Tau phosphorylation in cells transfected with wild-type or an Alzheimer's disease mutant Presenilin 1

A Novel PSEN1 M139L Mutation Found in a Chinese Pedigree with Early-Onset Alzheimer's Disease Increases Aβ42/Aβ40 ratio

BACKGROUND

Presenilin1 (PSEN1) is the most common gene related to familial Alzheimer's disease (AD). Only several mutation types from Chinese have been reported, with less biological function research conducted.

OBJECTIVES

We explore the pathological function of PSEN1 M139L, a mutation located at α-helix of PSEN1 transmembrane 2, using predictive programs and in vitro study and compare its effects on Aβ production to those of an artificial PSEN1 S141G located at non α-helix mutation face.

METHODS

APP, PSEN1, and PSEN2 genes were screened for mutations using Sanger sequencing in the DNA samples of the proband and additional available family members. Disease-mutation cosegregation analysis and three software programs were performed to predict the mutation's pathogenicity. In vitro, we investigated the impact of these mutations on Aβ production in HEK293-APPswe cells using lentiviral vectors harboring PSEN1 WT, PSEN1 M139L, the positive control (PSEN1 M139V) and the non α-helical mutation (PSEN1 S141G). In addition, we co-transfected PSEN1 and tau into cells to determine the mutations' impact on tau phosphorylation.

RESULTS

PSEN1 M139L mutation was discovered in the index patient and four affected siblings. Cosegregation analysis and silicon prediction suggested the mutation was probably disease causing. In vitro studies demonstrated that both PSEN1 M139L and PSEN1 S141G caused elevated ratios of Aβ42/Aβ40, but changes of tau phosphorylation were not detected.

CONCLUSION

The novel PSEN1 M139L mutation found in familial AD increases the Aβ42/Aβ40 ratio significantly. Mutations at non α-helical mutation face of PSEN1 TM2 can affect Aβ production and the region may play a key role in PSEN1 function.

Exploring the Role of PSEN Mutations in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia. Mutations of presenilin (PSEN) genes that encode presenilin proteins have been found as the vital causal factors for early-onset familial AD (FAD). AD pathological features such as memory loss, synaptic dysfunction, and formation of plaques have been successfully mimicked in the transgenic mouse models that coexpress FAD-related presenilin and amyloid precursor protein (APP) variants. γ-Secretase (GS) is an enzyme that plays roles in catalyzing intramembranous APP proteolysis to release pathogenic amyloid beta (Aβ). It has been found that presenilins can play a role as the GS's catalytic subunit. FAD-related mutations in presenilins can modify the site of GS cleavage in a way that can elevate the production of longer and highly fibrillogenic Aβ. Presenilins can interact with β-catenin to generate presenilin complexes. Aforesaid interactions have also been studied to observe the mutational and physiological activities in the catenin signal transduction pathway. Along with APP, GS can catalyze intramembrane proteolysis of various substrates that play a vital role in synaptic function. PSEN mutations can cause FAD with autosomal dominant inheritance and early onset of the disease. In this article, we have reviewed the current progress in the analysis of PSENs and the correlation of PSEN mutations and AD pathogenesis.

PS1 activates PI3K thus inhibiting GSK-3 activity and tau overphosphorylation: effects of FAD mutations

Phosphatidylinositol 3-kinase (PI3K) promotes cell survival and communication by activating its downstream effector Akt kinase. Here we show that PS1, a protein involved in familial Alzheimer's disease (FAD), promotes cell survival by activating the PI3K/Akt cell survival signaling. This function of PS1 is unaffected by gamma-secretase inhibitors. Pharmacological and genetic evidence indicates that PS1 acts upstream of Akt, at or before PI3K kinase. PS1 forms complexes with the p85 subunit of PI3K and promotes cadherin/PI3K association. Furthermore, conditions that inhibit this association prevent the PS1-induced PI3K/Akt activation, indicating that PS1 stimulates PI3K/Akt signaling by promoting cadherin/PI3K association. By activating PI3K/Akt signaling, PS1 promotes phosphorylation/inactivation of glycogen synthase kinase-3 (GSK-3), suppresses GSK-3-dependent phosphorylation of tau at residues overphosphorylated in AD and prevents apoptosis of confluent cells. PS1 FAD mutations inhibit the PS1-dependent PI3K/Akt activation, thus promoting GSK-3 activity and tau overphosphorylation at AD-related residues. Our data raise the possibility that PS1 may prevent development of AD pathology by activating the PI3K/Akt signaling pathway. In contrast, FAD mutations may promote AD pathology by inhibiting this pathway.

Forebrain degeneration and ventricle enlargement caused by double knockout of Alzheimer's presenilin-1 and presenilin-2

Early-onset familial Alzheimer's disease is the most aggressive form of Alzheimer's, striking patients as early as their 30s; those patients typically carry mutations in presenilin-1 and presenilin-2. To investigate the coordinated functions of presenilin in the adult brain, we generated double knockout mice, in which both presenilins were deleted in the forebrain. We found that concurrent loss of presenilins in adulthood resulted in massive cortical shrinkage, atrophy of hippocampal molecular layers and corpus callosum, and enlargement of the lateral and third ventricles. We further revealed that deficiency of presenilins caused a series of biochemical alterations, including neuronal atrophy, astrogliosis, caspase-3-mediated apoptosis, and tau hyperphosphorylation. Thus, our study demonstrates that presenilins are essential for the ongoing maintenance of cortical structures and function.

Mutant presenilin 1 proteins induce cell death and reduce tau-dependent processes outgrowth

The expression of familial Alzheimer's disease mutants of presenilin-1 (PS1) proteins has been observed to induce cell death in cellular systems. To investigate how this phenomenon might be associated to alterations of the microtubule network, we have studied the effect of wild-type and mutant (C263R, P264L and delta9) PS1 proteins expression on the formation of microtubule-dependent processes outgrowth and the association of PS1 to the insoluble cytoskeletal fraction in a cell line expressing the tau microtubule-associated protein. Expression of wild-type and mutant PS1 was associated with increased cell death, most marked for the P264L and delta9 mutants. The three PS1 mutants induced a significant reduction of the length of cell processes. These effects were not associated to a change in tau phosphorylation. However, the mutant PS1 proteins increased the proportion of insoluble tau in the cytoskeletal fraction and they were concentrated in the same fraction. These results suggest that PS1 proteins interact with the microtubule network, affect its organization and that this phenomenon, more marked for the PS1 mutants, might play a role in the cell dysfunction induced by mutant PS1 proteins.

Inhibition of proteasomal activity causes inclusion formation in neuronal and non-neuronal cells overexpressing Parkin

Association between protein inclusions and neurodegenerative diseases, including Parkinson's and Alzheimer's diseases, and polyglutamine disorders, has been widely documented. Although ubiquitin is conjugated to many of these aggregated proteins, the 26S proteasome does not efficiently degrade them. Mutations in the ubiquitin-protein ligase Parkin are associated with autosomal recessive juvenile Parkinsonism. Although Parkin-positive inclusions are not detected in brains of autosomal recessive juvenile Parkinsonism patients, Parkin is found in Lewy bodies in sporadic disease. This suggests that loss of Parkin ligase activity via mutation, or sequestration to Lewy bodies, is a contributory factor to sporadic disease onset. We now demonstrate that decreased proteasomal activity causes formation of large, noncytotoxic inclusions within the cytoplasm of both neuronal and nonneuronal cells overexpressing Parkin. This is not a general phenomenon as there is an absence of similar inclusions when HHARI, a structural homolog of Parkin, is overexpressed. The inclusions colocalize with ubiquitin and with proteasomes. Furthermore, Parkin inclusions colocalize with gamma-tubulin, acetylated alpha-tubulin, and cause redistribution of vimentin, suggesting aggresome-like properties. Our data imply that lower proteasomal activity, previously observed in brain tissue of Parkinson's disease patients, leads to Parkin accumulation and a concomitant reduction in ligase activity, thereby promoting Lewy body formation.

Cyclin-dependent kinase-5/p35 phosphorylates Presenilin 1 to regulate carboxy-terminal fragment stability

Mutations in the Presenilin 1 gene are the cause of the majority of autosomal dominant familial forms of Alzheimer's disease. Presenilin 1 (PS1) is produced as a holoprotein but is then rapidly processed to amino- (N-PS1) and carboxy-terminal (C-PS1) fragments that are incorporated into stable high molecular mass complexes. The mechanisms that control PS1 cleavage and stability are not properly understood but sequences within C-PS1 have been shown to regulate both of these properties. Here we demonstrate that cyclin dependent kinase-5/p35 (cdk5/p35) phosphorylates PS1 on threonine(354) within C-PS1 both in vitro and in vivo. Threonine(354) phosphorylation functions to selectively stabilize C-PS1. Our results demonstrate that cdk5/p35 is a regulator of PS1 metabolism.

Increased tau phosphorylation but absence of formation of neurofibrillary tangles in mice double transgenic for human tau and Alzheimer mutant (M146L) presenilin-1

Neurofibrillary tangles, composed of tau proteins, are a key lesion observed in sporadic forms of Alzheimer's disease and in familial forms associated with mutations of presenilin-1 (PS1). We have generated a double transgenic mouse line expressing a human tau isoform and a mutated form of PS1 (M146L) in neurons. Increased expression of the PS1 holoprotein was observed in the tau/PS1 transgenic mice and the proteolytic fragments of PS1 did not appear to be modified. A somatodendritic accumulation of the transgenic tau and an increase in tau phosphorylation were observed in both tau- and tau/PS1 transgenic mice. Neurofibrillary tangles were not observed in animals analyzed up to 17 months. Immunoprecipitation of tau from brain homogenates demonstrated its binding with active glycogen synthase kinase-3beta in control, tau- and tau/PS1 transgenic lines. These results suggest that overexpression of this Alzheimer mutant PS1 in vivo is not by itself sufficient to induce the formation of neurofibrillary tangles, even in neurons co-expressing and accumulating a human tau isoform.

X11 alpha and x11 beta interact with presenilin-1 via their PDZ domains

X11 alpha and X11 beta are two neuronal adaptor proteins that interact with the Alzheimer's disease amyloid precursor protein (APP). X11 alpha and X11 beta stabilise APP and inhibit production of proteolytic APP fragments including the A beta peptide that is deposited in the brains of Alzheimer's disease patients. The mechanisms by which X11 alpha and X11 beta modulate APP processing are not clear but one possibility is that they influence the activity of the secretases that cleave APP to give rise to A beta. Presenilin-1 is required for gamma-secretase activity and here we demonstrate that both X11 alpha and X11 beta interact with presenilin-1. X11/presenilin-1 binding is via two X11 PDZ domains and sequences within the carboxy-terminus of presenilin-1. We also demonstrate that both X11 alpha and X11 beta mediate the formation of complexes between APP and presenilin-1. These results suggest that the X11 regulation of APP processing is controlled, at least in part, via their interactions with APP and presenilin-1.

Presenilin structure, function and role in Alzheimer disease

Numerous missense mutations in the presenilins are associated with the autosomal dominant form of familial Alzheimer disease. Presenilin genes encode polytopic transmembrane proteins, which are processed by proteolytic cleavage and form high-molecular-weight complexes under physiological conditions. The presenilins have been suggested to be functionally involved in developmental morphogenesis, unfolded protein responses and processing of selected proteins including the beta-amyloid precursor protein. Although the underlying mechanism by which presenilin mutations lead to development of Alzheimer disease remains elusive, one consistent mutational effect is an overproduction of long-tailed amyloid beta-peptides. Furthermore, presenilins interact with beta-catenin to form presenilin complexes, and the physiological and mutational effects are also observed in the catenin signal transduction pathway.

Does dysregulation of the Notch and wingless/Wnt pathways underlie the pathogenesis of Alzheimer's disease?

Alzheimer's disease is characterized by the presence of neurofibrillary tangles and senile neuritic plaques in the brain. Tangles are aggregates of paired helical filaments composed of the microtubule-associated protein, tau, in a hyperphosphorylated state. Senile plaques have a core of amyloid beta-peptide derived by proteolysis of the amyloid precursor protein. A major hurdle in defining the pathogenic mechanisms in Alzheimer's disease is to understand how both amyloid beta-peptide deposition and paired helical filament formation are biochemically linked. Recent genetic discoveries provide some clues, suggesting that components of two developmentally important signalling pathways, Notch and wingless, or the vertebrate homologue of wingless, Wnt, are involved.

No influence of presenilin1 I143T and G384A mutations on endogenous tau phosphorylation in human and mouse neuroblastoma cells

Presenilin1 (PSEN1) 1143T and G384A mutations give rise to severe early-onset Alzheimer's disease in two extensively studied Belgian families. In the present study, we examined the effect of PSEN1 1143T and G384A mutations on tau phosphorylation in human SH-SY5Y and mouse Neuro-2a neuroblastoma cell lines that were transiently transfected with wild type (WT) or mutant PSEN1. With a phosphorylation independent antibody, no alteration in the electrophoretic mobility of tau was observed between wild type and mutant PSEN1 transfectants. Also, densitometric analysis of Tau1 immunoreactivity, characteristic of unphosphorylated tau, demonstrated no significant differences between WT and mutant PSEN1 transfectants. Our data suggest that in the cellular models we used, transient overexpression of 1143T and G384A mutant PSEN1 does not lead to increased tau phosphorylation.

Mint2/X11-like colocalizes with the Alzheimer's disease amyloid precursor protein and is associated with neuritic plaques in Alzheimer's disease

Aberrant metabolism of the amyloid precursor protein (APP) is believed to be at least part of the pathogenic process in Alzheimer's disease. The carboxy-terminus of APP has been shown to interact with the Mint/X11 family of phosphotyrosine binding (PTB) domain-bearing proteins. It is via their PTB domains that the Mints/X11s bind to APP. Here we report the cloning of full-length mouse Mint2 and demonstrate that in primary cortical neurons, Mint2 and APP share highly similar distributions. Mint2 also colocalizes with APP in transfected CHO cells. In Mint2/APP-cotransfected cells, Mint2 reorganizes the subcellular distribution of APP and also increases the steady-state levels of APP. Finally, we demonstrate that Mint2 is associated with the neuritic plaques found in Alzheimer's disease but not with neurofibrillary tangles. These results are consistent with a role for Mint2 in APP metabolism and trafficking, and suggest a possible role for the Mints/X11s in the pathogenesis of Alzheimer's disease.

Biology of presenilins as causative molecules for Alzheimer disease

Many missense mutations in the presenilins are associated with autosomal dominant forms of familial Alzheimer disease (AD). Presenilin genes encode polytopic transmembrane proteins, which are processed by proteolytic cleavage and form high-molecular-weight complexes under physiological conditions. The presenilins have been suggested to be functionally involved in developmental morphogenesis, apoptosis signal pathways, and processing of selected proteins including beta-amyloid precursor protein. Although the underlying mechanism in which presenilin mutations lead to development of AD remains elusive, one consistent mutational effect is an overproduction of long-tailed amyloid beta-peptides. Furthermore, presenilins interact with beta-catenin to form presenilin complexes and presenilin mutations effect beta-catenin signalling pathways.

Presenilin mutations associated with Alzheimer disease cause defective intracellular trafficking of beta-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 beta-catenin. We report here that presenilin mutations associated with familial Alzheimer disease (but not the non-pathogenic Glu318Gly polymorphism) alter the intracellular trafficking of beta-catenin after activation of the Wnt/beta-catenin signal transduction pathway. As with their effect on betaAPP processing, the effect of PS1 mutations on trafficking of beta-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-beta-catenin protein complexes is central to this process.

Cloning of the presenilin 2 cDNA and its distribution in brain of the primate, Microcebus murinus: coexpression with betaAPP and Tau proteins

A 1340-bp cDNA fragment encoding the lemurian presenilin 2 protein (PS2) was isolated from a Microcebus murinus brain cDNA library by PCR using oligonucleotide primers based on the nucleotide sequence of the human gene. Analysis of five isolated clones showed that the sequence encoded a 448-amino-acid open reading frame, 95.5% identical to the human and 93.5% identical to the mouse presenilin 2 sequences. However, neither the localization of the 2 positions in PS2 nor that of the 43 positions in PS1 associated with early onset Alzheimer's disease were changed. Expression of the presenilin 2 was detected by RT-PCR and compared with that of presenilin 1 and betaAPP in the brain and in peripheral tissues (liver, kidney, and spleen). Immunohistochemistry with a specific polyclonal antiserum raised against a synthetic peptide from the N-terminal part of the human PS2 showed that the protein is distributed throughout the microcebe brain, in vascular and nerve structures. In cortical and in subcortical areas, PS2 labeling was weak and granular in appearance and was scattered throughout the cytoplasm of many neurones, extending into neurites. The gene expression of PS2 increased with age but was not affected by the presence of numerous amyloid plaques. Double labeling immunocytochemistry detected very few neurones with combined immunoreactivity PS2 and APP, or PS2 and Tau.

Presenilin 1 associates with glycogen synthase kinase-3beta and its substrate tau

Families bearing mutations in the presenilin 1 (PS1) gene develop Alzheimer's disease. Previous studies have shown that the Alzheimer-associated mutations in PS1 increase production of amyloid beta protein (Abeta1-42). We now show that PS1 also regulates phosphorylation of the microtubule-associated protein tau. PS1 directly binds tau and a tau kinase, glycogen synthase kinase 3beta (GSK-3beta). Deletion studies show that both tau and GSK-3beta bind to the same region of PS1, residues 250-298, whereas the binding domain on tau is the microtubule-binding repeat region. The ability of PS1 to bring tau and GSK-3beta into close proximity suggests that PS1 may regulate the interaction of tau with GSK-3beta. Mutations in PS1 that cause Alzheimer's disease increase the ability of PS1 to bind GSK-3beta and, correspondingly, increase its tau-directed kinase activity. We propose that the increased association of GSK-3beta with mutant PS1 leads to increased phosphorylation of tau.

Localization and possible functions of presenilins in brain

Presenilin-1 (PS-1) is localized to chromosome 14 and presenilin-2 (PS-2) to chromosome 1. Mutations in these genes, primarily in PS-1, account for an estimated 60% of early onset familial Alzheimer's disease cases (FAD), while FAD cases account for about 10% of all Alzheimer's disease (AD) cases. The mutations are minor but are 100% penetrant, suggesting that the proteins have acquired a toxic gain in function. The proteins have multiple transmembrane domains and have been reported to be localized to the Golgi apparatus, endoplasmic reticulum, nuclear membranes and cell surface membranes. They are thought to have functions associated with vesicular trafficking, Notch signaling and apoptosis. PS mutants show relative increases in the amount of A beta42/43 compared with A beta40 in plasma, fibroblasts and brain, observations which have been taken as a possible mechanism of their role in AD. In brain, the mRNAs for these two genes are localized primarily in neurons, with the strongest in situ hybridization signals being observed in the hippocampus, cerebellum and cerebral cortex. In AD, signals detected in the hippocampus are weaker than those in normals, while signals in the cerebellum are comparable. Immunohistochemical localization of the proteins is also primarily in neurons, and, at least for PS-1, is reduced in AD affected areas. PS-1 is localized to granular structures which are most abundant in cell bodies and dendrites. The functions of the presenilins are not yet known, but available evidence points to pyramidal neurons as the most logical site for pathological change in AD.