Role of membrane interactions in the functional assembling of the γ-secretase and APP cleavage

The Alzheimer’s disease (AD) is a neurodegenerative disease characterized by memory deficits and a progressive decline of cognitive functions. The typical histological hallmarks of this disease are the senile plaques, formed by extracellular aggregates of the Amyloid-beta (Aβ) peptide, and the intracellular neurofibrillary tangles, formed by aggregation of hyperphosphorylated tau protein. The Aβ peptides, notably the Aβ 40 and Aβ 42, are produced and released from the cell by the amyloidogenic processing of its precursor protein (APP). The key enzyme involved in the amyloidogenic pathway is the γ-secretase, a tetrameric complex in which the active core is the Presenilin (PS1 or PS2), a nine transmembrane domain protein. Once the complex is correctly assembled, the Presenilin undergoes endoproteolysis in its cytosolic loop, converting the γ- secretase to its mature and enzymatically active form. Noteworthy the different familiar form of Alzheimer’s disease (FAD) are strictly dependent on mutations located either in APP or the Presenilin genes. An important number of mutations are clustered in PS1 and PS2 transmembrane domains (TMD). Aminoacid sequence analysis of PS revealed the presence of two conserved in frame GxxxG-like motif in PS1 and PS2 TMD8. GxxxG motifs are considered as fundamental determinants which allow a close contact between two facing transmembrane helices. In the present study, by several biochemical techniques we demonstrate the involvement of these motifs in the final maturation of PS and in their fundamental contribution to the enzymatic activity in particular for APP cleavage. Interestingly, cytological investigations showed that the GxxxG-like motif do not affect the interaction between the APP and PS. Our results strongly suggest that the TMD8 interaction motifs are critical for the formation of a functional γ-secretase. We are currently investigating in different cellular models how the mutations impact on PS1 and PS2 function