Inhibition of neurite outgrowth by familial Alzheimer's disease-linked presenilin-1 mutations

Preferential Involvement of BRCA1/BARD1, Not Tip60/Fe65, in DNA Double-Strand Break Repair in Presenilin-1 P117L Alzheimer Models

Recently, we showed that DNA double-strand breaks (DSBs) are increased by the Aβ₄₂-amyloid peptide and decreased by all-trans retinoic acid (RA) in SH-SY5Y cells and C57BL/6J mice. The present work was aimed at investigating DSBs in cells and murine models of Alzheimer's disease carrying the preseniline-1 (PS1) P117L mutation. We observed that DSBs could hardly decrease following RA treatment in the mutated cells compared to the wild-type cells. The activation of the amyloidogenic pathway is proposed in the former case as Aβ₄₂- and RA-dependent DSBs changes were reproduced by an α-secretase and a γ-secretase inhibitions, respectively. Unexpectedly, the PS1 P117L cells showed lower DSB levels than the controls. As the DSB repair proteins Tip60 and Fe65 were less expressed in the mutated cell nuclei, they do not appear to contribute to this difference. On the contrary, full-length BRCA1 and BARD1 proteins were significantly increased in the chromatin compartment of the mutated cells, suggesting that they decrease DSBs in the pathological situation. These Western blot data were corroborated by in situ proximity ligation assays: the numbers of BRCA1-BARD1, not of Fe65-Tip60 heterodimers, were increased only in the mutated cell nuclei. RA also enhanced the expression of BARD1 and of the 90 kDa BRCA1 isoform. The increased BRCA1 expression in the mutated cells can be related to the enhanced difficulty to inhibit this pathway by BRCA1 siRNA in these cells. Overall, our study suggests that at earlier stages of the disease, similarly to PS1 P117L cells, a compensatory mechanism exists that decreases DSB levels via an activation of the BRCA1/BARD1 pathway. This supports the importance of this pathway in neuroprotection against Alzheimer's disease.

Brain-derived neurotrophic factor increases cell number of neural progenitor cells derived from human induced pluripotent stem cells

Background

Several pieces of evidence from in vitro studies showed that brain-derived neurotrophic factor (BDNF) promotes proliferation and differentiation of neural stem/progenitor cells (NSCs) into neurons. Moreover, the JAK2 pathway was proposed to be associated with mouse NSC proliferation. BDNF could activate the STAT-3 pathway and induce proliferation in mouse NSCs. However, its effects on proliferation are not fully understood and JAK/STAT pathway was proposed to play a role in this activity.

Methods

In the present study, the effects of BDNF on cell proliferation and neurite outgrowth of Alzheimer’s disease (AD) induced pluripotent stem cells (iPSCs)-derived human neural progenitor cells (hNPCs) were examined. Moreover, a specific signal transduction pathway important in cell proliferation was investigated using a JAK2 inhibitor (AG490) to clarify the role of that pathway.

Results

The proliferative effect of BDNF was remarkably observed as an increase in Ki-67 positive cells. The cell number of hNPCs was significantly increased after BDNF treatment represented by cellular metabolic activity of the cells measured by MTT assay. This noticeable effect was statistically shown at 20 ng/ml of BDNF treatment. BDNF, however, did not promote neurite outgrowth but increased neuronal cell number. It was found that AG490 suppressed hNPCs proliferation. However, this inhibitor partially decreased BDNF-induced hNPCs proliferation. These results demonstrated the potential role of BDNF for the amelioration of AD through the increase of AD-derived hNPCs number.

Mulberry Fruit Cultivar 'Chiang Mai' Prevents Beta-Amyloid Toxicity in PC12 Neuronal Cells and in a Drosophila Model of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, characterized by chronic neuron loss and cognitive problems. Aggregated amyloid beta (Aβ) peptides, a product of cleaved amyloid precursor protein (APP) by beta-secretase 1 (BACE-1), have been indicated for the progressive pathogenesis of AD. Currently, screening for anti-AD compounds in foodstuffs is increasing, with promising results. Hence, the purpose of this study was to investigate the extraction conditions, phytochemical contents, and anti-AD properties, targeting Aβ peptides of Morus cf. nigra 'Chiang Mai' (MNCM) both in vitro and in vivo. Data showed that the aqueous extract of MNCM contained high amounts of cyanidin, keracyanin, and kuromanin as anthocyanidin and anthocyanins. The extract also strongly inhibited cholinesterases and BACE-1 in vitro. Moreover, MNCM extract prevented Aβ-induced neurotoxicity and promoted neurite outgrowth in neuronal cells. Interestingly, MNCM extract reduced Aβ1-42 peptides and improved locomotory coordination of Drosophila co-expressing human APP and BACE-1, specifically in the brain. These findings suggest that MNCM may be useful as an AD preventive agent by targeting Aβ formation.

PSEN1 p.Thr116Ile Variant in Two Korean Families with Young Onset Alzheimer's Disease

An in depth study of PSEN1 mutation p.Thr116Ile (c.335C>T) is presented from two Korean families with autosomal dominant inheritance. Clinical manifestation of our patients included memory loss, attention deficits, visuospatial dysfunction, agnosia, aphasia, apraxia, and personality changes, which occurred in their 30s. PSEN1 Thr116Ile was initially discovered in an Italian patient and two French families with early onset Alzheimer’s disease (EOAD) with similar age of onset. To verify the possible pathogenic mechanisms of mutation, in silico predictions and 3D modeling were performed. Structure predictions revealed significant aberrations in first hydrophilic loop (HL-I loop). The hydrophobic isoleucine could alter the loop orientation through increased hydrophobic contacts with the surrounding amino acids. Mutation could destroy a possible hydrogen bond between tyrosine 115 and threonine 116, which may affect the loop conformation. HL-I was confirmed as a conservative region of PSEN1, which may be critical in PSEN1 functions. An additional pathogenic mutation, PSEN1 Thr116Asn, was also found for the same residue, where the patient presented young onset AD (YOND). Other mutations in HL-I loop, such as Tyr115His and Glu120Asp, were described in patients with YOND, supporting the critical role of HL-I loop in PSEN1 activity.

Overexpression of SNX3 Decreases Amyloid-β Peptide Production by Reducing Internalization of Amyloid Precursor Protein

BACKGROUND

Sorting nexins (SNXs) have diverse functions in protein sorting and membrane trafficking. Recently, single-nucleotide polymorphisms in SNX3 were found to be associated with Alzheimer disease. However, it remains unknown whether SNX3 participates in amyloid (A)β peptide production.

OBJECTIVE

To examine the role of SNX3 in Aβ production and APP processing.

METHODS

The effect of increased expression of SNX3 was studied in HEK293T cells. Aβ peptides were measured by immunoassay. Protein-protein association was analyzed by a bimolecular fluorescence complementation (BiFC) assay. APP uptake was measured with an α-bungarotoxin-binding assay, and flow cytometry was used to measure cell surface APP levels.

RESULTS

We found that overexpression of SNX3 in HEK293T cells decreases the levels of secreted Aβ and soluble N-terminal APP fragments (sAPPβ). The reduction correlated with a decreased association of APP with BACE1, as revealed by BiFC. This effect may, in part, be explained by a reduced internalization of APP; SNX3 overexpression reduced APP internalization as determined by an α-bungarotoxin-binding assay, and caused increased APP levels on the cell surface, as shown by flow cytometry. In addition, SNX3 overexpression increased the cellular levels of full-length APP.

CONCLUSION

These results provide evidence that SNX3 regulates Aβ production by influencing the internalization of APP.

17β-trenbolone, an anabolic-androgenic steroid as well as an environmental hormone, contributes to neurodegeneration

Both genetic and environmental factors contribute to neurodegenerative disorders. In a large number of neurodegenerative diseases (for example, Alzheimer's disease (AD)), patients do not carry the mutant genes. Other risk factors, for example the environmental factors, should be evaluated. 17β-trenbolone is a kind of environmental hormone as well as an anabolic-androgenic steroid. 17β-trenbolone is used as a growth promoter for livestock in the USA. Also, a large portion of recreational exercisers inject 17β-trenbolone in large doses and for very long time to increase muscle and strength. 17β-trenbolone is stable in the environment after being excreted. In the present study, 17β-trenbolone was administered to adult and pregnant rats and the primary hippocampal neurons. 17β-trenbolone's distribution and its effects on serum hormone levels and Aβ42 accumulation in vivo and its effects on AD related parameters in vitro were assessed. 17β-trenbolone accumulated in adult rat brain, especially in the hippocampus, and in the fetus brain. It altered Aβ42 accumulation. 17β-trenbolone induced apoptosis of primary hippocampal neurons in vitro and resisted neuroprotective function of testosterone. Presenilin-1 protein expression was down-regulated while β-amyloid peptide 42 (Aβ42) production and caspase-3 activities were increased. Both androgen and estrogen receptors mediated the processes. 17β-trenbolone played critical roles in neurodegeneration. Exercisers who inject large doses of trenbolone and common people who are exposed to 17β-trenbolone by various ways are all influenced chronically and continually. Identification of such environmental risk factors will help us take early prevention measure to slow down the onset of neurodegenerative disorders.

Axonal Transport and Alzheimer's Disease

In contrast to most eukaryotic cells, neurons possess long, highly branched processes called axons and dendrites. In large mammals, such as humans, some axons reach lengths of over 1 m. These lengths pose a major challenge to the movement of proteins, vesicles, and organelles between presynaptic sites and cell bodies. To overcome this challenge axons and dendrites rely upon specialized transport machinery consisting of cytoskeletal motor proteins generating directed movements along cytoskeletal tracks. Not only are these transport systems crucial to maintain neuronal viability and differentiation, but considerable experimental evidence suggests that failure of axonal transport may play a role in the development or progression of neurological diseases such as Alzheimer's disease.

Specific inhibition of CBP/beta-catenin interaction rescues defects in neuronal differentiation caused by a presenilin-1 mutation

Wnt/beta-catenin signaling has been shown to promote self-renewal in a variety of tissue stem cells, including neuronal stem cells and hematopoietic stem cells. However, activation of the Wnt/beta-catenin pathway promoted and inhibition of the pathway prevented differentiation of neuronal precursor cells. A clear explanation for the differential effects of Wnt/beta-catenin activation on neuronal precursors is not available at present. Presenilin-1 (PS-1) is a polytopic protein comprised of six to eight transmembrane domains. PS-1, as part of the gamma-secretase complex, is required for the intramembrane proteolysis of both amyloid precursor protein (APP) and Notch. Additionally, through interactions with beta-catenin, PS-1 is associated with modulation of Wnt/beta-catenin signaling. A familial Alzheimer's disease-associated PS-1 mutant, PS-1(L286V), causes a dramatic increase in T cell factor (TCF)/beta-catenin transcription in PC-12 cells, which prevents normal nerve growth factor (NGF)-induced neuronal differentiation and neurite outgrowth. Selective inhibition of TCF/beta-catenin/cAMP-response element-binding protein (CREB)-binding protein (CBP)-mediated transcription, but not TCF/beta-catenin/p300, with the recently described small molecule antagonist ICG-001 corrects these defects in neuronal differentiation, highlighting the importance of Wnt/beta-catenin signaling in this process. We propose that increased TCF/beta-catenin/CBP-mediated transcription, as well as a failure to switch to TCF/beta-catenin/p300-mediated transcription, play an important role in decreasing neuronal differentiation.

Molecular mechanisms involved in the adenosine A1 and A2A receptor-induced neuronal differentiation in neuroblastoma cells and striatal primary cultures

Adenosine A1 receptors (A1Rs) and adenosine A(2A) receptors (A(2A)Rs) are the major mediators of the neuromodulatory actions of adenosine in the brain. In the striatum A1Rs and A(2A)Rs are mainly co-localized in the GABAergic striatopallidal neurons. In this paper we show that agonist-induced stimulation of A1Rs and A(2A)Rs induces neurite outgrowth processes in the human neuroblastoma cell line SH-SY5Y and also in primary cultures of striatal neuronal precursor cells. The kinetics of adenosine-mediated neuritogenesis was faster than that triggered by retinoic acid. The triggering of the expression of TrkB neurotrophin receptor and the increase of cell number in the G1 phase by the activation of adenosine receptors suggest that adenosine may participate in early steps of neuronal differentiation. Furthermore, protein kinase C (PKC) and extracellular regulated kinase-1/2 (ERK-1/2) are involved in the A1R- and A(2A)R-mediated effects. Inhibition of protein kinase A (PKA) activity results in a total inhibition of neurite outgrowth induced by A(2A)R agonists but not by A1R agonists. PKA activation is therefore necessary for A(2A)R-mediated neuritogenesis. Co-stimulation does not lead to synergistic effects thus indicating that the neuritogenic effects of adenosine are mediated by either A1 or A(2A) receptors depending upon the concentration of the nucleoside. These results are relevant to understand the mechanisms by which adenosine receptors modulate neuronal differentiation and open new perspectives for considering the use of adenosine agonists as therapeutic agents in diseases requiring neuronal repair.

Fatty acids increase presenilin-1 levels and γ-secretase activity in PSwt-1 cells

Presenilin-1 (PS1) is an important determinant of the gamma-secretase activity necessary for the generation of beta-amyloid (Abeta), likely the central pathogenic molecule in Alzheimer's disease. Most presenilin is rapidly degraded, and determinants of the level of the active cleaved form are unknown. We examined the influence of fatty acids on PS1 levels and gamma-secretase activity using stably transfected CHO cells that express human PS1 and the human amyloid precursor protein. Cells cultured with 0.4 mM oleic acid (OA), with 0.1 mM linoleic acid, or with a triglyceride emulsion expressed increased PS1 and Abeta. This effect was independent of any secondary increase in cellular cholesterol. Cells cultured in 0.4 mM OA also exhibited significantly increased gamma-secretase activity. PS1 mRNA levels were unchanged, and pulse-chase experiments indicated that OA slowed presenilin holoprotein degradation. Nontransfected human neuroblastoma cells also showed increased presenilin when cultured in 0.4 mM OA. Lipids may be important biological determinants of PS1 level and gamma-secretase activity.

Altered morphological and electrophysiological properties of Cajal-Retzius cells in cerebral cortex of embryonic Presenilin-1 knockout mice

Mutations of Presenilin-1 are the major cause of familial Alzheimer's disease. Presenilin-1 knockout (PS1-/-) mice develop severe cortical dysplasia related to human type 2 lissencephaly. This overmigration syndrome has been attributed to the premature loss of Cajal-Retzius cells (CRcs), pioneer neurons required for the termination of radial neuronal migration. To elucidate the potential cellular mechanisms responsible for this premature neuronal loss, we investigated the morphological and electrophysiological properties of visually identified CRcs of wild-type (WT) and PS1-/- mouse brains at embryonic day 16.5. The density of CRcs was substantially reduced in the cerebral cortex of PS1-/-. In PS1-/- CRcs the number of axonal branches was significantly increased to 12.5 +/- 4.9 (n = 8; WT, 4.0 +/- 1.4, n = 12), while no differences in dendritic branching and total length of dendritic and axonal compartments were observed. Additionally, the resting membrane potential of PS1-/- CRcs was significantly depolarized (-48.3 +/- 1.7 mV; n = 23) in contrast to WT CRcs (-57.9 +/- 2.1 mV; n = 38). Active membrane properties were not affected by PS1 deficiency. CRcs of both genotypes showed spontaneous postsynaptic currents that could be completely blocked by 100 microM bicuculline and were unaffected by glutamatergic antagonists, suggesting that they were mediated by GABAA receptors. These results demonstrate that axonal branching and resting membrane potential of CRcs was affected in embryonic cerebral cortex of PS1-/- mice. The depolarized membrane potential observed in PS1-/- CRcs may increase the susceptibility to neuronal death, thus facilitating the premature loss of CRcs in PS1-/- mice.

Presenilin expression during induced differentiation of the human neuroblastoma SH-SY5Y cell line

Human neuroblastoma SH-SY5Y cells stably transfected with both wild-type and exon-9 deleted (deltaE9) presenilin constructs were used to study the role of the presenilin proteins during differentiation. Cells transfected with either wild-type or deltaE9 PS1, of which the latter abolishes normal endoproteolytic cleavage of the protein, showed no obvious differences in their ability to differentiate to a neuronal-like phenotype upon treatment with retinoic acid (RA). A defined pattern of PS1 expression was observed during differentiation with both RA and the phorbol ester TPA. Full-length PS1 was shown to increase dramatically within 5-24 h of RA treatment. TPA gave an earlier and longer lasting increase in full-length PS1 levels. The intracellular distribution pattern of PS1 was markedly altered following RA treatment. Within 24h PS1 was highly up-regulated throughout the cell body around the nucleus. Between 2 and 4 weeks PS1 staining appeared punctate and also localised to the nucleus. Increases in PS1 expression upon treatment with RA and TPA were blocked by treatment with cycloheximide, indicating a role of de-novo protein synthesis in this effect. PS2 expression remained unchanged during differentiation. Levels of full-length PS1 were also seen to increase during neurogenesis and neuronal differentiation in the forebrain of first trimester human foetuses between 6.5 and 11 weeks. These combined observations support the idea that PS1 is involved in neuronal differentiation by a mechanism likely independent of endoproteolysis of the protein.

beta-Amyloid peptide induces formation of actin stress fibers through p38 mitogen-activated protein kinase

Based on the critical role of actin in the maintenance of synaptic function, we examined whether expression of familial beta-amyloid precursor protein APP-V642I (IAPP) or mutant presenilin-1 L286V (mPS1) affects actin polymerization in rat septal neuronal cells. Expression of either IAPP or mPS1 but not wild-type amyloid precursor protein or presenilin-1induced formation of actin stress fibers in SN1 cells, a septal neuronal cell line. Treatment with beta-amyloid (Abeta) peptide also caused formation of actin stress fibers in SN1 cells and primary cultured hippocampal neurons. Treatment with a gamma-secretase inhibitor completely blocked formation of actin stress fibers, indicating that overproduction of Abeta peptide induces actin stress fibers. Because activation of the p38 mitogen-activated protein kinase (p38MAPK)-mitogen-associated protein kinase-associated protein kinase (MAPKAPK)-2-heat-shock protein 27 signaling pathway mediates actin polymerization, we explored whether Abeta peptide activates p38MAPK and MAPKAPK-2. Expression of IAPP or mPS1 induced activation of p38MAPK and MAPKAPK-2. Treatment with a p38MAPK inhibitor completely inhibited formation of actin stress fibers mediated by Abeta peptide, IAPP or mPS1. Moreover, treatment with a gamma-secretase inhibitor completely blocked activation of p38MAPK and MAPKAPK-2. In summary, our data suggest that overproduction of Abeta peptide induces formation of actin stress fibers through activation of the p38MAPK signaling pathway in septal neuronal cells.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Abeta as a bioflocculant: implications for the amyloid hypothesis of Alzheimer's disease

Research into Alzheimer's disease (AD) has been guided by the view that deposits of fibrillar amyloid-beta peptide (Abeta) are neurotoxic and are largely responsible for the neurodegeneration that accompanies the disease. This 'amyloid hypothesis' has claimed support from a wide range of molecular, genetic and animal studies. We critically review these observations and highlight inconsistencies between the predictions of the amyloid hypothesis and the published data. We show that the data provide equal support for a 'bioflocculant hypothesis', which posits that Abeta is normally produced to bind neurotoxic solutes (such as metal ions), while the precipitation of Abeta into plaques may be an efficient means of presenting these toxins to phagocytes. We conclude that if the deposition of Abeta represents a physiological response to injury then therapeutic treatments aimed at reducing the availability of Abeta may hasten the disease process and associated cognitive decline in AD.

Transgenic mice expressing the human presenilin 1 gene demonstrate enhanced hippocampal reorganization following entorhinal cortex lesions

We have examined the effects of the presence of the mutated human presenilin 1 gene (M146L; hps1*) on lesion-induced sprouting in the hippocampus of the mouse (C57/CBA). The entorhinal cortex was unilaterally lesioned with ibotenic acid in adult, male mice. Four weeks later the subsequent axonal sprouting in the dentate gyrus was analysed, by measuring the density of the synaptophysin immunocytochemical staining in the termination area of the entorhinal cortex axons. The data demonstrate that mice expressing either the human presenilin 1 gene (hps1) or the hps1* gene display a significantly increased density of immunocytochemical staining for synaptophysin, indicative of axonal sprouting, compared to the control mice. No (or a very small) sprouting response is observed in mice expressing the normal mouse ps1 gene. Taken together, these data indicate that the presence of a human ps1 gene, normal or with an Alzheimer's disease mutation, leads to enhanced plasticity in the mouse brain.

Alzheimer's disease presenilin-1 expression modulates the assembly of neurofilaments

Mutations in presenilin-1 gene are responsible for the majority of early-onset familial Alzheimer's disease cases. The function of this protein and the mechanism underlying the pathogenicity of its mutations are still unclear. To elucidate the role of presenilin-1 in the Alzheimer's disease pathology, we tested two such mutations (P117L and M146L) for their effect in stably transfected mouse neuroblastoma cell lines. Over-expression of the wild-type presenilin-1 gene induced formation of a well-extended, orderly organized network consisting of neurofilaments assembled from the L and H subunits, while in cells with the mutant gene this network was markedly reduced to short filaments concentrated in structures resembling cups. Cells expressing the mutant gene displayed altered processing of the transgene protein and neurofilament-H, suggesting that presenilin-1 is the mediator of changes targeted at neurofilaments. The two different mutations produced similar alterations, implying that this is a common pathogenic mechanism. Presenilin-1, neurofilament-H and tau proteins showed co-localization as evidenced by confocal microscopy, suggesting a possible physiological connection between these three proteins. Presenilin-1 appears to influence assembly of the H subunit into neurofilaments and the subsequent formation of new neurites. Mutations impair this function of presenilin-1, resulting in inhibition of neurite outgrowth. That presenilin-1 influences the assembly of neurofilaments may represent a novel pathway through which presenilin-1 mutations are involved in Alzheimer's disease pathology. In this hypothesis, presenilin-1 mutations will be associated with aberrant sprouting leading to synaptic loss, a key neuropathological feature of Alzheimer's disease.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

The role of Alzheimer's disease-related presenilin 1 in intercellular adhesion

Most cases of familial early-onset Alzheimer's disease are caused by mutations in the presenilin 1 (PS1) gene. However, the cellular functions of PS1 are unknown. We showed predominant localization of PS1 to cell-cell contacts of the plasma membrane in human prostate epithelial tissue and in a human epithelial cell line HEp2 stably transfected with an inducible PS1 construct. PS1 co-immunoprecipitated with beta-catenin from cell lysates of stable transfectants. Conversely, PS1 lacking the PS1-beta-catenin interaction site did not co-immunoprecipitate with beta-catenin and was not recruited to the cell-cell contacts. L cells, which do not form tight intercellular contacts, formed clusters of adhered cells after stable transfection with GFP-PS1 cDNA and demonstrated a clear preference for independent aggregation in the mixed cultures. However, L cells transfected with mutant GFP-PS1 constructs, which had a truncated N-terminus of PS1 or deleted PS1-beta-catenin interaction site, failed to form intercellular contacts. In addition, in primary cultures of mouse cortical neurons PS1 was highly concentrated on the surface of extended growth cones. Taken together, our results suggest an important role of PS1 in intercellular adhesion in epithelial cells and neurons.

Genetic susceptibility factors for Alzheimer's disease

Alzheimer's disease is the most frequent cause of dementia. Family and twin studies have suggested that genetic factors play a role in Alzheimer's disease development. Some Alzheimer's disease cases show an autosomal dominant inheritance pattern and thus allow the discovery of major disease genes. However, most Alzheimer's disease cases are sporadic. These cases are mainly due to the effects of several different genes and of interactions between genetic susceptibility factors and environmental factors. Such interactions are illustrated by the apolipoprotein E epsilon4 allele, associated with a higher risk of Alzheimer's disease. Other genetic susceptibility factors have been reported but variously confirmed in Alzheimer's disease: apolipoprotein E receptors, alpha2-macroglobulin or angiotensin I converting enzyme genes. Thus, except for a small percentage of Alzheimer's disease cases with a dominant inheritance pattern, the genetic component of the vast majority of cases is underlain by complex interactions of genetic susceptibility factors and environmental conditions.

Presenilin-1 regulates the neuronal threshold to excitotoxicity both physiologically and pathologically

A direct pathophysiological role of Familial Alzheimer's Disease (FAD)-associated Presenilin 1 (PS1) mutations in neuronal vulnerability remains a controversial matter. We evaluated the relationship between PS1 and excitotoxicity in four different experimental models of neurotoxicity by using primary neurons from (i) transgenic (tg) mice overexpressing a human FAD-linked PS1 variant (L286V mutation), (ii) tg mice overexpressing human wild-type (wt) PS1, (iii) PS1 knockout mice, and (iv) wt mice in which PS1 gene expression was knocked down by antisense treatment. We found that primary neurons overexpressing mutated PS1 showed an increased vulnerability to both excitotoxic and hypoxic-hypoglycemic damage when compared with neurons obtained from either mice overexpressing human wt PS1 or in wt mice. In addition, reduced excitotoxic damage was obtained in neurons in which PS1 expression was absent or diminished. Data obtained in in vivo experimental models of excitotoxicity partially supported the in vitro observations. Accelerated neuronal death was demonstrated in the hippocampus of mice overexpressing mutated PS1 after peripheral administration of kainic acid in comparison with wt animals. However, measurement of the infarct volume after middle cerebral artery occlusion did not show significant difference between the two animal groups. The results altogether suggest that expression of FAD-linked PS1 variants increases the vulnerability of neurons to a specific type of damage in which excitotoxicity plays a relevant role. In addition, they support the view that reduction of endogenous PS1 expression results in neuroprotection.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.