G. Levesque

The Presenilin 1 Protein Is a Component of a High Molecular Weight Intracellular Complex That Contains β-Catenin

The presenilin (PS) genes associated with Alzheimer disease encode polytopic transmembrane proteins which undergo physiologic endoproteolytic cleavage to generate stable NH2- and COOH-terminal fragments (NTF or CTF) which co-localize in intracellular membranes, but are tightly regulated in their stoichiometry and abundance. We have used linear glycerol velocity and discontinuous sucrose gradient analysis to investigate the distribution and native conformation of PS1 and PS2 during this regulated processing in cultured cells and in brain. The PS1 NTF and CTF co-localize in the endoplasmic reticulum (ER) and in the Golgi apparatus, where they are components of a ∼250-kDa complex. This complex also contains β-catenin but not β-amyloid precursor protein (APP). In contrast, the PS1 holoprotein precursor is predominantly localized to the rough ER and smooth ER, where it is a component of a ∼180-kDa native complex. PS2 forms similar but independent complexes. Restricted incorporation of the presenilin NTF and CTF along with a potentially functional ligand (β-catenin) into a multimeric complex in the ER and Golgi apparatus may provide an explanation for the regulated accumulation of the NTF and CTF.

Presenilin mutations associated with Alzheimer disease cause defective intracellular trafficking of β-catenin, a component of the presenilin protein complex

The presenilin proteins are components of high–molecular–weight protein complexes in the endoplasmic reticulum and Golgi apparatus that also contain β-catenin. We report here that presenilin mutations associated with familial Alzheimer disease (but not the non–pathogenic Glu318Gly polymorphism) alter the intracellular trafficking of β-catenin after activation of the Wnt/β-catenin signal transduction pathway. As with their effect on βAPP processing, the effect of PS1 mutations on trafficking of β-catenin arises from a dominant gain of aberrant function activity. These results indicate that mistrafficking of selected presenilin ligands is a candidate mechanism for the genesis of Alzheimer disease associated with presenilin mutations, and that dysfunction in the presenilin–β-catenin protein complexes is central to this process.

Presenilins Interact with Armadillo Proteins Including Neural‐Specific Plakophilin‐Related Protein and β‐Catenin

Abstract : Missense substitutions in the presenilin 1 (PS1) and presenilin 2 (PS2) proteins are associated with early‐onset familial Alzheimers disease. We have used yeast‐two‐hybrid and coimmunoprecipitation methods to show that the large cytoplasmic loop domains of PS1 and PS2 interact specifically with three members of the armadillo protein family, including β‐catenin, p0071, and a novel neuronal‐specific armadillo protein—neural plakophilin‐related armadillo protein (NPRAP). The PS1 : NPRAP interaction occurs between the arm repeats of NPRAP and residues 372‐399 at the C‐terminal end of the large cytoplasmic loop of PS1. The latter residues contain a single arm‐like domain and are highly conserved in the presenilins, suggesting that they form a functional armadillo protein binding site for the presenilins.

Presenilin 1 is Actively Degraded by the 26S Proteasome

The metabolic pathways governing the turnover of presenilin 1 (PS1) have been incompletely worked out. The PS1 holoprotein has low abundance in many cells and appears to undergo endoproteolytic cleavage near residue 298. We provide evidence that one mechanism by which the PS1 holoprotein is degraded is through the action of the 26S proteasome. We also show that the proteasome does not participate in the endoproteolytic cleavage.

Absence of association between Alzheimer disease and the −491 regulatory region polymorphism of APOE

A novel polymorphism (-491 A/T) within the regulatory region on the apolipoprotein E gene has recently been reported to be associated with risk for Alzheimer disease (AD). To test this association in an independent data set, we have examined this polymorphism in a sample of 88 well-characterized AD cases and compared the allele frequency and genotype frequencies for this polymorphism with those observed in 112 cognitively normal subjects drawn from the same ethnic group. These results suggest that in the current data set at least, the -491 A/T polymorphism is not associated with risk for AD, but may be in partial linkage disequilibrium with the APOE epsilon2/epsilon3/epsilon4 polymorphism.

Amyloid-β-protein isoforms in brain of subjects with PS1-linked, βAPP-linked and sporadic alzheimer disease

To determine whether similar abnormalities of various soluble full-length and N-terminal truncated Abeta peptides occur in postmortem cerebral cortex of affected PS1 mutation carriers, we examined the amounts of two amyloid species ending at residue 40 or at residues 42(43) using sandwich ELISA systems. Our results indicate that PS1 mutations effect a dramatic accumulation in brain of the highly insoluble potentially neurotoxic long-tailed isoforms of the Abeta peptide such as Abeta1-42(43) and Abetax-42(43). This enhancing effect of PS1 mutation on Abetax-42(43) deposition was highly similar to that of a betaAPP mutation (Val717Ile) but the effects on Abetax-40 production were significantly different between these two causal genes. In contrast to previous studies of soluble Abeta in plasma and in supernatants from cultured fibroblasts of subjects with PS1 mutations, our studies also show that there is an increase in insoluble Abetax-40 peptides in brain of subjects with PS1 mutations.

Failure to detect missense mutations in the S182 gene in a series of late-onset Alzheimer's disease cases

The possibility of an interaction of multiple genes has been speculated in pathogenesis of Alzheimers disease (AD). Because we have recently cloned a novel gene S182 bearing five different missense mutations which segregate with early-onset familial AD, we sought the frequency of these mutations in familial and sporadic late-onset AD to clarify the incidence of these mutations in the disease. The current study showed lack of these mutations in 118 independent subjects affected with late-onset Alzheimers disease.

Presenilin Proteins and the Pathogenesis of Early-Onset Familial Alzheimer’s Disease: β-Amyloid Production and Parallels to Prion Diseases

In contrast to rare mutations in the amyloid presursor protein (APP) gene, missense mutations in the presenilin 1 (PS1) and presenilin 2 (PS2) genes, on chromosomes 14 and 1 respectively, are the most common causes of early-onset familial Alzheimer’s disease (AD)(Sherrington et al. 1995)(Rogaev et al. 1995)(Levy-Lahad et al. 1995). Presenilin genes encode proteins with at least seven putative transmembrane domains and an extruded cytoplasmic “loop”, the latter with a preponderance of acidic amino acid residues: these proteins are expressed in a variety of cell types. While the physiologic function of these genes is unknown, their protein products have been demonstrated to accumulate in intracellular sites including the endoplasmic reticulum, and the Golgi apparatus. Similarly, the mechanism by which the 30 different point mutations have been identified in PS1 and PS2 to date cause the clinical and neuropathological hallmarks of Alzheimer disease is unknown. However, fibroblasts from heterozygous carriers of PS1 and PS2 mutations secrete increased levels of the amyloidogenic long-tailed amyloid β-peptides ending at residues 42 or 43 (Aβ42)(Martin 1995)(Scheuner et al. 1996). Increased levels of Aβ42 and other Aβ-peptides can also be measured in postmortem brain tissue from human patients dying with early-onset FAD associated with PS1 mutations(Lemere et al. 1996). To determine whether overproduction of Aβ peptides occurs in brain as an early biochemical event prior to the onset of neurodegeneration, we constructed transgenic mice with either mutant or wild-type human PS1 and mated them with another line of transgenic mice overexpressing wild-type human βAPP695 under the control of the same transcriptional regulatory element. These studies reveal that mutant PS1 transgenes but not wild-type PS1 transgenes act in a dominant fashion to programme over-production of long-tailed Aβ42 peptides in brain, and that this biochemical difference is present by at least 2–4 months of age and in the absence of any detectable neuropathologic lesions. These advances in our understanding of presenilin function are discussed in relation to the two schools of thought on AD pathogenesis, “tau-ist”and “saptist”, and also with regards the hypothesis that similarities between AD and prion diseases reflect the existence of shared pathogenic pathways.