The nonconserved hydrophilic loop domain of presenilin (PS) is not required for PS endoproteolysis or enhanced Aβ42 production mediated by familial early onset Alzheimer's disease-linked PS variants

Abstract

Presenilin 1 (PS1) and presenilin 2 (PS2) are polytopic membrane proteins that are mutated in the majority of early onset familial Alzheimer's disease (FAD) cases. Two lines of evidence establish a critical role for PS in the production of β-amyloid peptides (Aβ). FAD-linked PS mutations elevate the levels of highly amyloidogenic Aβ ending at residue 42 (Aβ42), and cells with ablated PS1 alleles secrete low levels of Aβ. Several recent reports have shown that the hydrophilic loop (HL) domain, located between transmembrane domains 6 and 7, contains sites for phosphorylation, caspase cleavage, and sequences that bind several PS-interacting proteins. In the present report, we examined the metabolism of PS polypeptides lacking the HL domain and the influence of these molecules on Aβ production. We report that the deletion of the HL domain does not have a deleterious effect on the regulated endoproteolysis of PS, saturable accumulation of PS fragments, or the self-association of PS fragments. Aβ production was not significantly altered in cells expressing HL-deleted PS polypeptides compared with cells expressing full-length PS. Importantly, deletion of the HL domain did not affect FAD mutation-mediated elevation in the production of Aβ42. Furthermore, the deletion of the HL domain did not impair the role of PSI or PS2 in facilitating Notch processing. Thus, our results argue against a biologically or pathologically relevant role for the HL domain phosphorylation and caspase cleavage and the association of PS HL domain- interacting proteins, in amyloid precursor protein metabolism and Aβ production or Notch cleavage.