Destabilization of beta-catenin by mutations in presenilin-1 potentiates neuronal apoptosis

Apoptotic activities of wild-type and Alzheimer's disease-related mutant presenilins in Drosophila melanogaster

Mutant human presenilins cause early-onset familial Alzheimer's disease and render cells susceptible to apoptosis in cultured cell models. We show that loss of presenilin function in Drosophila melanogaster increases levels of apoptosis in developing tissues. Moreover, overexpression of presenilin causes apoptotic and neurogenic phenotypes resembling those of Presenilin loss-of-function mutants, suggesting that presenilin exerts a dominant negative effect when expressed at high levels. In Drosophila S2 cells, Psn overexpression leads to reduced Notch receptor synthesis affecting levels of the intact approximately 300-kD precursor and its approximately 120-kD processed COOH-terminal derivatives. Presenilin-induced apoptosis is cell autonomous and can be blocked by constitutive Notch activation, suggesting that the increased cell death is due to a developmental mechanism that eliminates improperly specified cell types. We describe a genetic model in which the apoptotic activities of wild-type and mutant presenilins can be assessed, and we find that Alzheimer's disease-linked mutant presenilins are less effective at inducing apoptosis than wild-type presenilin.

Targeted deficiency or cytosolic truncation of the VE-cadherin gene in mice impairs VEGF-mediated endothelial survival and angiogenesis

Vascular endothelial cadherin, VE-cadherin, mediates adhesion between endothelial cells and may affect vascular morphogenesis via intracellular signaling, but the nature of these signals remains unknown. Here, targeted inactivation (VEC-/-) or truncation of the beta-catenin-binding cytosolic domain (VECdeltaC/deltaC) of the VE-cadherin gene was found not to affect assembly of endothelial cells in vascular plexi, but to impair their subsequent remodeling and maturation, causing lethality at 9.5 days of gestation. Deficiency or truncation of VE-cadherin induced endothelial apoptosis and abolished transmission of the endothelial survival signal by VEGF-A to Akt kinase and Bcl2 via reduced complex formation with VEGF receptor-2, beta-catenin, and phosphoinositide 3 (PI3)-kinase. Thus, VE-cadherin/ beta-catenin signaling controls endothelial survival.

Lithium induces morphological differentiation of mouse neuroblastoma cells

Neuroblastoma cells are used as a model system to study neuronal differentiation. Here we describe the induction of morphological differentiation of mouse neuroblastoma Neuro 2a (N2a) cells by treatments with either chemical inhibitors of cyclin-dependent kinases or lithium, which inhibits glycogen synthase kinase-3. Cyclin-dependent kinase inhibitors cause a rapid cell cycle block as well as the extension of multiple neurites per cell. These multipolar differentiated cells then undergo a massive death. However, lithium promotes a delayed mitotic arrest and the extension of one or two long neurites per cell. This differentiation is maximal after 48 hours of lithium treatment and the differentiated cells remain viable for long periods of time. Neuronal differentiation in lithium-treated cells is preceded by the accumulation of beta-catenin, a protein which is efficiently proteolyzed when it is phosphorylated by glycogen synthase kinase-3. Both neuronal differentiation and beta-catenin accumulation are observed in lithium-treated cells either in the absence or in the presence of supraphysiological concentrations of inositol. The results are consistent with the hypothesis that inhibition of glycogen synthase kinase-3 by lithium triggers the differentiation of neuroblastoma N2a cells.

Mutant presenilin-1 induces apoptosis and downregulates Akt/PKB

Most early onset cases of familial Alzheimer's disease (AD) are caused by mutations in presenilin-1 (PS1) and presenilin-2 (PS2). These mutations lead to increased beta-amyloid formation and may induce apoptosis in some model systems. Using primary cultured hippocampal neurons (HNs) and rat pheochromocytoma (PC12) cells transiently transfected with replication-defective recombinant adenoviral vectors expressing wild-type or mutant PS1, we demonstrate that mutant PS1s induce apoptosis, downregulate the survival factor Akt/PKB, and affect several Akt/PKB downstream targets, including glycogen synthase kinase-3beta and beta-catenin. Expression of a constitutively active Akt/PKB rescues HNs from mutant PS1-induced neuronal cell death, suggesting a potential therapeutic target for AD. Downregulation of Akt/PKB may be a mechanism by which mutant PS1 induces apoptosis and may play a role in the pathogenesis of familial AD.

Phosphorylation of tau protein by recombinant GSK-3beta: pronounced phosphorylation at select Ser/Thr-Pro motifs but no phosphorylation at Ser262 in the repeat domain

Glycogen synthase kinase-3beta (GSK-3beta) has been described as a proline-directed kinase which phosphorylates tau protein at several sites that are elevated in Alzheimer paired helical filaments. However, it has been claimed that GSK-3beta can also phosphorylate the non-proline-directed KXGS motifs in the presence of heparin, including Ser262 in the repeat domain of tau, which could induce the detachment of tau from microtubules. We have analyzed the activity of recombinant GSK-3beta and of GSK-3beta preparations purified from tissue, using two-dimensional phosphopeptide mapping, immunoblotting with phosphorylation-sensitive antibodies, and phosphopeptide sequencing. The most prominent phosphorylation sites on tau are Ser396 and Ser404 (PHF-1 epitope), Ser46 and Thr50 in the first insert, followed by a less efficient phosphorylation of other Alzheimer phosphoepitopes (antibodies AT-8, AT-270, etc). We also show that the non-proline-directed activity at KXGS motifs is not due to GSK-3beta itself, but to kinase contaminations in common GSK-3beta preparations from tissues which are activated upon addition of heparin.

Presenilin overexpression arrests cells in the G1 phase of the cell cycle. Arrest potentiated by the Alzheimer's disease PS2(N141I)mutant

To investigate the mechanism by which presenilin (PS) overexpression induces apoptosis, we studied the effects of these proteins on cell cycle progression. Transiently transfected HeLa cells were bromodeoxyuridine (BrdU) labeled to visualize DNA synthesis by immunofluorescence and stained with propidium iodide to measure DNA content by fluorescence-activated cell sorting (FACS). BrdU labeling was decreased in cells expressing presenilin-1 (PS1), presenilin-2 (PS2), an Alzheimer's disease-associated missense mutation PS2(N141I), and the carboxyl-terminally deleted PS2 construct PS2(166aa), compared with mock and neurofilament-light (NF-L) transfected cells. Analysis of BrdU incorporation in mitotically synchronized HeLa cells suggested that cells were arresting in the G1 phase of the cell cycle, and this was confirmed by FACS analysis. Interestingly, cell cycle progression was more inhibited by the expression of PS2(N141I) compared with wild-type PS2. In addition, ATM, the gene product mutated in ataxia-telangiectasia, does not appear to be a downstream effector of PS-induced cell cycle arrest as transfection of PS constructs into an ataxia-telangiectasia cell line also resulted in cell cycle inhibition. Quantitative immunoblotting of whole-cell lysates from PS-transfected cells did not reveal increases or decreases in the steady-state levels of p21, p27, p53, pRb, or c-myc, suggesting that the presenilins mediate cell cycle arrest by mechanisms other than simple changes in the steady-state levels of these cell-cycle-related proteins.

Localisation of presenilin 2 in human and rodent pancreatic islet beta-cells; Met239Val presenilin 2 variant is not associated with diabetes in man

Mutations in presenilin 1 and 2 are causative factors for early onset familial Alzheimer's disease and possible roles for presenilins include protein trafficking, regulation of apoptosis and/or calcium homeostasis. Presenilin 2 mRNA is expressed in brain, muscle and pancreas but the role of pancreatic presenilin 2 and its relationship to diabetes are unknown. Presenilin 2 immunoreactivity was localised in human and rodent pancreas to islet cells and found in granules of beta-cells. Presenilin 2 was identified in primitive islet and duct cells of human foetal pancreas and in proliferating exocrine duct cells in human pancreatitis but not found in islet amyloid deposits in Type 2 diabetic subjects. Full length, approximately 50 kDa, and the approximately 30 kDa N-terminal fragment of presenilin 2 were identified by western blotting in extracted rodent pancreas but only the 30 kDa fragment was detected in mouse islets and human insulinoma. Post-mortem pancreatic morphology was normal in a subject with the presenilin 2 Met239Val variant and early onset familial Alzheimer's disease. Oral glucose tolerance tests on subjects with the presenilin 2 Met239Val mutation unaffected by early onset familial Alzheimer's disease (mean age 35 years) and on their first-degree relatives without the mutation demonstrated no evidence of glucose intolerance or increased proinsulin secretion. PS2 is a novel β-cell protein with potential roles in development or protein processing but pancreatic islet structure and function appear to be unaffected by the Met239Val mutation.

Presenilin 1 facilitates the constitutive turnover of beta-catenin: differential activity of Alzheimer's disease-linked PS1 mutants in the beta-catenin-signaling pathway

Although an association between the product of the familial Alzheimer's disease (FAD) gene, presenilin 1 (PS1), and beta-catenin has been reported recently, the cellular consequences of this interaction are unknown. Here, we show that both the full length and the C-terminal fragment of wild-type or FAD mutant PS1 interact with beta-catenin from transfected cells and brains of transgenic mice, whereas E-cadherin and adenomatous polyposis coli (APC) are not detected in this complex. Inducible overexpression of PS1 led to increased association of beta-catenin with glycogen synthase kinase-3beta (GSK-3beta), a negative regulator of beta-catenin, and accelerated the turnover of endogenous beta-catenin. In support of this finding, the beta-catenin half-life was dramatically longer in fibroblasts deficient in PS1, and this phenotype was completely rescued by replacement of PS1, demonstrating that PS1 normally stimulates the degradation of beta-catenin. In contrast, overexpression of FAD-linked PS1 mutants (M146L and DeltaX9) failed to enhance the association between GSK-3beta and beta-catenin and interfered with the constitutive turnover of beta-catenin. In vivo confirmation was demonstrated in the brains of transgenic mice in which the expression of the M146L mutant PS1 was correlated with increased steady-state levels of endogenous beta-catenin. Thus, our results indicate that PS1 normally promotes the turnover of beta-catenin, whereas PS1 mutants partially interfere with this process, possibly by failing to recruit GSK-3beta into the PS1-beta-catenin complex. These findings raise the intriguing possibility that PS1-beta-catenin interactions and subsequent activities may be consequential for the pathogenesis of AD.

A myristoylated calcium-binding protein that preferentially interacts with the Alzheimer's disease presenilin 2 protein

It is well established that mutations in the presenilin 1 and 2 genes cause the majority of early onset Alzheimer's disease (AD). However, our understanding of the cellular functions of the proteins they encode remains rudimentary. Knowledge of proteins with which the presenilins interact should lead to a better understanding of presenilin function in normal and disease states. We report here the identification of a calcium-binding protein, calmyrin, that interacts preferentially with presenilin 2 (PS2). Calmyrin is myristoylated, membrane-associated, and colocalizes with PS2 when the two proteins are overexpressed in HeLa cells. Yeast two-hybrid liquid assays, affinity chromatography, and coimmunoprecipitation experiments confirm binding between PS2 and calmyrin. Functionally, calmyrin and PS2 increase cell death when cotransfected into HeLa cells. These results allude to several provocative possibilities for a dynamic role of calmyrin in signaling, cell death, and AD.

Proteolytic release and nuclear translocation of Notch-1 are induced by presenilin-1 and impaired by pathogenic presenilin-1 mutations

The Notch family of proteins consists of transmembrane receptors that play a critical role in the determination of cell fate. Genetic studies in Caenorhabditis elegans suggest that the presenilin proteins, which are associated with familial Alzheimer's disease, regulate Notch signaling. Here we show that proteolytic release of the Notch-1 intracellular domain (NICD), an essential step in the activation of Notch signaling, is markedly reduced in presenilin-1 (PS1)-deficient cells and is restored by PS1 expression. Nuclear translocation of the NICD is also markedly reduced in PS1-deficient cells, resulting in reduced transcriptional activation. Mutations in PS1 that are associated with familial Alzheimer's disease impair the ability of PS1 to induce proteolytic release of the NICD and nuclear translocation of the cleaved protein. These results suggest that PS1 plays a central role in the proteolytic activation of the Notch-1-signaling pathway and that this function is impaired by pathogenic PS1 mutations. Thus, dysregulation of proteolytic function may underlie the mechanism by which presenilin mutations cause Alzheimer's disease.

Presenilins: structural aspects and posttranslational events

Most of early-onset forms of Alzheimer's disease (AD) are caused by inherited mutations located on chromosomes 14 and 1, the gene products of which have been recently identified and referred to as presenilins 1 (PS1) and 2 (PS2), respectively. The first phenotypic alterations triggered by mutated PS were reported to be an increased production of the amyloid peptide (Abeta) and, more precisely, its 42 amino-acids long counterpart Abeta42. This overproduction is thought to be responsible for the genesis of the senile plaques that invade the cortical and subcortical areas of these AD-affected brains. The discovery of PSs has triggered numerous studies aimed at better understanding their normal physiology and the dysfunctions brought by the mutations that could explain, at least in part, the neurodegenerative process taking place in this syndrome. In this review, I will focus on the structural aspects of PS and on the various posttranscriptional events they undergo. I will also briefly discuss that current hypotheses concerning their normal functions and the influence of FAD-linked mutations.

beta-Trcp couples beta-catenin phosphorylation-degradation and regulates Xenopus axis formation

Regulation of beta-catenin stability is essential for Wnt signal transduction during development and tumorigenesis. It is well known that serine-phosphorylation of beta-catenin by the Axin-glycogen synthase kinase (GSK)-3beta complex targets beta-catenin for ubiquitination-degradation, and mutations at critical phosphoserine residues stabilize beta-catenin and cause human cancers. How beta-catenin phosphorylation results in its degradation is undefined. Here we show that phosphorylated beta-catenin is specifically recognized by beta-Trcp, an F-box/WD40-repeat protein that also associates with Skp1, an essential component of the ubiquitination apparatus. beta-catenin harboring mutations at the critical phosphoserine residues escapes recognition by beta-Trcp, thus providing a molecular explanation for why these mutations cause beta-catenin accumulation that leads to cancer. Inhibition of endogenous beta-Trcp function by a dominant negative mutant stabilizes beta-catenin, activates Wnt/beta-catenin signaling, and induces axis formation in Xenopus embryos. Therefore, beta-Trcp plays a central role in recruiting phosphorylated beta-catenin for degradation and in dorsoventral patterning of the Xenopus embryo.

Transgenic mice with Alzheimer presenilin 1 mutations show accelerated neurodegeneration without amyloid plaque formation

Familial Alzheimer disease mutations of presenilin 1 (PS-1) enhance the generation of A beta1-42, indicating that PS-1 is involved in amyloidogenesis. However, PS-1 transgenic mice have failed to show amyloid plaques in their brains. Because PS-1 mutations facilitate apoptotic neuronal death in vitro, we did careful quantitative studies in PS-1 transgenic mice and found that neurodegeneration was significantly accelerated in mice older than 13 months (aged mice) with familial Alzheimer disease mutant PS-1, without amyloid plaque formation. However, there were significantly more neurons containing intracellularly deposited A beta42 in aged mutant transgenic mice. Our data indicate that the pathogenic role of the PS-1 mutation is upstream of the amyloid cascade.

Direct interaction of Alzheimer's disease-related presenilin 1 with armadillo protein p0071

Alzheimer's disease-related presenilins are thought to be involved in Notch signaling during embryonic development and/or cellular differentiation. Proteins mediating the cellular functions of the presenilins are still unknown. We utilized the yeast two-hybrid system to identify an interacting armadillo protein, termed p0071, that binds specifically to the hydrophilic loop of presenilin 1. In vivo, the presenilins constitutively undergo proteolytic processing, forming two stable fragments. Here, we show that the C-terminal fragment of presenilin 1 directly binds to p0071. Nine out of 10 armadillo repeats in p0071 are essential for mediating this interaction. Since armadillo proteins, like beta-catenin and APC, are known to participate in cellular signaling, p0071 may function as a mediator of presenilin 1 in signaling events.

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.

Regulation of LEF-1/TCF transcription factors by Wnt and other signals

LEF-1/TCF transcription factors mediate a nuclear response to Wnt signals by interacting with beta-catenin. Wnt signaling and other cellular events that increase the stability of beta-catenin result in transcriptional activation by LEF-1/TCF proteins in association with beta-catenin. In the absence of Wnt signaling, LEF-1/TCF proteins repress transcription in association with Groucho and CBP. The LEF-1/TCF transcription factors can also interact with other cofactors and play an architectural role in the assembly of multiprotein enhancer complexes, which may allow for the integration of multiple signaling pathways.

The role of cell cycle-mediated events in Alzheimer's disease

The mechanism(s) underlying selective neuronal death in Alzheimer's disease remain unresolved. However, recently, we and others showed that susceptible hippocampal neurones in Alzheimer's disease express markers common to cells in various phases of the cell cycle. Since neuronal maturation is associated with effective escape from the cell division cycle, emergence out of quiescence may be deleterious. Here, we review a number of current findings indicating that disregulated ectopic re-activation of cell cycle-mediated events, akin to neoplasia, represent an important early pathway associated with neuronal death and, more importantly, one that involves virtually the entire spectrum of the pathological events described in Alzheimer's disease.

Alzheimer's disease: clues from flies and worms

Presenilin mutations give rise to familial Alzheimer's disease and result in elevated production of amyloid beta peptide. Recent evidence that presenilins act in developmental signalling pathways may be the key to understanding how senile plaques, neurofibrillary tangles and apoptosis are all biochemically linked.