Familial Alzheimer's disease-associated mutations block translocation of full-length presenilin 1 to the nuclear envelope

Chromosome Instability and Mosaic Aneuploidy in Neurodegenerative and Neurodevelopmental Disorders

Evidence from multiple laboratories has accumulated to show that mosaic neuronal aneuploidy and consequent apoptosis characterizes and may underlie neuronal loss in many neurodegenerative diseases, particularly Alzheimer’s disease and frontotemporal dementia. Furthermore, several neurodevelopmental disorders, including Seckel syndrome, ataxia telangiectasia, Nijmegen breakage syndrome, Niemann–Pick type C, and Down syndrome, have been shown to also exhibit mosaic aneuploidy in neurons in the brain and in other cells throughout the body. Together, these results indicate that both neurodegenerative and neurodevelopmental disorders with apparently different pathogenic causes share a cell cycle defect that leads to mosaic aneuploidy in many cell types. When such mosaic aneuploidy arises in neurons in the brain, it promotes apoptosis and may at least partly underlie the cognitive deficits that characterize the neurological symptoms of these disorders. These findings have implications for both diagnosis and treatment/prevention.

Role of Trisomy 21 Mosaicism in Sporadic and Familial Alzheimer's Disease

Trisomy 21 and the consequent extra copy of the amyloid precursor protein (APP) gene and increased beta-amyloid (Aβ) peptide production underlie the universal development of Alzheimer's disease (AD) pathology and high risk of AD dementia in people with Down syndrome (DS). Trisomy 21 and other forms of aneuploidy also arise among neurons and peripheral cells in both sporadic and familial AD and in mouse and cell models thereof, reinforcing the conclusion that AD and DS are two sides of the same coin. The demonstration that 90% of the neurodegeneration in AD can be attributed to the selective loss of aneuploid neurons generated over the course of the disease indicates that aneuploidy is an essential feature of the pathogenic pathway leading to the depletion of neuronal cell populations. Trisomy 21 mosaicism also occurs in neurons and other cells from patients with Niemann-Pick C1 disease and from patients with familial or sporadic frontotemporal lobar degeneration (FTLD), as well as in their corresponding mouse and cell models. Biochemical studies have shown that Aβ induces mitotic spindle defects, chromosome mis-segregation, and aneuploidy in cultured cells by inhibiting specific microtubule motors required for mitosis. These data indicate that neuronal trisomy 21 and other types of aneuploidy characterize and likely contribute to multiple neurodegenerative diseases and are a valid target for therapeutic intervention. For example, reducing extracellular calcium or treating cells with lithium chloride (LiCl) blocks the induction of trisomy 21 by Aβ. The latter finding is relevant in light of recent reports of a lowered risk of dementia in bipolar patients treated with LiCl and in the stabilization of cognition in AD patients treated with LiCl.

Alzheimer amyloid beta inhibition of Eg5/kinesin 5 reduces neurotrophin and/or transmitter receptor function

The mechanism by which amyloid beta (Aβ) causes neuronal dysfunction and/or death in Alzheimer's disease (AD) is unclear. Previously, we showed that Aβ inhibits several microtubule-dependent kinesin motors essential for mitosis and also present in mature neurons. Here, we show that inhibition of kinesin 5 (Eg5) by Aβ blocks neuronal function by reducing transport of neurotrophin and neurotransmitter receptors to the cell surface. Specifically, cell-surface NGF/NTR(p75) and NMDA receptors decline in cells treated with Aβ or the kinesin 5 inhibitor monastrol, or expressing APP. Aβ and monastrol also inhibit NGF-dependent neurite outgrowth from PC12 cells and glutamate-dependent Ca++ entry into primary neurons. Like Aβ, monastrol inhibits long-term potentiation, a cellular model of NMDA-dependent learning and memory, and kinesin 5 activity is absent from APP/PS transgenic mice brain or neurons treated with Aβ. These data imply that cognitive deficits in AD may derive in part from inhibition of neuronal Eg5 by Aβ, resulting in impaired neuronal function and/or survival through receptor mislocalization. Preventing inhibition of Eg5 or other motors by Aβ may represent a novel approach to AD therapy.

Phlorotannin-rich Ecklonia cava reduces the production of beta-amyloid by modulating alpha- and gamma-secretase expression and activity

Beta-amyloid (Aβ) is a major pathogenic peptide in Alzheimer's disease (AD) and is generated by the processing of amyloid precursor protein (APP). We have previously reported that the brown algae Ecklonia cava, which has anti-oxidant and anti-inflammatory functions, decreased Aβ production and further aggregation in HEK293 cells expressing the APP Swedish mutation. Here, we show the reduction mechanism of Aβ production using the butanol extract of Ecklonia cava through the examination of expression and activity of alpha-, beta-, and gamma-secretase. Treatment with the extract resulted in the activation of alpha-secretase with a contrasting decrease in its mRNA and protein expression. This activation was consistent with the translocation of the extract into the plasma membrane of the secretase. Gamma-secretase activity was lowered by E. cava, and this effect may be due to the decreased expression of PSEN1 mRNA and protein. In addition, the basal nuclear location of PSEN1, which may affect chromosome missegregation in neurodegenerative disease, was reduced by the extract, despite the significance of this finding remains unclear. Taken together, these results led us to conclude that E. cava regulated the expression and activity of gamma-secretase and alpha-secretase, leading to a reduction in Aβ production by the stable cells. Our data indicate that E. cava is a novel natural-product candidate for AD treatment, although further in vivo studies are needed.

Alzheimer Abeta peptide induces chromosome mis-segregation and aneuploidy, including trisomy 21: requirement for tau and APP

Chromosome aneuploidy, especially trisomy 21, arises in both familial and sporadic Alzheimer's disease. Expression of FAD genes or exposure to Aβ peptide induces aneuploidy in tg-mice and cultured cells. The requirement for GSK-3β, calpain, and Tau in Aβ-induced chromosome mis-segregation points to MT dysfunction as contributing to AD pathogenesis.

Both sporadic and familial Alzheimer's disease (AD) patients exhibit increased chromosome aneuploidy, particularly trisomy 21, in neurons and other cells. Significantly, trisomy 21/Down syndrome patients develop early onset AD pathology. We investigated the mechanism underlying mosaic chromosome aneuploidy in AD and report that FAD mutations in the Alzheimer Amyloid Precursor Protein gene, APP, induce chromosome mis-segregation and aneuploidy in transgenic mice and in transfected cells. Furthermore, adding synthetic Aβ peptide, the pathogenic product of APP, to cultured cells causes rapid and robust chromosome mis-segregation leading to aneuploid, including trisomy 21, daughters, which is prevented by LiCl addition or Ca²⁺ chelation and is replicated in tau KO cells, implicating GSK-3β, calpain, and Tau-dependent microtubule transport in the aneugenic activity of Aβ. Furthermore, APP KO cells are resistant to the aneugenic activity of Aβ, as they have been shown previously to be resistant to Aβ-induced tau phosphorylation and cell toxicity. These results indicate that Aβ-induced microtubule dysfunction leads to aneuploid neurons and may thereby contribute to the pathogenesis of AD.

Mitochondrial disorders in the nervous system

Mitochondrial diseases (encephalomyopathies) have traditionally been ascribed to defects of the respiratory chain, which has helped researchers explain their genetic and clinical complexity. However, other mitochondrial functions are greatly important for the nervous system, including protein importation, organellar dynamics, and programmed cell death. Defects in genes controlling these functions are attracting increasing attention as causes not only of neurological (and psychiatric) diseases but also of age-related neurodegenerative disorders. After discussing some pathogenic conundrums regarding the neurological manifestations of the respiratory chain defects, we review altered mitochondrial dynamics in the etiology of specific neurological diseases and in the physiopathology of more common neurodegenerative disorders.

Down’s syndrome and Alzheimer’s disease: two sides of the same coin

All Down’s syndrome individuals develop Alzheimer’s disease (AD) neuropathology by the age of 40 years. To unite the two diseases under one hypothesis, we have suggested that classical AD, both of the genetic and late-onset sporadic forms, might be promoted by small numbers of trisomy 21 cells developing during the life of the affected individual. Recent evidence from several laboratories will be presented, which strongly supports the trisomy 21 hypothesis that defects in mitosis, and particularly in chromosome segregation, may be a part of the AD process. Specifically, genetic mutations that cause familial AD disrupt the cell cycle and lead to chromosome aneuploidy, including trisomy 21, in transgenic mice and transfected cells; cells from both familial and sporadic AD patients exhibit chromosome aneuploidy, including trisomy 21. The possibility that many cases of AD are mosaic for trisomy 21 suggests novel approaches to diagnosis and therapy.

Alzheimer's presenilin 1 causes chromosome missegregation and aneuploidy

Mutations in the presenilin 1 gene cause most early onset familial Alzheimer's disease (FAD). Here, we report that a defect in the cell cycle - improper chromosome segregation - can be caused by abnormal presenilin function and therefore may contribute to AD pathogenesis. Specifically we find that either over-expression or FAD mutation in presenilin 1 (M146L and M146V) leads to chromosome missegregation and aneuploidy in vivo and in vitro: (1) Up to 20% of lymphocytes and neurons of FAD-PS-1 transgenic and knocking mice are aneuploid by metaphase chromosome analysis and in situ hybridization. (2) Transiently transfected human cells over-expressing normal or mutant PS-1 develop similar aneuploidy within 48 h, including trisomy 21. (3) Mitotic spindles in the PS-1 transfected cells contain abnormal microtubule arrays and lagging chromosomes. Several mechanisms by which chromosome missegregation induced by presenilin may contribute to Alzheimer's disease are discussed.

Characterization of presenilin complexes from mouse and human brain using Blue Native gel electrophoresis reveals high expression in embryonic brain and minimal change in complex mobility with pathogenic presenilin mutations

The presenilin proteins are required for intramembrane cleavage of a subset of type 1 membrane proteins including the Alzheimer's disease amyloid precursor protein. Previous studies indicate presenilin proteins form enzymatically active high molecular mass complexes consisting of heterodimers of N- and C-terminal fragments in association with nicastrin, presenilin enhancer-2 and anterior pharynx defective-1 proteins. Using Blue Native gel electrophoresis (BN/PAGE) we have studied endogenous presenilin 1 complex mass, stability and association with nicastrin, presenilin enhancer-2 and anterior pharynx defective-1. Solubilization of mouse or human brain membranes with dodecyl-d-maltoside produced a 360-kDa species reactive with antibodies to presenilin 1. Presenilin 1 complex levels were high in embryonic brain. Complex integrity was sensitive to Triton X-100 and SDS, but stable to reducing agent. Addition of 5 M urea caused complex dissolution and nicastrin to migrate as a subcomplex. Nicastrin and presenilin enhancer-2 were detected in the presenilin 1 complex following BN/PAGE, electroelution and second-dimension analysis. Anterior pharynx defective-1 was detected as an 18-kDa form and 9-kDa C-terminal fragment by standard SDS/PAGE of mouse tissues, and as a predominant 36-kDa band after presenilin 1 complex second-dimension analysis. Membranes from brain cortex of Alzheimer's disease patients, or from cases with presenilin 1 missense mutations, indicated no change in presenilin 1 complex mobility. Higher molecular mass presenilin 1-reactive species were detected in brain containing presenilin 1 exon 9 deletion mutation. This abnormality was confirmed using cells transfected with the same presenilin deletion mutation.

Presenilin-2 in the cynomolgus monkey brain: investigation of age-related changes

Localization of presenilin-2 (PS-2), a transmembrane protein implicated in early onset familial Alzheimer's disease, was examined in the brains of 30 cynomolgus monkeys aged 4 to 36 years. Anti-PS-2 antibody N20, which recognizes PS-2 amino acid residues 2-20, and anti-PS-2 antibody C20, which recognizes PS-2 amino acid residues 535-554, stained mainly the cytoplasm of large pyramidal neurons and large neurites. This finding was also confirmed by double immunohistochemical investigations using N20 or C20 and anti-NeuN antibody. In the brain of the oldest monkey, swollen neurites containing senile plaques were immunostained with C20, but not with N20. Western blot analyses of microsomal fractions isolated from the brains of three adult monkeys revealed that much less PS-2 was present compared to presenilin-1 (PS-1). Age-related assessment of PS-2 in brain homogenates from young and adult monkeys showed that PS-2 levels and PS-2 subcellular localization were unchanged with increasing age. Because PS-2 expression was much less robust than that of PS-1, we conclude that PS-2 mainly localizes to large neurons and does not show so drastic age-related changes as PS-1.

Hypoxia signaling to genes: significance in Alzheimer's disease

Aberrations in neural signaling, converging to and diverging from oxidative metabolism and blood supply, contribute to the initiation and maintenance of inflammatory responses, neuronal degeneration, and age-related cognitive decline in Alzheimer's disease (AD). Hypoxia/ischemia triggers phospholipase A2, leading to the accumulation of free arachidonic and docosahexaenoic acids (AA, DHA), as well as that of lysophospholipids. Some of these bioactive lipid messengers in turn give rise to several downstream lipid messengers, such as platelet-activating factor (PAF) and ecosanoids (prostaglandins and leukotrienes). Eicosanoid synthesis is highly regulated in hypoxia and in reperfusion subsequent to ischemia. As one of the consequences, mitochondrial function is disrupted and reactive oxygen species (ROS) both contribute to the expansion of cellular inflammatory responses and reduce the expression of genes required to maintain synaptic structure and function. On the other hand, pro-inflammatory genes are up-regulated. One of these, the inducible cyclooxygenase-2 (COX-2), along with oxygen-starved mitochondria, comprise the major sources of ROS in the brain during hypoxia, ischemia, and reperfusion. One outcome is a sustained metabolic stress that drives progressive dysfunction, apoptosis and/or necrosis, and brain cell death. How hypoxia modulates oxygen-sensitive gene expression is not well understood. Pro-inflammatory gene families that contribute to neurodegeneration are transiently activated in part by the heterodimeric oxygen-sensitive DNA-binding proteins nuclear factor for kappa B (NF-kappaB) and hypoxia-inducible factor-alpha (HIF-1alpha). Here the authors summarize current studies supporting the hypothesis that synaptically-derived lipid messengers play significant roles in ischemic stroke and that hypoxia is an important contributor to the onset and progression of AD neurodegeneration.

Cyclooxygenase-2 and presenilin-1 gene expression induced by interleukin-1beta and amyloid beta 42 peptide is potentiated by hypoxia in primary human neural cells

Lipid messengers and amyloid beta (Abeta) peptides generated by cyclooxygenase-2 (COX-2) and presenilin-1 (PS1) mediate pro-inflammatory signaling and neural degeneration in Alzheimer's disease (AD) brain. This study provides data showing that the COX-2 and PS1 genes each transcribe rare, highly labile RNA species that display early response gene behavior in human neural (HN) cells in primary culture, down-regulation during human neural development, and up-regulation in AD neocortex and hippocampal CA1. Together, interleukin-1beta and amyloid beta42 peptide [IL-1beta+Abeta42] synergistically activated COX-2 and PS1 gene expression preceded by increases in AP1-, STAT1alpha-, and in particular NF-kappaBp50/p65- and HIF-1alpha-DNA binding. These events were markedly potentiated by hypoxia and blocked by the antioxidant alpha-phenyl-N-tert-butyl nitrone. Broad transcription profiling further indicated that hypoxia-induced, [IL-1beta+Abeta42]-treated HN cells display robust induction of COX-2 and PS1 as well as a pro-inflammatory gene family that includes NF-kappaBp50/p105, IL-1beta precursor, and cytosolic phospholipase A2 genes. These findings indicate a novel [IL-1beta+Abeta42]-mediated, hypoxia-enhanced, free radical-triggered gene program that drives inflammatory gene signaling and suggest a mechanism by which hypoxia during aging contributes episodically to amyloidogenesis, inflammation, and AD pathophysiology.

Age-related changes in the localization of presenilin-1 in cynomolgus monkey brain

Age-related changes in PS-1 localization were examined in the brains of 22 cynomolgus monkeys ranging in age from embryonic day 87 to 35 years. In embryonic monkey brains, anti-PS-1 antibody N12, which recognizes the PS-1 N-terminal fragment (Ntf) and holo protein, stained immature neuronal cells. In juvenile monkeys, N12 stained large pyramidal neurons, cerebral neocortical neurons, and cerebellar Purkinje's cells. Cytoplasmic staining of these cells was granular in appearance. In aged monkeys, N12 stained neurons in all layers of the neocortex. In contrast, regardless of the age of the animals examined, M5, an anti-PS-1 antibody that specifically recognizes only the PS-1 C-terminal fragment (Ctf), stained neurons in all layers of the neocortex and neurons in the cerebellum. M5 also stained neuropil and white matter, and in aged monkeys, M5 stained swollen neurites of mature senile plaques. Age-related changes in PS-1 expression were further examined using Western blot analysis of mitochondrial, myelin, microsomal, nuclear, synaptosomal, and cytosol fractions isolated from 10 monkey brains ranging in age from embryonic day 87 to 32 years. In all brains, Ntf and Ctf were expressed most abundantly in the microsome fraction. The amount of PS-1 in the nuclear fraction dramatically increased with age. We conclude that the transport of PS-1 diminished with age and that PS-1 fragments accumulated in endoplasmic reticulum (ER) associated with the nuclear membrane.

Presenilin 1 regulates beta-catenin-mediated transcription in a glycogen synthase kinase-3-independent fashion

Presenilin 1 (PS1) is linked with Alzheimer's disease but exhibits functional roles regulating growth and development. For instance, PS1 binds to beta-catenin and modulates beta-catenin signaling. In the current study, we observed that knockout of PS1 inhibited beta-catenin-mediated transcription by 35%, as shown by a luciferase reporter driven by the hTcf-4 promoter. Overexpressing wild-type PS1 increased beta-catenin-mediated transcription by 37.5%, and overexpressing PS1 with mutations associated with Alzheimer's disease decreased beta-catenin-mediated transcription by 66%. To examine whether regulation of beta-catenin by PS1 requires phosphorylation by glycogen synthase kinase 3beta (GSK 3beta), we examined whether inhibiting GSK 3beta activity overcomes the inhibition of beta-catenin transcription induced by mutant PS1 constructs. Cells expressing wild-type or mutant PS1 were treated with LiCl, which inhibits GSK 3beta, or transfected with beta-catenin constructs that lack the GSK 3beta phosphorylation sites. Neither treatment overcame PS1-mediated inhibition of beta-catenin signaling, suggesting that regulation of beta-catenin by PS1 was not affected by the activity of GSK 3beta. To investigate how PS1 might regulate beta-catenin signaling, we determined whether PS1 interacts with other elements of the beta-catenin signaling cascade, such as the Tcf-4 transcription factor. Coimmunoprecipitation studies showed binding of PS1 and hTcf-4, and examining nuclear isolates indicated that nuclear hTcf-4 was decreased in cells expressing mutant PS1. These data show that PS1 interacts with multiple components of the beta-catenin signaling cascade and suggest that PS1 regulates beta-catenin in a manner independent of GSK 3beta activity.