Presenilin-1 P264L Knock-In Mutation: Differential Effects on Aβ Production, Amyloid Deposition, and Neuronal Vulnerability

Abstract

The pathogenic mechanism linking presenilin-1 (PS-1) gene mutations to familial Alzheimer's disease (FAD) is uncertain, but has been proposed to include increased neuronal sensitivity to degeneration and enhanced amyloidogenic processing of the ␤-amyloid precursor protein (APP). We investigated this issue by using gene targeting with the Cre-lox system to introduce an FAD-linked P264L mutation into the endogenous mouse PS-1 gene, an approach that maintains normal regulatory controls over expression. Primary cortical neurons derived from PS-1 homozygous mutant knock-in mice exhibit basal neurodegeneration similar to their PS-1 wild-type counterparts. Staurosporine and A␤1-42 induce apoptosis, and neither the dose dependence nor maximal extent of cell death is altered by the PS-1 knock-in mutation. Similarly, glutamate-induced neuronal necrosis is unaffected by the PS-1P264L mutation. The lack of effect of the PS-1P264L mutation is confirmed by measures of basal-and toxin-induced caspase and calpain activation, biochemical indices of apoptotic and necrotic signaling, respectively. To analyze the influence of the PS-1P264L knock-in mutation on APP processing and the development of AD-type neuropathology, we created mouse lines carrying mutations in both PS-1 and APP. In contrast to the lack of effect on neuronal vulnerability, cortical neurons cultured from PS-1P264L homozygous mutant mice secrete A␤42 at an increased rate, whereas secretion of A␤40 is reduced. Moreover, the PS-1 knock-in mutation selectively increases A␤42 levels in the mouse brain and accelerates the onset of amyloid deposition and its attendant reactive gliosis, even as a single mutant allele. We conclude that expression of an FAD-linked mutant PS-1 at normal levels does not generally increase cortical neuronal sensitivity to degeneration. Instead, enhanced amyloidogenic processing of APP likely is critical to the pathogenesis of PS-1-linked FAD. Key words: presenilin; amyloid; plaque, neuronal necrosis; neuronal apoptosis; plaque; amyloid precursor protein; A␤; familial Alzheimer's Disease; gene targetingMutations in the ␤-amyloid precursor protein (APP), presenilin-1 (PS-1), and presenilin-2 (PS-2) genes are a leading cause of early onset familial Alzheimer's disease (FAD) and cosegregate with FAD in an autosomal dominant manner (Price et al., 1998;Selkoe, 1998). All forms of AD are characterized by loss of neurons and synapses in specific brain regions, and deposition of protein aggregates as A␤-containing amyloid plaques in the brain parenchyma, leptomeninges, and cerebrovasculature, and as tau-containing intraneuronal neurofibrillary tangles. At least two leading hypotheses have emerged for the pathogenic mechanisms of the mutations. According to the "amyloid cascade" hypothesis, APP and PS mutations promote formation from APP of the highly insoluble A␤42 variant, whose progressive aggregation triggers the amyloid and synaptic abnormalities and neuronal loss. It is supported by findings that all of the FAD-linked mutations examined so far i...

Wild-type Presenilin 1 Protects against Alzheimer Disease Mutation-induced Amyloid Pathology

“…Several interesting findings emerged: (i) similar to our previous analysis of endogenous A␤ (35), reduction of 50% PS1 dosage resulted in a significant decrease in both A␤40 and A␤42 without altering the ratio of A␤42/A␤40 (compare ϩ/ϩ with ϩ/Ϫ; Fig. 3, C-E); (ii) consistent with the A␤ data obtained from PS1 A264L knock-in mice (23), expression of the PS1 M146V FAD mutation was accompanied by a simultaneous reduction of A␤40 and increase in A␤42 (compare ϩ/ϩ with M146V/ϩ; Fig. 3, C and D); (iii) removal of the remaining wild-type PS1 from the APP/PS1 M146V/ϩ mice led to a further reduction of A␤40 without significantly affecting A␤42 (compare M146V/Ϫ with M146V/ϩ; Fig.…”

“…This can result from elevated production of A␤42, a reduction of A␤40, or both combined. Indeed, several publications indicate that increase in A␤42 is at the expense of reduced A␤40 (23,24), suggesting that the apparent gain of A␤42 could be secondary to partial loss of cleavage at A␤40 or other ␥-secretase sites. Furthermore, because the wild-type PS1 is present in all reported studies, whether the observed changes in A␤42/A␤40 ratio are due to intrinsic properties of the mutant protein or the interactions between the wild type and the mutant PS1 has not been explored.…”

The presenilin hypothesis of Alzheimer's disease: Evidence for a loss-of-function pathogenic mechanism

“…Perhaps more surprisingly, numerous mutations in PS1 (e.g., N135D, L166P, M233T, P264L, G384A, and C410Y) and PS2 (e.g., T122P, N141I, M239V, and M239I) can cause significant reductions in the production of A␤40, often despite a concomitant increase in the production of A␤42 (12)(13)(14)(16)(17)(18)(19). The differential effect of PS mutations on alternative cleavage positions in the APP transmembrane domain seems not to represent a simple shift in substrate preference or specificity, because the degree of elevation in A␤42 levels with individual mutations does not correlate with the degree of depression in A␤40 or AICD levels (e.g., ref.…”

Regulation of tyrosinase trafficking and processing by presenilins: Partial loss of function by familial Alzheimer's disease mutation

“…More than 100 mutations have been identified in PS1 and PS2, and they spread throughout the entire molecules. These mutations are known to alter the ␥-secretase activity, often resulting in increased A␤42 production at the expense of A␤40 (2,3). Through similar mechanisms defined as regulated intramembrane proteolysis (4), PS are required for processing and signaling of Notch (5), and this pathway likely contributes to various PS developmental activities (6).…”