Effects of presenilin N-terminal fragments on production of amyloid beta peptide and accumulation of endogenous presenilins

Three-dimensional structure of the signal peptide peptidase

Signal peptide peptidase (SPP) is an atypical aspartic protease that hydrolyzes peptide bonds within the transmembrane domain of substrates and is implicated in several biological and pathological functions. Here, we analyzed the structure of human SPP by electron microscopy and reconstructed the three-dimensional structure at a resolution of 22 Å. Enzymatically active SPP forms a slender, bullet-shaped homotetramer with dimensions of 85 × 85 × 130 Å. The SPP complex has four concaves on the rhombus-like sides, connected to a large chamber inside the molecule. Intriguingly, the N-terminal region of SPP is sufficient for the tetrameric assembly. Moreover, overexpression of the N-terminal region inhibited the formation of the endogenous SPP tetramer and the proteolytic activity within cells. These data suggest that the homotetramer is the functional unit of SPP and that its N-terminal region, which works as the structural scaffold, has a novel modulatory function for the intramembrane-cleaving activity of SPP.

A family of membrane proteins associated with presenilin expression and gamma-secretase function

Presenilin 1 (PS1) forms the gamma-secretase complex with at least three components: nicastrin, APH-1, and PEN-2. This complex mediates intramembrane cleavage of amyloid precursor protein (APP) to generate beta-amyloid protein (Abeta) as well as other type 1 transmembrane proteins. Although PS1 mutations linked to familial Alzheimer's disease influence these cleavages, their biological consequences have not been fully understood. In this study, we used mRNA differential display analysis to identify a gene, denoted adoplin-1/ORMDL-1, which displays significantly reduced expression in association with PS1 mutations. Adoplin-1 and two highly homologous genes (adoplin-2, -3) constitute a gene family that encodes transmembrane proteins. The mRNA and protein levels of adoplins (particularly adoplin-1, -2) were markedly elevated in PS-deficient fibroblasts, compared to wild-type cells. Moreover, knockdown of the three adoplins by RNA interference affected maturation of nicastrin and its association with PS1. Adoplin knockdown additionally resulted in elevated levels of APP C-terminal fragments and decreased Abeta production, suggestive of reduced gamma-secretase activity. Our data collectively indicate that adoplins are unique molecules with PS-related expression and functions that may play important role(s) in the maturation and activity of the gamma-secretase complex.

Abnormal intracellular trafficking of high affinity nerve growth factor receptor, Trk, in stable transfectants expressing presenilin 1 protein

The pathogenesis of Alzheimer's disease (AD) is now thought to be tightly linked to Abeta deposition and oxidative stress, but it is still unknown how these factors result in neuronal dysfunction and cell death. Mutations of presenilin 1 (PS1) gene are the causative gene for early onset familial AD (FAD) due to the overproduction and deposition of pathogenic Abeta1-42 peptides. We report here the molecular influences of the overexpression of PS1 protein by stable transfection of PS1 cDNA into SH-SY5Y neuroblastoma cells on the function of high affinity nerve growth factor receptor, Trk, that is essential for neuronal survival and differentiation. We examined the sensitivity of these transfectants to oxidative stress and found that mutant (I143T) PS1-expressing clones showed the highest vulnerability to an oxidative stress inducer, hydrogen peroxide treatment compared with that of mock-transfected clones, whereas wild PS1-expressing cells were less vulnerable to the treatment than mutant PS1 transfectants. Because nerve growth factor (NGF) is known to protect neuronal cells from oxidative stress-induced cell death, we examined the NGF-Trk-mediated intracellular signaling pathway in these transfectants. In the wild and mutant PS1 cDNA-transfected cells, NGF did not elicit the autophosphorylation response of Trk, although their basal levels of tyrosine phosphorylation were higher than those of mock-transfected cells. Immunocytochemical and subcellular fractionation studies revealed that most of Trk proteins are abnormally located in the cytoplasm as well as in the nucleus in PS1-overexpressing clones irrespective of wild and mutant forms. These results strongly indicate that the expression level of PS1 protein has a cross talk with the Trk-dependent neuroprotective intracellular signaling pathway.

Enhanced generation of intracellular Aβ42 amyloid peptide by mutation of presenilins PS1 and PS2

The accumulation of amyloid beta-peptide (Abeta) in the brain is a critical pathological process in Alzheimer's disease (AD). Recent studies have implicated intracellular Abeta in neurodegeneration in AD. To investigate the generation of intracellular Abeta, we established human neuroblastoma SH-SY5Y cells stably expressing wild-type amyloid precursor protein (APP), Swedish mutant APP, APP plus presenilin 1 (PS1) and presenilin 2 (PS2; wild-type or familial AD-associated mutant), and quantified intracellular Abeta40 and Abeta42 in formic acid extracts by sensitive Western blotting. Levels of both intracellular Abeta40 and Abeta42 were 2-3-fold higher in cells expressing Swedish APP, compared with those expressing wild-type APP. Intracellular Abeta42/Abeta40 ratios were approximately 0.5 in these cells. These ratios were increased markedly in cells expressing mutant PS1 or PS2 compared with those expressing their wild-type counterparts, consistent with the observed changes in secreted Abeta42/Abeta40 ratios. High total levels of intracellular Abeta were observed in cells expressing mutant PS2 because of a marked elevation of Abeta42. Immunofluorescence staining additionally revealed more intense Abeta42 immunoreactivity in mutant PS2-expressing cells than in wild-type cells, which was partially colocalized with immunoreactivity for the trans-Golgi network and endosomes. The data collectively indicate that PS mutations promote the accumulation of intracellular Abeta42, which appears to be localized in multiple subcellular compartments.

Pro-apoptotic effect of presenilin 2 (PS2) overexpression is associated with down-regulation of Bcl-2 in cultured neurons

Presenilin 2 (PS2) is a polytopic membrane protein that is mutated in some cases of familial Alzheimer's disease (AD). The normal functions of PS2 and its pathogenic role in AD remain unclear. We investigated the biological role of this protein in neurons, using adenovirus-mediated transduction of the PS2 gene into rat primary cortical neurons. Immunocytochemical analyses demonstrated increased PS2 immunoreactivity in most neurons infected with recombinant adenoviruses expressing PS2. Neurons infected with wild-type or mutant (N141I) PS2-expressing adenoviruses showed a significant increase in basal cell death, compared with those infected with control beta-galactosidase-expressing adenovirus. Moreover, PS2 overexpression markedly increased neuronal susceptibility to staurosporine-induced apoptosis. Mutant PS2 was more effective in enhancing apoptosis than its wild-type counterpart. Staurosporine-induced death was significantly inhibited by a specific caspase 3 inhibitor. Western analyses revealed that Bcl-2 protein expression was specifically down-regulated in neurons overexpressing PS2, which temporally corresponded to the accumulation of C- and N-terminal fragments of PS2. Additionally, expression of mutant, but not wild-type PS2, increased the production of beta-amyloid protein (Abeta) 42. These data collectively suggest that the pro-apoptotic effect of PS2 is mediated by down-regulation of Bcl-2. PS2 mutations may increase the susceptibility of neurons to apoptotic stimuli by perturbing the regulation of cell death.

Metabolism of presenilins

Understanding mechanisms involved in the production of Abeta has long been the central focus of cell biologists engaged in molecular AD research. The discovery of two genes that encode homologous polytopic membrane proteins termed Presenilins (PS), has lead to several exciting recent findings on the proteolytic processes responsible for generating the COOH-terminus of Abeta. What we now know is that PS proteins play an important role in Abeta production and are considered one of the therapeutic targets. Here I have reviewed the vast literature on the biology of PS, especially focusing on PS endoproteolysis and the accumulation of stable PS derivatives that are likely the functional units.

Mutational analysis of intrinsic regions of presenilin 2 that determine its endoproteolytic cleavage and pathological function

To investigate the significance of endoproteolytic processing of presenilin 2 (PS2) on its pathological function, we constructed PS2 cDNAs causing amino acid substitutions or deletions around the cleavage site. We found that a PS2 mutant (Del3) with a 20-amino acid deletion was not endoproteolytically processed, while other PS2s with amino acid substitutions and short deletions were cleaved. Overproduction of all the mutant proteins led to a compensatory decrease of endogenous PS1 fragments, but did not affect the amyloid beta peptide X-42/Abeta X-40 ratio without the familial Alzheimer's disease mutation. The Del3 mutant did not exhibit significant deficits in gamma-secretase activity. The turnover rate of the Del3 holoprotein was the same as that of full-length PS2. These data suggest that the determinants of the PS2 cleavage site reside within a large region and that the pathological function of PS2 is exerted by familial Alzheimer's disease mutations not related to the cleavage of holoproteins. We also found that PS2 with an 18-amino acid deletion at the C-terminal end was not processed. Its overexpression led neither to diminished accumulation of endogenous PS1 fragments nor to increased production of amyloid beta peptide X-42. The C-terminal end of PS2 seems to possess the signal for entry into the processing pathway.

Amyloidogenic function of the Alzheimer's disease-associated presenilin 1 in the absence of endoproteolysis

Alzheimer's disease (AD) is characterized by the invariant accumulation of senile plaques predominantly composed of the pathologically relevant 42-amino acid amyloid beta-peptide (Abeta42). The presenilin (PS) proteins play a key role in Abeta generation. FAD-associated mutations in PS1 and PS2 enhance the production of Abeta42, and PS1 is required for physiological Abeta production, since a gene knockout of PS1 and dominant negative mutations of PS1 abolish Abeta generation. PS proteins undergo endoproteolytic processing, and current evidence indicates that fragment formation may be required for the amyloidogenic function of PS. We have now determined the sequence requirements for endoproteolysis of PS1. Mutagenizing amino acids at the previously determined major cleavage site (amino acid 298) had no effect on PS1 endoproteolysis. In contrast, mutations or deletions at the additional cleavage site around amino acid 292 blocked endoproteolysis. The uncleavable PS1 derivatives accumulated as full-length proteins and replaced the endogenous PS1 proteins. In contrast to the previously described aspartate mutations within transmembrane domains 6 and 7, the uncleaved PS1 variants do not act as dominant negative inhibitors of Abeta production. Moreover, when a FAD-associated mutation (M146L) was combined with a mutation blocking endoproteolysis, Abeta42 production still reached pathological levels. These data therefore demonstrate that endoproteolysis of presenilins is not an absolute prerequisite for the amyloidogenic function of PS1. These data also show that accumulation of the PS1 holoprotein is not associated with the pathological activity of PS1 mutations as suggested previously.