Generation and deposition of Aβ43 by the virtually inactive presenilin‐1 L435F mutant contradicts the presenilin loss‐of‐function hypothesis of Alzheimer's disease

Abstract

As stated by the prevailing amyloid cascade hypothesis, Alzheimer's disease (AD) is caused by the aggregation and cerebral deposition of long amyloid‐β peptide (Aβ) species, which are released from a C‐terminal amyloid precursor protein fragment by γ‐secretase. Mutations in its catalytic subunit presenilin‐1 (PS1) increase the Aβ42 to Aβ40 ratio and are the major cause of familial AD (FAD). An opposing hypothesis states that loss of essential presenilin functions underlies the disease. A major argument for this hypothesis is the observation that the nearly inactive PS1 L435F mutant, paradoxically, causes FAD. We now show that the very little Aβ generated by PS1 L435F consists primarily of Aβ43, a highly amyloidogenic species which was overlooked in previous studies of this mutant. We further demonstrate that the generation of Aβ43 is not due to a trans‐dominant effect of this mutant on WT presenilin. Furthermore, we found Aβ43‐containing plaques in brains of patients with this mutation. The aberrant generation of Aβ43 by this particular mutant provides a direct objection against the presenilin hypothesis.

Aβ43‐producing PS1 FAD mutants cause altered substrate interactions and respond to γ‐secretase modulation

“…However, since deficiencies in the initial e-site cleavage are largely compensated by the unaffected wt PS1 and PS2 alleles in human FAD brain, while pathogenic Ab ratios persist [20], it is unlikely that FAD mutant c-secretases show an impact on signaling functions. This is also and prominently seen for the PS1 R278I and PS1 L435F mutants [16][17][18][19] indicating that their strong deficiency in presenilin endoproteolysis may contribute to their loss in processivity thereby resulting in the formation of longer Ab43 species. This is also and prominently seen for the PS1 R278I and PS1 L435F mutants [16][17][18][19] indicating that their strong deficiency in presenilin endoproteolysis may contribute to their loss in processivity thereby resulting in the formation of longer Ab43 species.…”

“…In support of their pathogenicity, changes in the ratios of Ab42 to Ab40 correlate with the clinical age of onset of the particular PS1 FAD mutant [13]. For these mutants, the processivity changes appear also to be associated with a stronger loss of function in the esite cleavage of C99 [16,17,19]. For these mutants, the processivity changes appear also to be associated with a stronger loss of function in the esite cleavage of C99 [16,17,19].…”

“…They lead to the generation of abnormal relative amounts of the longer Ab forms as a result of a reduced carboxyterminal processivity [11,12]. While it has been well established that Ab42/Ab40 ratios are increased for PS1 and PS2 FAD mutants [14,15], in a few cases such as the PS1 R278I or PS1 L435F mutants increased levels of Ab43 rather than Ab42 are observed [16][17][18]. While it has been well established that Ab42/Ab40 ratios are increased for PS1 and PS2 FAD mutants [14,15], in a few cases such as the PS1 R278I or PS1 L435F mutants increased levels of Ab43 rather than Ab42 are observed [16][17][18].…”

Substrate recruitment of γ‐secretase and mechanism of clinical presenilin mutations revealed by photoaffinity mapping

“…The following polyclonal and monoclonal antibodies were used: N1660 against NCT (Sigma), O2C2 against APH‐1aL (Thermo Scientific), 1638 against PEN‐2 (Steiner et al , ), PS1NT against the PS1 NTF (Capell et al , ; Covance), 5E12 against the PS1 CTF (Kretner et al , ), and penta‐His (Qiagen).…”

The amyloid hypothesis of Alzheimer's disease at 25 years

“…They have studied presenilin‐1 mutations that generally lower Aβ and hardly raise relative Aβ42 levels, but this work may overlook an elevation of the Aβ43 and other longer species, which are highly amyloidogenic (Saito et al , ). Although AD‐causing presenilin mutations can indeed be interpreted as partial loss of function from a genetics perspective, pinpointing the function of presenilin as an aspartyl endopeptidase allows one instead to speak in biochemical terms of a functional shift of the principal proteolytic cleavages to more C‐terminal bonds in the substrate (Kretner et al , ). Humans with pathogenic presenilin mutations are heterozygotes and experience no loss of function of Notch cleavage; rather, they have accelerated Aβ42 and Aβ43 accumulation that long precedes their AD‐typical memory syndrome.…”