes as a source of amyloidogenic peptides in Alzheimer’s disease. Acta Neuropathol. 2019;137(2):239–57. http://dx.doi.org/10.1007/s00401-018-1929-5
25. Kalback W, Watson MD, Kokjohn TA, Kuo YM, Weiss N, Luehrs DC, et al. APP transgenic mice Tg2576 accumulate Abeta peptides that are distinct from the chemically modified and insoluble peptides deposited in Alzheimer’s disease senile plaques. Biochemistry. 2002;41(3):922–8. http://dx.doi.org/10.1021/bi015685+
26. Kuo YM, Kokjohn TA, Beach TG, Sue LI, Brune D, Lopez JC, et al. Comparative analysis of amyloid-beta chemical structure and amyloid plaque morphology of transgenic mouse and Alzheimer’s disease brains. J Biol Chem. 2001;276(16):12991–8. http://dx.doi.org/10.1074/jbc.M007859200
27. Hsiao KK, Chapman P, Nilsen S, Eckman C, Harigaya Y, Younkin S, et al. Correlative memory deficits, Abeta elevation and amyloid plaques in transgenic mice. Science. 1996;274:99–102. http://dx.doi.org/10.1126/science.274.5284.99
28. Sturchler-Pierrat C, Abramowski D, Duke M, Wiederhold KH, Mistl C, Rothacher S, et al. Two amyloid precursor protein transgenic mouse models with Alzheimer disease-like pathology. Proc Natl Acad Sci U S A. 1997;94(24):13287–92. http://dx.doi.org/10.1073/pnas.94.24.13287
29. Oakley H, Cole SL, Logan S, Maus E, Shao P, Craft J, et al. Intraneuronal beta-amyloid aggregates, neurodegeneration, and neuron loss in transgenic mice with five familial Alzheimer’s disease mutations: Potential factors in amyloid plaque formation. J Neurosci. 2006;26(40):10129–40. http://dx.doi.org/10.1523/JNEUROSCI.1202-06.2006
30. Jankowsky JL, Fadale DJ, Anderson J, Xu GM, Gonzales V, Jenkins NA, et al. Mutant presenilins specifically elevate the levels of the 42 residue β-amyloid peptide in vivo: Evidence for augmentation of a 42-specific γ secretase. Human Molecular Genetics. 2003;13(2):159–70. http://dx.doi.org/10.1093/hmg/ddh019
31. Cai X, Golde T, Younkin S. Release of excess amyloid beta protein from a mutant amyloid beta protein precursor. Science. 1993;259(5094):514–16. http://dx.doi.org/10.1126/science.8424174
32. Citron M, Vigo-Pelfrey C, Teplow DB, Miller C, Schenk D, Johnston J, et al. Excessive production of amyloid beta-protein by peripheral cells of symptomatic and presymptomatic patients carrying the Swedish familial Alzheimer disease mutation. Proc Natl Acad Sci U S A. 1994;91(25):11993–7. http://dx.doi.org/10.1073/pnas.91.25.11993
33. Weggen S, Beher D. Molecular consequences of amyloid precursor protein and presenilin mutations causing autosomal-dominant Alzheimer’s disease. Alzheimers Res Ther. 2012;4(2):9. http://dx.doi.org/10.1186/alzrt107
34. Schieb H, Kratzin H, Jahn O, Mobius W, Rabe S, Staufenbiel M, et al. Beta-amyloid peptide variants in brains and cerebrospinal fluid from amyloid precursor protein (APP) transgenic mice: Comparison with human Alzheimer amyloid. J Biol Chem. 2011;286(39):33747–58. http://dx.doi.org/10.1074/jbc.M111.246561
35. Rüfenacht P, Güntert A, Bohrmann B, Ducret A, Döbeli H. Quantification of the Aβ peptide in Alzheimer’s plaques by laser dissection microscopy combined with mass spectrometry. J Mass Spectrom. 2005;40(2):193–201. http://dx.doi.org/10.1002/jms.739
36. Wiltfang J, Esselmann H, Cupers P, Neumann M, Kretzschmar H, Beyermann M, et al. Elevation of beta-amyloid peptide 2–42 in sporadic and familial Alzheimer’s disease and its generation in PS1 knockout cells. J Biol Chem. 2001;276(46):42645–57. http://dx.doi.org/10.1074/jbc.M102790200
37. Savastano A, Klafki H, Haussmann U, Oberstein TJ, Muller P, Wirths O, et al. N-Truncated abeta 2-X starting with position two in sporadic Alzheimer’s disease cases and two Alzheimer mouse models. J Alzheimers Dis. 2016;49(1):101–10. http://dx.doi.org/10.3233/JAD-150394
38. Sevalle J, Amoyel A, Robert P, Fournie-Zaluski MC, Roques B, Checler F. Aminopeptidase A contributes to the N-terminal truncation of amyloid beta-peptide. J Neurochem. 2009;109(1):248–56. http://dx.doi.org/10.1111/j.1471-4159.2009.05950.x
39. Bien J, Jefferson T, Čaušević M, Jumpertz T, Munter L, Multhaup G, et al. The metalloprotease meprin β generates amino terminal-truncated amyloid β peptide species. J Biol Chem. 2012;287(40):33304–13. http://dx.doi.org/10.1074/jbc.M112.395608
40. Schönherr C, Bien J, Isbert S, Wichert R, Prox J, Altmeppen H, et al. Generation of aggregation prone N-terminally truncated amyloid β peptides by meprin β depends on the sequence specificity at the cleavage site. Mol Neurodegen. 2016;11:19. http://dx.doi.org/10.1186/s13024-016-0084-5
41. Schlenzig D, Cynis H, Hartlage-Rubsamen M, Zeitschel U, Menge K, Fothe A, et al. Dipeptidyl-peptidase activity of meprin beta links N-truncation of abeta with glutaminyl cyclase-catalyzed pGlu-abeta formation. J Alzheimers Dis. 2018;66(1):359–75. http://dx.doi.org/10.3233/JAD-171183
42. Ahmed RR, Holler CJ, Webb RL, Li F, Beckett TL, Murphy MP. BACE1 and BACE2 enzymatic activities in Alzheimer’s disease. J Neurochem. 2010;112(4):1045–53. http://dx.doi.org/10.1111/j.1471-4159.2009.06528.x
43. Kehoe PG, Hibbs E, Palmer LE, Miners JS. Angiotensin-III is increased in Alzheimer’s disease in association with amyloid-beta and tau pathology. J Alzheimers Dis. 2017;58(1):203–14. http://dx.doi.org/10.3233/JAD-161265
44. Miners JS, Baig S, Tayler H, Kehoe PG, Love S. Neprilysin and insulin-degrading enzyme levels are increased in Alzheimer disease in relation to disease severity. J Neuropathol Exp Neurol. 2009;68(8):902–14. http://dx.doi.org/10.1097/NEN.0b013e3181afe475
45. Miners JS, Van Helmond Z, Chalmers K, Wilcock G, Love S, Kehoe PG. Decreased expression and activity of neprilysin in Alzheimer disease are associated with cerebral amyloid angiopathy. J Neuropathol Exp Neurol. 2006;65(10):1012–21. http://dx.doi.org/10.1097/01.jnen.0000240463.87886.9a
46. Howell S, Nalbantoglu J, Crine P. Neutral endopeptidase can hydrolyze β-amyloid(1–40) but shows no effect on β-amyloid precursor protein metabolism. Peptides. 1995;16(4):647–52. http://dx.doi.org/10.1016/0196-9781(95)00021-B
47. Leissring MA, Lu A, Condron MM, Teplow DB, Stein RL, Farris W, et al. Kinetics of amyloid β-protein degradation determined by novel fluorescence- and fluorescence polarization-based assays. J Biol Chem. 2003;278(39):37314–20. http://dx.doi.org/10.1074/jbc.M305627200
48. Hornung K, Zampar S, Engel N, Klafki H, Liepold T, Bayer TA, et al. N-terminal truncated Abeta4-42 is a substrate for neprilysin degradation in vitro and in vivo. J Alzheimers Dis. 2019;67(3):849–58. http://dx.doi.org/10.3233/JAD-181134
49. Mital M, Bal W, Frączyk T, Drew SC. Interplay between copper, neprilysin, and N-truncation of β-amyloid. Inorg Chem. 2018;57(11):6193–7. http://dx.doi.org/10.1021/acs.inorgchem.8b00391
50. Harigaya Y, Saido TC, Eckman CB, Prada CM, Shoji M, Younkin SG. Amyloid beta protein starting pyroglutamate at position 3 is a major component of the amyloid deposits in the Alzheimer’s disease brain. Biochem Biophys Res Commun. 2000;276(2):422–7. http://dx.doi.org/10.1006/bbrc.2000.3490
51. Saido TC, Iwatsubo T, Mann DM, Shimada H, Ihara Y, Kawashima S. Dominant and differential deposition of distinct beta-amyloid peptide species, A beta N3(pE), in senile plaques. Neuron. 1995;14(2):457–66. http://dx.doi.org/10.1016/0896-6273(95)90301-1
52. Schilling S, Lauber T, Schaupp M, Manhart S, Scheel E, Bohm G, et al. On the seeding and oligomerization of pGlu-amyloid peptides (in vitro). Biochemistry. 2006;45(41):12393–9. http://dx.doi.org/10.1021/bi0612667
53. Wirths O, Erck C, Martens H, Harmeier A, Geumann C, Jawhar S, et al. Identification of low molecular weight pyroglutamate abeta oligomers in Alzheimer disease: A novel tool for therapy and diagnosis. J Biol Chem. 2010;285(53):41517–24. http://dx.doi.org/10.1074/jbc.M110.178707
54. Dammers C, Schwarten M, Buell AK, Willbold D. Pyroglutamate-modified Abeta3-42 affects aggregation kinetics of Abeta1-42 by accelerating primary and secondary pathways. Chem Sci. 2017;8(7):4996–5004. http://dx.doi.org/10.1039/C6SC04797A
55. Kuo YM, Webster S, Emmerling MR, De Lima N, Roher AE. Irreversible dimerization/tetramerization and post-translational modifications inhibit proteolytic degradation of A beta peptides of Alzheimer’s disease. Biochim Biophys Acta. 1998;1406(3):291–8. http://dx.doi.org/10.1016/S0925-4439(98)00014-3
56. Piccini A, Russo C, Gliozzi A, Relini A, Vitali A, Borghi R, et al. Beta-amyloid is different in normal aging and in Alzheimer disease. J Biol Chem. 2005;280(40):34186–92. http://dx.doi.org/10.1074/jbc.M501694200
57. Frost JL, Le KX, Cynis H, Ekpo E, Kleinschmidt M, Palmour RM, et al. Pyroglutamate-3 amyloid-β deposition in the brains of humans, non-human primates, canines, and Alzheimer disease–like transgenic mouse models. Am J Pathol. 2013;183(2):369–81. http://dx.doi.org/10.1016/j.ajpath.2013.05.005
58. Mandler M, Walker L, Santic R, Hanson P, Upadhaya AR, Colloby SJ, et al. Pyroglutamylated amyloid-beta is associated with hyperphosphorylated tau and severity of Alzheimer’s disease. Acta Neuropathol. 2014;128(1):67–79. http://dx.doi.org/10.1007/s00401-014-1296-9
59. Galante D, Corsaro A, Florio T, Vella S, Pagano A, Sbrana F, et al. Differential toxicity, conformation and morphology of typical initial aggregation states of Abeta1-42 and Abetapy3-42 beta-amyloids. Int J Biochem Cell Biol. 2012;44(11):2085–93. http://dx.doi.org/10.1016/j.biocel.2012.08.010
60. Tekirian TL, Yang AY, Glabe C, Geddes JW. Toxicity of pyroglutaminated amyloid beta-peptides 3(pE)-40 and -42 is similar to that of A beta1-40 and -42. J Neurochem. 1999;73(4):1584–9. http://dx.doi.org/10.1046/j.1471-4159.1999.0731584.x
61. Bouter Y, Dietrich K, Wittnam JL, Rezaei-Ghaleh N, Pillot T, Papot-Couturier S, et al. N-truncated amyloid beta (Abeta) 4-42 forms stable aggregates and induces acute and long-lasting behavioral deficits. Acta Neuropathol. 2013;126(2):189–205. http://dx.doi.org/10.1007/s00401-013-1129-2
62. Goldblatt G, Cilenti L, Matos JO, Lee B, Ciaffone N, Wang QX, et al. Unmodified and pyroglutamylated amyloid β peptides form hypertoxic hetero-oligomers of unique secondary structure. FEBS J. 2017;284(9):1355–69. http://dx.doi.org/10.1111/febs.14058
63. Ghiso J, Rostagno A, Tomidokoro Y, Lashley T, Bojsen-Moller M, Braendgaard H, et al. Genetic alterations of the BRI2 gene: Familial British and Danish dementias. Brain Pathol. 2006;16(1):71–9. http://dx.doi.org/10.1111/j.1750-3639.2006.tb00563.x
64. Saul A, Lashley T, Revesz T, Holton J, Ghiso JA, Coomaraswamy J, et al. Abundant pyroglutamate-modified ABri and ADan peptides in extracellular and vascular amyloid deposits in familial British and Danish dementias. Neurobiol Aging. 2013;34:1416–25. http://dx.doi.org/10.1016/j.neurobiolaging.2012.11.014
65. Schilling S, Hoffmann T, Manhart S, Hoffmann M, Demuth HU. Glutaminyl cyclases unfold glutamyl cyclase activity under mild acid conditions. FEBS Lett. 2004;563(1–3):191–6. http://dx.doi.org/10.1016/S0014-5793(04)00300-X
66. Schilling S, Zeitschel U, Hoffmann T, Heiser U, Francke M, Kehlen A, et al. Glutaminyl cyclase inhibition attenuates pyroglutamate Abeta and Alzheimer’s disease-like pathology. Nat Med. 2008;14(10):1106–11. http://dx.doi.org/10.1038/nm.1872
67. Jawhar S, Wirths O, Schilling S, Graubner S, Demuth HU, Bayer TA. Overexpression of glutaminyl cyclase, the enzyme responsible for pyroglutamate abeta formation, induces behavioral deficits, and glutaminyl cyclase knock-out rescues the behavioral phenotype in 5XFAD mice. J Biol Chem. 2011;286(6):4454–60. http://dx.doi.org/10.1074/jbc.M110.185819
68. Wirths O, Bethge T, Marcello A, Harmeier A, Jawhar S, Lucassen PJ, et al. Pyroglutamate abeta pathology in APP/PS1KI mice, sporadic and familial Alzheimer’s disease cases. J Neural Transm. 2010;117(1):85–96. http://dx.doi.org/10.1007/s00702-009-0314-x
69. Breyhan H, Wirths O, Duan K, Marcello A, Rettig J, Bayer TA. APP/PS1KI bigenic mice develop early synaptic deficits and hippocampus atrophy. Acta Neuropathol. 2009;117(6):677–85. http://dx.doi.org/10.1007/s00401-009-0539-7
70. Moro ML, Phillips AS, Gaimster K, Paul C, Mudher A, Nicoll JAR, et al. Pyroglutamate and isoaspartate modified amyloid-beta in ageing and Alzheimer’s disease. Acta Neuropathol Comm. 2018;6(1):3. http://dx.doi.org/10.1186/s40478-017-0505-x
71. Wirths O, Breyhan H, Cynis H, Schilling S, Demuth HU, Bayer TA. Intraneuronal pyroglutamate-abeta 3-42 triggers neurodegeneration and lethal neurological deficits in a transgenic mouse model. Acta Neuropathol. 2009;118(4):487–96. http://dx.doi.org/10.1007/s00401-009-0557-5
72. Alexandru A, Jagla W, Graubner S, Becker A, Bauscher C, Kohlmann S, et al. Selective hippocampal neurodegeneration in transgenic mice expressing small amounts of truncated abeta is induced by pyroglutamate-abeta formation. J Neurosci. 2011;31(36):12790–801. http://dx.doi.org/10.1523/JNEUROSCI.1794-11.2011
73. Wittnam JL, Portelius E, Zetterberg H, Gustavsson MK, Schilling S, Koch B, et al. Pyroglutamate amyloid β (Aβ) aggravates behavioral deficits in transgenic amyloid mouse model for Alzheimer disease. J Biol Chem. 2012;287(11):8154–62. http://dx.doi.org/10.1074/jbc.M111.308601
74. Cynis H, Schilling S, Bodnar M, Hoffmann T, Heiser U, Saido TC, et al. Inhibition of glutaminyl cyclase alters pyroglutamate formation in mammalian cells. Biochim Biophys Acta. 2006;1764(10):1618–25. http://dx.doi.org/10.1016/j.bbapap.2006.08.003
75. Cabrera E, Mathews P, Mezhericher E, Beach TG, Deng J, Neubert TA, et al. Aβ truncated species: Implications for brain clearance mechanisms and amyloid plaque deposition. Biochim Biophys Acta. 2018;1864(1):208–25. http://dx.doi.org/10.1016/j.bbadis.2017.07.005
76. Wirths O, Walter S, Kraus I, Klafki HW, Stazi M, Oberstein TJ, et al. N-truncated Aβ4–x peptides in sporadic Alzheimer’s disease cases and transgenic Alzheimer mouse models. Alzheimers Res Ther. 2017;9:80. http://dx.doi.org/10.1186/s13195-017-0309-z
77. Portelius E, Lashley T, Westerlund A, Persson R, Fox NC, Blennow K, et al. Brain amyloid-beta fragment signatures in pathological ageing and Alzheimer’s disease by hybrid immunoprecipitation mass spectrometry. Neurodegen Dis. 2015;15(1):50–7. http://dx.doi.org/10.1159/000369465
78. Iwata N, Tsubuki S, Takaki Y, Watanabe K, Sekiguchi M, Hosoki E, et al. Identification of the major Abeta1-42-degrading catabolic pathway in brain parenchyma: Suppression leads to biochemical and pathological deposition. Nat Med. 2000;6(2):143–50. http://dx.doi.org/10.1038/72237
79. Apte SS. A disintegrin-like and metalloprotease (reprolysin-type) with thrombospondin type 1 motif (ADAMTS) superfamily: Functions and mechanisms. J Biol Chem. 2009;284(46):31493–7. http://dx.doi.org/10.1074/jbc.R109.052340
80. Tortorella MD, Pratta M, Liu R-Q, Austin J, Ross OH, Abbaszade I, et al. Sites of aggrecan cleavage by recombinant human aggrecanase-1 (ADAMTS-4). J Biol Chem. 2000;275(24):18566–73. http://dx.doi.org/10.1074/jbc.M909383199
81. Tortorella MD, Burn TC, Pratta MA, Abbaszade I, Hollis JM, Liu R, et al. Purification and cloning of aggrecanase-1: A member of the ADAMTS family of proteins. Science. 1999;284(5420):1664–6. http://dx.doi.org/10.1126/science.284.5420.1664
82. Nasrabady SE, Rizvi B, Goldman JE, Brickman AM. White matter changes in Alzheimer’s disease: A focus on myelin and oligodendrocytes. Acta Neuropathol Comm. 2018;6(1):22. http://dx.doi.org/10.1186/s40478-018-0515-3
83. Ringman JM, O’Neill J, Geschwind D, Medina L, Apostolova LG, Rodriguez Y, et al. Diffusion tensor imaging in preclinical and presymptomatic carriers of familial Alzheimer’s disease mutations. Brain. 2007;130(Pt 7):1767–76. http://dx.doi.org/10.1093/brain/awm102
84. Mathys H, Davila-Velderrain J, Peng Z, Gao F, Mohammadi S, Young JZ, et al. Single-cell transcriptomic analysis of Alzheimer’s disease. Nature. 2019;570(7761):332–7. http://dx.doi.org/10.1038/s41586-019-1195-2
85. Jansen IE, Savage JE, Watanabe K, Bryois J, Williams DM, Steinberg S, et al. Genome-wide meta-analysis identifies new loci and functional pathways influencing Alzheimer’s disease risk. Nat Genet. 2019;51(3):404–13. http://dx.doi.org/10.1038/s41588-018-0311-9
86. Hüttenrauch M, Brauss A, Kurdakova A, Borgers H, Klinker F, Liebetanz D, et al. Physical activity delays hippocampal neurodegeneration and rescues memory deficits in an Alzheimer disease mouse model. Transl Psych. 2016;6:e800. http://dx.doi.org/10.1038/tp.2016.65
87. Dietrich K, Bouter Y, Muller M, Bayer TA. Synaptic alterations in mouse models for Alzheimer disease – A special focus on N-truncated Abeta 4-42. Molecules (Basel, Switzerland). 2018;23:718. http://dx.doi.org/10.3390/molecules23040718
88. Takeda K, Araki W, Akiyama H, Tabira T. Amino-truncated amyloid beta-peptide (Abeta5-40/42) produced from caspase-cleaved amyloid precursor protein is deposited in Alzheimer’s disease brain. FASEB J. 2004;18(14):1755–7. http://dx.doi.org/10.1096/fj.03-1070fje
89. Haußmann U, Jahn O, Linning P, Janßen C, Liepold T, Portelius E, et al. Analysis of amino-terminal variants of amyloid-β peptides by capillary isoelectric focusing immunoassay. Anal Chem. 2013;85(17):8142–9. http://dx.doi.org/10.1021/ac401055y
90. Mattsson N, Rajendran L, Zetterberg H, Gustavsson M, Andreasson U, Olsson M, et al. BACE1 inhibition induces a specific cerebrospinal fluid β-amyloid pattern that identifies drug effects in the central nervous system. PLoS One. 2012;7(2):e31084. http://dx.doi.org/10.1371/journal.pone.0031084
91. Portelius E, Dean RA, Andreasson U, Mattsson N, Westerlund A, Olsson M, et al. Beta-site amyloid precursor protein-cleaving enzyme 1(BACE1) inhibitor treatment induces Abeta5-X peptides through alternative amyloid precursor protein cleavage. Alzheimers Res Ther. 2014;6:75. http://dx.doi.org/10.1186/s13195-014-0075-0
92. Guzman EA, Bouter Y, Richard BC, Lannfelt L, Ingelsson M, Paetau A, et al. Abundance of Abeta5-x like immunoreactivity in transgenic 5XFAD, APP/PS1KI and 3xTG mice, sporadic and familial Alzheimer’s disease. Mol Neurodegen. 2014;9(1):13. http://dx.doi.org/10.1186/1750-1326-9-13
93. Huse JT, Liu K, Pijak DS, Carlin D, Lee VM-Y, Doms RW. β-Secretase processing in the Trans-Golgi Network preferentially generates truncated amyloid species that accumulate in Alzheimer’s disease brain. J Biol Chem. 2002;277(18):16278–84. http://dx.doi.org/10.1074/jbc.M111141200
94. Liu K, Solano I, Mann D, Lemere C, Mercken M, Trojanowski JQ, et al. Characterization of Abeta11-40/42 peptide deposition in Alzheimer’s disease and young Down’s syndrome brains: Implication of N-terminally truncated Abeta species in the pathogenesis of Alzheimer’s disease. Acta Neuropathol. 2006;112(2):163–74. http://dx.doi.org/10.1007/s00401-006-0077-5
95. Lee EB, Skovronsky DM, Abtahian F, Doms RW, Lee VM. Secretion and intracellular generation of truncated Abeta in beta-site amyloid-beta precursor protein-cleaving enzyme expressing human neurons. J Biol Chem. 2003;278(7):4458–66. http://dx.doi.org/10.1074/jbc.M210105200
96. Sullivan CP, Berg EA, Elliott-Bryant R, Fishman JB, McKee AC, Morin PJ, et al. Pyroglutamate-Abeta 3 and 11 colocalize in amyloid plaques in Alzheimer’s disease cerebral cortex with pyroglutamate-Abeta 11 forming the central core. Neurosci Lett. 2011;505(2):109–12. http://dx.doi.org/10.1016/j.neulet.2011.09.071