Beta-amyloid and tau in Alzheimer's disease

​​Explore beta-amyloid and tau in Alzheimer's disease and access the tools you need to better understand the role they play within this complex condition.  

Healthy amyloid precursor protein (APP) is cleaved to pathogenic beta-amyloid (Aβ) peptides through the sequential action of β- and γ-secretases. Similarly, tau protein is hyperphosphorylated by GSK3 and other kinases to first form paired helical filaments and subsequently neurofibrillary tangles (NFTs).

Here you can find an overview of beta-amyloid and tau, including their functions and structures, as well as find the antibodies and kits you need for your Alzheimer's disease research.

​​​​Contents

• Beta-amyloid in Alzheimer's disease
  ​
  • APP processing
  • Conformational variation
  • Conformation-specific amyloid beta antibodies
• Tau in Alzheimer's disease
  ​
  • Function of tau
  • Structure of tau
  • Role of tau in Alzheimer's disease
  • Antibodies and kits for tau research
• References

​​Beta-amyloid in Alzheimer's disease

​Alzheimer's disease is characterized by the presence of neurotoxic Aβ plaques in the brain. These plaques are formed by monomeric Aβ spontaneously assembling into soluble oligomers, which cluster together to form insoluble fibrils. Evidence suggests a role for both soluble oligomers and insoluble fibrils in Alzheimer’s disease pathology, but their exact contributions are still under debate. Here we cover the generation of Aβ from APP and how structural variation of Aβ may explain the complex pathology of Alzheimer’s disease.

APP processing

Pathogenic Aβ peptides are produced by the proteolytic cleavage of APP by enzyme complexes β- and γ-secretases. APP cleavage occurs via two distinct pathways (Figure 1). The non-amyloidogenic pathway provides beneficial neurotrophic effects and the amyloidogenic pathway produces neurotoxic Aβ peptides. The Aβ peptides formed via the amyloidogenic pathway can misfold and aggregate into deposits that contribute to Alzheimer’s disease pathology.

​​​​​Figure 1. The non-amyloidogenic and amyloidogenic pathways of APP processing.

The non-amyloidogenic pathway

The non-amyloidogenic pathway involves cleavage of APP by α-secretase to generate two fragments: an 83 amino acid C-terminal fragment (C83) that remains in the membrane and an N-terminal ectodomain (sAPPα) that is released into the extracellular medium.

Three enzymes have been identified with α-secretase activity: ADAM9, ADAM10, and ADAM17¹. Importantly, cleavage of APP by α-secretase occurs within the Aβ domain and consequently prohibits Aβ peptide production.

Of note, the C83 membrane fragment can be subsequently cleaved by γ-secretase to produce a short fragment called P3 peptide and an APP intracellular domain (AICD). To date, the P3 peptide is believed to be pathologically irrelevant².

The amyloidogenic pathway

The amyloidogenic pathway leads to neurotoxic Aβ generation. β-secretase (BACE1) mediates the first proteolysis step, which releases a large N-terminal ectodomain (sAPPβ) into the extracellular medium. A 99-amino acid C terminal fragment (C99) remains in the membrane^3–5.

The newly exposed C99 N-terminus corresponds to the first amino acid of Aβ. Successive cleavage of this fragment by γ-secretase (between residues 38 and 43) releases the Aβ peptide. γ-secretase is a complex of enzymes consisting of presenilin 1 or 2 (PS1 and PS2), nicastrin, anterior pharynx defective (APH-1) and presenilin enhancer 2 (PEN2) ^6–10.

Most of the Aβ peptides are 40 residues in length (Aβ 1–40), with a small percentage containing 42 residues (Aβ 1–42). Aβ 1–42 is considered the more neurotoxic form because the extra two amino acids provide a greater tendency to misfold and subsequently aggregate¹¹. Elevated plasma levels of Aβ 1–42 have been correlated with Alzheimer’s disease¹².

BACE inhibitors

Targeting Aβ accumulation by inhibiting its production is important to slow down the progression of Alzheimer’s disease. Blocking APP cleavage is made possible due to access to several β-secretase inhibitors.

Table 1. Commonly used inhibitors targeting β-secretase and Aβ production.

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Table 2. Recommended tools to study Aβ in Alzheimer's disease.

Conformational variation of beta-amyloid

One barrier to understanding the role of Aβ in Alzheimer’s disease is the lack of correlation between Aβ in the brain and the cognitive ability of patients. For example, some patients with Aβ deposits show no symptoms of Alzheimer’s disease at all^20,21.

The answer to Alzheimer’s disease heterogeneity may lie in structural variations of Aβ, which can form polymorphic Aβ oligomers in a process known as segmental polymorphism. This is where the segments that form beta sheets vary between different fibril structures^22–24.

Therefore, like in prion diseases, unique forms of structurally distinct Aβ might be deposited in different places and at different times in the brains of patients with Alzheimer’s disease. However, it remains unclear which types of deposit are more closely linked with the cognitive symptoms of the disease²⁵.

The need for conformation-specific antibodies

With increasing evidence to support the biomedical importance of Aβ structural variation, conformation-specific Aβ imaging reagents will play a central role in the future of Alzheimer’s research.

Research in humans has shown the clinical relevance of Aβ structural variation. Tissue taken from two Alzheimer’s disease patients with distinct clinical histories revealed that each patient had a predominant Aβ fibril structure; however, the dominant structure was different in each patient²⁶.

Studies in mice and cultured cells have also supported the biological relevance of Aβ structural variation. Structurally distinct Aβ fibrils cause varying levels of toxicity in neuronal cultures, while mice given Aβ from different sources develop distinct patterns of Aβ deposition within the brain^27,28.

Furthermore, the complexity of Aβ structure is convincingly reflected by the immune system, with research showing that the antibodies produced in response to Aβ fibrils are diverse, reflecting their structural variation^25,29.

Considering all evidence, it is becoming increasingly clear that a single antibody will not be enough to study or target all the possible pathological aggregates of Aβ contributing to Alzheimer’s disease. This makes conformation-specific Aβ antibodies an essential tool for the future of Alzheimer’s disease research^30–33.

​​Conformation-specific amyloid beta antibodies

In collaboration with Professor Charles Glabe (UC Irvine), we developed rabbit monoclonal antibodies against Aβ 1–42 fibrils that can distinguish conformational variation in amyloid structures.

• Human Aβ (1–42) fibril immunogen
• Rabbit monoclonal antibodies for high affinity and specificity
• Validated using dot blot and IHC-P
• Published in The Journal of Biological Chemistry

Table 3. Antibody reactivity in human and mouse Alzheimer's disease brain.

^{*14 month-old 3xTg-AD mouse model of Alzheimer's disease
** IHC shown in Hatami et al. 2014}

• Browse all conformation-specific Aß antibodies

Tau in Alzheimer's disease

A brief introduction to the tau protein and how it contributes to Alzheimer's disease.

Function of tau

Under normal circumstances, tau is a microtubule-associated protein (MAP) involved in microtubule stabilization. However, it is also a multi-functional protein with a critical role in certain neurodegenerative disorders including Alzheimer's disease³⁴.

The tau protein is highly soluble, expressed in neurons, oligodendrocytes, and astrocytes within the central nervous system (CNS) and peripheral nervous system (PNS)^35,36.

Tau is primarily found in axons where it regulates microtubule polymerization and stabilization. However, its broad selection of binding partners suggests that it has multiple functions, including postnatal brain maturation, regulation of axonal transport and signaling cascades, cellular response to heat shock, and adult neurogenesis³⁷.​​

Structure of tau

Tau can be divided into four regions: an N-terminal region, a proline-rich domain, a microtubule-binding domain (MBD), and a C-terminal region³⁸.  The human tau gene (MAPT) contains 16 exons, and alternative splicing of exons 2, 3, and 10 yields six isoforms (Figure 2).

The tau protein contains 85 potential serine (S), threonine (T), and tyrosine (Y) phosphorylation sites, and under normal conditions, phosphorylation helps to maintain the cytoskeletal structure³⁹.  Abnormal phosphorylation of tau is known to contribute to Alzheimer's disease pathology, with approximately 45 specific phosphorylation sites identified in the Alzheimer's disease brain^39,40.

In addition to phosphorylation, tau undergoes multiple post-translational modifications, including glycosylation, glycation, truncation, nitration, oxidation, polyamination, ubiquitination, SUMOylation and aggregation⁴¹.

​​Figure 2. Alternative splicing of tau produces isoforms ranging in length from 352 to 441 amino acids. Exons 2 and 3 of the tau gene encode two N terminal inserts (N1 and N2). Absence of exons 2 and 3 gives rise to 0N tau isoforms, the inclusion of exon 2 results in 1N isoforms and inclusion of both exons 2 and 3 produces 2N isoforms. R1–R4 represent the four microtubule-binding domains with R2 being encoded by exon 10. Inclusion of exon 10 results in 4R isoforms, while exclusion results in 3R isoforms.​

Role of tau in Alzheimer's disease

Accumulation of plaques and intracellular NFTs (Figure 3) is correlated with Alzheimer's disease symptoms and results in neuron damage and death⁴².  It is now believed that soluble Aβ and tau work in tandem, independently of their accumulation into plaques and tangles, to push neurons towards a diseased state⁴².​​

Figure 3. Formation of neurofibrillary tangles (NFTs) by the tau protein in tauopathies such as Alzheimer’s disease. Under pathological conditions, tau becomes hyperphosphorylated and detaches from microtubules. Phosphorylated tau then aggregates to form paired helical filaments (PHFs) and NFTs.

In Alzheimer’s disease, the elevation of intracellular soluble Aβ leads to the abnormal phosphorylation of tau and its release from microtubules in a soluble monomeric form^39,43.  In response to Aβ, tau is relocated from axons to the somatodendritic compartments of neurons⁴³.  Here, tau can bind and sequester the Src tyrosine kinase, fyn, altering its localization⁴⁴.

Elevated levels of fyn accompany the elevated levels of tau in dendritic spines, allowing the phosphorylation and stabilization of excitatory GluN2B NMDA receptors. This enhances glutamate signaling and causes an intracellular flood of Ca²⁺, which enhances Aβ toxicity^42,44,45.  Calcium-induced excitotoxicity can damage post-synaptic sites and cause mitochondrial Ca²⁺ overload, membrane depolarization, oxidative stress and apoptotic cell death^42,46,47.

Extracellular vesicles may be involved in the dissemination of pathological Aβ and tau in a prion-like propagation of Alzheimer's disease plagues and NFTs^48,49.

Novel therapeutic strategies for the treatment of Alzheimer's disease may include preventing the Aβ-induced, tau-dependent enhancement of NMDA receptor activity, by reducing dendritic levels of fyn¹⁸ or targeting tau directly⁵⁰.

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​​​​Antibodies and kits for tau research

Study tau from every angle with a comprehensive array of research tools to get new insights into tau pathology. From aggregation inhibitors to kits and antibodies, find everything you need to untangle tau, in one place.

Total tau detection

Tau is found in soluble form in the normal brain but in Alzheimer's disease, becomes aggregated and insoluble. Easily detect total tau with bovine serum albumin (BSA) and azide-free antibody, an antibody panel, or an ELISA kit from the table below.

Table 4. Tools to detect total tau.

^{Note: Studying insoluble tau can be problematic, consider using detergents such as RIPA and Sarkosyl.53}

Tau phosphorylation

Tau function is governed by phosphorylation, which becomes dysregulated during pathology, resulting in mislocalization, aggregation, and neuronal death. Effortlessly study all aspects of tau phosphorylation with antibodies against different post-translationally modified sites.

Table 5. Tools to study tau phosphorylation.

Browse all antibodies to detect phosphorylated tau.

If you need multiple phosphorylated tau antibodies, try our Tau antibody panel (ab226492).

Tau aggregation inhibitors

The presence of aggregated tau corresponds with pathology in diseases such as Alzheimer’s. Effectively inhibit the formation of neurofibrillary tangles with a potent tau aggregation inhibitor.

Table 6. Tau inhibitors.

Browse all tau inhibitors

Browse microtubule activators and inhibitors

Tau and neuroinflammation

There is a complex interplay between misfolded proteins and neuroinflammation in neurodegenerative disease.

Table 7. Tools to study tau in the context of neuroinflammation.

• Profile multiple cytokines with a pre-designed panel, or define your own -learn more about FirePlex immunoassays
• Get results in 90 minutes - discover the range of SimpleStep ELISA^® kits

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