R. Sherrington

Presenilin Proteins Undergo Heterogeneous Endoproteolysis between Thr291and Ala299and Occur as Stable N- and C-Terminal Fragments in Normal and Alzheimer Brain Tissue

Humans inheriting missense mutations in the presenilin (PS)1 and -2 genes undergo progressive cerebral deposition of the amyloid beta-protein at an early age and develop a clinically and pathologically severe form of familial Alzheimers disease (FAD). Because PS1 mutations cause the most aggressive known form of AD, it is important to elucidate the structure and function of this multitransmembrane protein in the brain. Using a panel of region-specific PS antibodies, we characterized the presenilin polypeptides in mammalian tissues, including brains of normal, AD, and PS1-linked FAD subjects, and in transfected and nontransfected cell lines. Very little full-length PS1 or -2 was detected in brain and untransfected cells; instead the protein occurred as a heterogeneous array of stable N- and C-terminal proteolytic fragments that differed subtly among cell types and mammalian tissues. Sequencing of the major C-terminal fragment from PS1-transfected human 293 cells showed that the principal endoproteolytic cleavage occurs at and near Met298 in the proximal portion of the large hydrophilic loop. Full-length PS1 in these cells is quickly turned over (T1/2 approximately 60 min), in part to the two major fragments. The sizes and amounts of the PS fragments were not significantly altered in four FAD brains with the Cys410Tyr PS1 missense mutation. Our results indicate that presenilins are rapidly processed to N- and C-terminal fragments in both neural and nonneural cells and that interference with this processing is not an obligatory feature of FAD-causing mutations.

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.