Generation and deposition of A43 by the virtually inactive presenilin-1 L435F mutant contradicts the presenilin loss-of-function hypothesis of Alzheimer's disease

As stated by the prevailing amyloid cascade hypothesis, Alzheimer's disease (AD) is caused by the aggregation and cerebral deposition of long amyloid- peptide (A) species, which are released from a C-terminal amyloid precursor protein fragment by -secretase. Mutations in its catalytic subunit presenilin-1 (PS1) increase the A42 to A40 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 A generated by PS1 L435F consists primarily of A43, a highly amyloidogenic species which was overlooked in previous studies of this mutant. We further demonstrate that the generation of A43 is not due to a trans-dominant effect of this mutant on WT presenilin. Furthermore, we found A43-containing plaques in brains of patients with this mutation. The aberrant generation of A43 by this particular mutant provides a direct objection against the presenilin hypothesis

Generation and deposition of A43 by the virtually inactive presenilin-1 L435F mutant contradicts the presenilin loss-of-function hypothesis of Alzheimer's disease

As stated by the prevailing amyloid cascade hypothesis, Alzheimer's disease (AD) is caused by the aggregation and cerebral deposition of long amyloid- peptide (A) species, which are released from a C-terminal amyloid precursor protein fragment by -secretase. Mutations in its catalytic subunit presenilin-1 (PS1) increase the A42 to A40 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 A generated by PS1 L435F consists primarily of A43, a highly amyloidogenic species which was overlooked in previous studies of this mutant. We further demonstrate that the generation of A43 is not due to a trans-dominant effect of this mutant on WT presenilin. Furthermore, we found A43-containing plaques in brains of patients with this mutation. The aberrant generation of A43 by this particular mutant provides a direct objection against the presenilin hypothesis

Attenuated Aβ42 Responses to Low Potency γ-Secretase Modulators Can Be Overcome for Many Pathogenic Presenilin Mutants by Second-generation Compounds*

Sequential processing of the β-amyloid precursor protein by β- and γ-secretase generates the amyloid β-peptide (Aβ), which is widely believed to play a causative role in Alzheimer disease. Selective lowering of the pathogenic 42-amino acid variant of Aβ by γ-secretase modulators (GSMs) is a promising therapeutic strategy. Here we report that mutations in presenilin (PS), the catalytic subunit of γ-secretase, display differential responses to non-steroidal anti-inflammatory drug (NSAID)-type GSMs and more potent second-generation compounds. Although many pathogenic PS mutations resisted lowering of Aβ42 generation by the NSAID sulindac sulfide, the potent NSAID-like second-generation compound GSM-1 was capable of lowering Aβ42 for many but not all mutants. We further found that mutations at homologous positions in PS1 and PS2 can elicit differential Aβ42 responses to GSM-1, suggesting that a positive GSM-1 response depends on the spatial environment in γ-secretase. The aggressive pathogenic PS1 L166P mutation was one of the few pathogenic mutations that resisted GSM-1, and Leu-166 was identified as a critical residue with respect to the Aβ42-lowering response of GSM-1. Finally, we found that GSM-1-responsive and -resistant PS mutants behave very similarly toward other potent second-generation compounds of different structural classes than GSM-1. Taken together, our data show that a positive Aβ42 response for PS mutants depends both on the particular mutation and the GSM used and that attenuated Aβ42 responses to low potency GSMs can be overcome for many PS mutants by second generation GSMs