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APP metabolism and Amyloid-β peptides
Amyloid-β (Aβ) peptides derive from the cleavage of Amyloid precursor protein (APP), coded by a gene with the same name (APP) located on chromosome 21 in humans. APP has about 10 different isoforms, from 639 to 770 amino acids, generated by alternative splicing. APP is ubiquitously expressed in all human tissues, mostly isoforms APP751 and APP770, with higher expression in the central nervous system (CNS), due to the predominant expression of isoform APP695 in neurons, and all these three isoforms can generate Aβ (X. Wang et al. 2017).
In neurons, APP has been associated with cell adhesion, cell-to-cell and cell-to-substratum interactions (Reinhard, Hébert, and De Strooper 2005), the modulation of cell survival, growth, motility, and neurite outgrowth (O’Brien and Wong 2011), and the regulation of synapse formation (Hick et al. 2015). APP is cleaved by two main pathways during maturation and processing, the amyloidogenic and the non-amyloidogenic pathways. During the non-amyloidogenic pathway, in the plasma membrane of the cell surface, APP is cleaved by a α-secretase (e.g. ADAM10), creating a soluble extracellular fragment termed sAPPα that is released, and a transmembranar C-terminal fragment α, known as CTFα, that is subsequently cleaved by a γ-secretase (i.e. enzyme complex composed by nicastrin, APH-1, PEN-2, and PSEN1 or 2), creating a fragment corresponding to APP intracellular domain (AICD) and a 3 kDa peptide corresponding to the amino acids between α- and γ-secretases cleavage (p3). The amyloidogenic pathway occurs in endosomes, where APP is cleaved by the β-secretase BACE1, creating and releasing to the endosome lumen the sAPPβ, a soluble fragment slightly shorter than sAPPα, and the transmembranar CTFβ, which is further cleaved by a γ-secretase to produce the same AICD and a 4 kDa peptide resulting from the cleavage by β- and γ-secretases called Aβ (Nhan, Chiang, and Koo 2015; van der Kant and Goldstein 2015), Figure 3.
Aβ can have several amino acids in length, due to N- or C-terminal truncation (Dunys, Valverde, and Checler 2018). Differences in the N-terminal are most commonly achieved by BACE1 cleavage of APP at two different sites: between Methionine 596 and Aspartate 597, giving origin to Aβ1-X, and between Tyrosine 606 and Glutamate 607, giving origin to Aβ11-X (Zhang and Song 2013). The production of Aβ11-X or truncation of Aβ1-X’ N-terminal can expose glutamate residues, located at positions 3 and 11, to cyclization by glutaminyl cyclase, and transform these residues into pyroglutamate (Gunn et al. 2016). The C-terminal of Aβ also varies in residue length, mostly due to γ-secretase differential endoproteolysis of the CTFβ: (i) γ-secretase cleavage starts at the endproteolytic ε sites, (between Leucine 49 and Valine 50 or Threonine 48 and Leucine 49) to generate Aβ49 or Aβ48, (ii) followed by trimming of the C-terminal mostly every three amino acids called ζ sites, and (iii) finishing at the γ sites (following the sequences Aβ49-46-43-40 and Aβ48-45-42-38) to produce Aβ forms with 43 to 38 residues long; but how the ε sites are recognized by γ-secretase to start the endoproteolysis process is still poorly understood (De Strooper 2010; Fernandez et al. 2016).
Microtubule associated protein Tau
Tau is part of the microtubule associated proteins (MAP) family, highly conserved in the animal kingdom, which have the canonical function of microtubule assembly regulation and stabilization, through direct binding of tubulin (Himmler et al. 1989; Al-Bassam et al. 2002). Tau is almost exclusively expressed in neurons, especially in the axons, and Tau proteins are expressed in the human brain in 6 functional isoforms, resulting from the alternative splicing of the gene MAPT (microtubule associated protein Tau) (Buée et al. 2000; G. Lee and Leugers 2012). These isoforms differ between them by the presence or absence of one or two inserts in the N-terminal part of the protein (0N, 1N, 2N), responsible for the interaction with other proteins of the cytoskeleton and plasma membrane, and the presence of three or four peptide repeats before the C-terminal part of Tau (3R, 4R), mainly responsible for the interaction with tubulin and identified as the microtubule binding domain (MBD). Linking the N-terminal region and the MDB, Tau has a proline-rich domain (PRD) between amino acids 151 and 244 (using as reference the longest isoform, 2N4R, with 441 amino acids) and after the MDB, from amino acid 369 to 441, Tau has a conserved C-terminal region, both of which are present in all Tau isoforms, Figure 4. The expression levels of Tau isoforms differ during neuronal development, brain region and between physiological and pathological conditions (Y. Wang and Mandelkow 2016; Guo, Noble, and Hanger 2017).
The PRD of Tau contains 7 sequences of amino acids comprised by a Proline, any two amino acids and another Proline, known as PxxP motifs, which are commonly recognized by, and are the binding sites of, Src-homology 3 domain (SH3)-containing proteins (Reynolds et al. 2008; Morris et al. 2011).
Interplay between Tau and Aβ
One of the major questions of debate in AD research is regarding which of the pathologies, Tau or Aβ, appears first and/or is more important in AD pathology (Nisbet et al. 2015; Selkoe and Hardy 2016).
It is proposed that the development and progression of the two hallmarks have distinct and independent spatiotemporal progression (Brettschneider et al. 2015), with Aβ-plaques accumulation starting several years prior to clinical symptoms in the neocortical region and progressing towards limbic regions, while NFT appears in association with neurodegeneration and develops initially in the locus coeruleus and entorhinal cortex (Iaccarino et al. 2018). Thus, it is accepted that AD starts much earlier than the detection of the first symptoms, which makes it very difficult to clinically identify and understand the biological mechanisms that lead to it (Heiko Braak and Del Tredici 2012; Villemagne et al. 2013).
Adding complexity to this question, Amyloid plaques were observed in the brains of “healthy” people, upon autopsy, and Tau is also hallmark of other neurodegenerative diseases. Tau was shown to be essential for Aβ-induced toxicity (Rapoport et al. 2002), either through interaction with Src kinase Fyn in the postsynaptic compartment of neurons (Ittner et al. 2010), and through CaMKII-AMPK pathway and Tau phosphorylation (Mairet-Coello et al. 2013). Conversely, Aβ was shown to induce mislocalization of Tau and destabilization of synapses and microtubules (Zempel et al. 2013), and reported to induce and enhance the formation of NFT (J. Götz et al. 2001; Hurtado et al. 2010; Bennett et al. 2017).
Table of contents :
SUMMARY
Résumé en Francais
Abstract in English
ACKNOWLEDGEMENTS
LIST OF ABREVIATIONS
INTRODUCTION
La Maladie d’Alzheimer
Alzheimer’s disease
APP metabolism and Amyloid-β peptides
Microtubule associated protein Tau
Interplay between Tau and Aβ
Genetics of Alzheimer’s disease
Bridging integrator 1
Animal models of Alzheimer’s disease
High Content Screening
Proximity Ligation Assay
Objectives
RESULTS
Tau phosphorylation regulates the interaction between BIN1’s SH3 domain and Tau’s proline-rich domain
BIN1 recovers tauopathy-induced long-term memory deficits in mice and interacts with Tau through Thr348 phosphorylation
DISCUSSION AND CONCLUSION
BIBLIOGRAPHY
