Phosphorylation of amyloid beta (Aβ) peptides - a trigger for formation of toxic aggregates in Alzheimer's disease

Abstract

Alzheimer's disease (AD) is the most common form of dementia and associated with the progressive accumulation of amyloid β-peptides (Aβ) in form of extracellular amyloid plaques in the human brain. A critical role of Aβ in the pathogenesis of AD is strongly supported by gene mutations that cause early-onset familial forms of the disease. Such mutations have been identified in the APP gene itself and in presenilin 1 and 2. Importantly, all the identified mutations commonly lead to early deposition of extracellular plaques likely by increasing the generation and/or aggregation of Aβ. However, such mutations are very rare and molecular mechanisms that might trigger aggregation and deposition of Aβ, in the most common late onset AD are largely unknown. We recently demonstrated that extracellular Aβ undergoes phosphorylation by a cell surface-localized or secreted form of protein kinase A. The phosphorylation of serine residue 8 promotes aggregation by stabilization of β-sheet conformation of Aβ and increased formation of oligomeric Aβ aggregates that represent nuclei for fibrillization. Phosphorylated Aβ was detected in the brains of transgenic mice and human AD brains and showed increased toxicity in Drosophila models as compared with non-phosphorylated Aβ. Together, these findings demonstrate a novel molecular mechanism that triggers aggregation and toxicity of Aβ. Thus, phosphorylation of Aβ could be relevant in the pathogenesis of late onset AD. The identification of extracellular protein kinase A should also stimulate pharmacological approaches to decrease Aβ phosphorylation in the therapy and/or prevention of AD.

Figure 1. Schematic representation of generation of Aβ by proteolytic processing of APP and the familial AD causing APP mutations

(A) Two pathways (β/γ and α/γ) of APP proteolysis. APP can be cleaved by either β- or α-secretase, which is then followed by γ-secretase cleavage results in the generation of either the p3-fragment (non-amyloidogenic) or an Aβ (amyloigenic pathway). The designation of secretases, substrates and products are depicted, (B) Representation of APP familial AD causing mutations that are identified around N- and C-terminal and in the middle region of Aβ. The amino acid residues are numbered according to Aβ sequence. The swedish mutation (KM>NL) at N-terminus of Aβ̣ near to β-secretase cleavage site increases the total production of Aβ, whereas the mutations C-terminus of Aβ results in increased production of Aβ42 by altering γ-secretase activity. The mutations in the middle region of Aβ might decrease the α-secretory cleavage, facilitate the amyloidogenic processing, promote the Aβ production and/or increases the propensity of Aβ aggregation or stabilizes the Aβ against clearance by different proteases.

Figure 2. Nucleation-dependent polymerization model of amyloid aggregation

Amyloid formation consists of two phases: (i) a nucleation phase/lag phase, in which monomers undergo conformational change/misfolding and associate to form oligomeric nuclei, and (ii) a elongation phase/growth phase, in which the nuclei rapidly grow by further addition of monomers and form larger polymers/fibrils until saturation. The ‘nucleation phase‘, is thermodynamically unfavourable and occurs gradually, whereas ‘elongation phase’, is much more favourable process and proceeds quickly. Thus, kinetics of amyloid formation is well represented by a sigmoidal curve with a lag phase followed by rapid growth phase (green curve). The rate limiting step in the process is the formation of nuclei/seeds to promote aggregation. Thus, amyloid formation can be substantially speedup by the addition of preformed seeds (nuclei). The addition of seeds reduces the lag time and induces faster aggregate formation (red curve).