Brain oxidative stress in a triple-transgenic mouse model of Alzheimer disease

Alzheimer disease (AD) is a neurodegenerative disease which is characterized by the presence of extracellular senile plaques mainly composed of amyloid-beta peptide (Abeta), intracellular neurofibrillary tangles, and selective synaptic and neuronal loss. AD brains revealed elevated levels of oxidative stress markers which have been implicated in Abeta-induced toxicity. In the present work we addressed the hypothesis that oxidative stress occurs early in the development of AD and evaluated the extension of the oxidative stress and the levels of antioxidants in an in vivo model of AD, the triple-transgenic mouse, which develops plaques, tangles, and cognitive impairments and thus mimics AD progression in humans. We have shown that in this model, levels of antioxidants, namely, reduced glutathione and vitamin E, are decreased and the extent of lipid peroxidation is increased. We have also observed increased activity of the antioxidant enzymes glutathione peroxidase and superoxide dismutase. These alterations are evident during the Abeta oligomerization period, before the appearance of Abeta plaques and neurofibrillary tangles, supporting the view that oxidative stress occurs early in the development of the disease.

NAP and ADNF-9 Protect Normal and Downs Syndrome Cortical Neurons from Oxidative Damage and Apoptosis

NAP (Asn-Ala-Pro-Val-Ser-Ile-Pro-Gln, single letter code: NAPVSIPQ) and ADNF-9 (activity-dependent neurotrophic factor-9; Ser-Ala-Leu-Leu-Arg-Ser-Ile-Pro-Ala; single letter code: SALLRSIPA) are peptides derived from naturally occurring glial proteins that have shown neuroprotection in rodent model systems. Here, the neuroprotective activity of ADNF-9 and NAP was tested in two human models of neuronal degeneration in culture mediated by oxidative stress: normal human cortical neurons treated with H2O2 and Down's syndrome (DS) cortical neurons. Incubation of normal cortical neurons with 50 microM H2O2 for 1 hour resulted in morphological and structural changes consistent with neuronal degeneration and loss of viability of more than 60% of the neurons present in the culture. Addition of ADNF-9 or NAP at femtomolar concentrations resulted in significant increases in survival of normal neurons treated with H2O2. Femtomolar concentrations of ADNF-9 or NAP exhibited a similar neuroprotective efficacy, comparable to the antioxidant N-tert-butyl-2-sulpho-phenylnitrone at 100 microM (s-PBN). Treatment of DS cortical neurons with ADNF-9 or NAP resulted in a significant increase in neuronal survival as well as reduction of degenerative morphological changes. The results suggest that ADNF-9 and NAP possess potent neuroprotective properties against oxidative damage in human neurons that may be useful to preserve neuronal function and prevent neuronal death associated with chronic neurodegenerative disorders.

Beta-amyloid, oxidative stress and down syndrome

Down syndrome (DS) provides a model for studying important aspects of Alzheimer disease (AD). Chromosome 21 contains several genes that have been implicated in neurodegenerative mechanisms. These include Cu/Zn superoxide dismutase (SOD-1), Ets-2 transcription factors, Down Syndrome Critical Region 1 (DSCR1) stress-inducible factor, and the amyloid precursor protein (APP). The accumulation of Abeta plaques is progressive across the lifespan in DS. Overexpression of APP in the obligate region for DS is associated with abundant Abeta plaques and tangles consistent with Braak stage V-VI. Intraneuronal Abeta in DS appears to trigger a pathological cascade leading to oxidative stress and a neurodegeneration typical of AD. There are suggestions that an increase in subcellular processing of APP and factors related to membrane APP cleavage favor the secretion of Abeta with age in DS. A misbalance between SOD-1 and glutathione perioxidase activity in DS has been linked to free radical generation. Ets-2 and DSCR1 overexpression in DS has been linked to cell degeneration. Age-related accumulation of somatic DNA mutations in both DS and AD contribute to oxidative stress that exacerbates the imbalance in gene expression. This leads to enhanced Abeta deposition and further neuronal vulnerability. The consequence of these factors and their temporal relationships is likely to be the subject of future research. Since the pathological processes leading to AD are seen across the lifespan in DS, an opportunity is afforded for early pharmacological intervention in the disorder.

Phosphorylation of actin-depolymerizing factor/cofilin by LIM-kinase mediates amyloid beta-induced degeneration: a potential mechanism of neuronal dystrophy in Alzheimer's disease

Deposition of fibrillar amyloid beta (fAbeta) plays a critical role in Alzheimer's disease (AD). We have shown recently that fAbeta-induced dystrophy requires the activation of focal adhesion proteins and the formation of aberrant focal adhesion structures, suggesting the activation of a mechanism of maladaptative plasticity in AD. Focal adhesions are actin-based structures that provide a structural link between the extracellular matrix and the cytoskeleton. To gain additional insight in the molecular mechanism of neuronal degeneration in AD, here we explored the involvement of LIM kinase 1 (LIMK1), actin-depolymerizing factor (ADF), and cofilin in Abeta-induced dystrophy. ADF/cofilin are actin-binding proteins that play a central role in actin filament dynamics, and LIMK1 is the kinase that phosphorylates and thereby inhibits ADF/cofilin. Our data indicate that treatment of hippocampal neurons with fAbeta increases the level of Ser3-phosphorylated ADF/cofilin and Thr508-phosphorylated LIMK1 (P-LIMK1), accompanied by a dramatic remodeling of actin filaments, neuritic dystrophy, and neuronal cell death. A synthetic peptide, S3 peptide, which acts as a specific competitor for ADF/cofilin phosphorylation by LIMK1, inhibited fAbeta-induced ADF/cofilin phosphorylation, preventing actin filament remodeling and neuronal degeneration, indicating the involvement of LIMK1 in Abeta-induced neuronal degeneration in vitro. Immunofluorescence analysis of AD brain showed a significant increase in the number of P-LIMK1-positive neurons in areas affected with AD pathology. P-LIMK1-positive neurons also showed early signs of AD pathology, such as intracellular Abeta and pretangle phosphorylated tau. Thus, LIMK1 activation may play a key role in AD pathology.

Different conformations of amyloid beta induce neurotoxicity by distinct mechanisms in human cortical neurons

Characterization of soluble oligomeric amyloid beta (Abeta) species in the brains of Alzheimer's disease (AD) patients and transgenic models has raised the possibility that different conformations of Abeta may contribute to AD pathology via different mechanisms. To characterize the toxic effect of different Abeta conformations, we tested side by side the effect of well characterized Abeta oligomers (AbetaOs), Abeta-derived diffusible ligands (ADDLs), and fibrillar Abeta (Abetaf) preparations in human cortical neurons (HCNs). Both AbetaOs and ADDLs bind rapidly and with high affinity to synaptic contacts and cellular membranes. AbetaOs (5 microm) induced rapid and massive neuronal death. Calcium influx accelerated, but was not required for, AbetaO toxicity. AbetaOs elicited a stereotyped succession of cellular changes consistent with the activation of a mitochondrial death apoptotic pathway. At low concentrations AbetaOs caused chronic and subtler mitochondrial alterations but minimal cell death. ADDLs induced similar toxic changes as AbetaOs but on a fivefold longer time scale. Higher concentrations of Abetaf and longer incubation times were required to produce widespread neuritic dystrophy but modest HCN cell death. Thus various Abeta species may play relevant roles in AD, causing neurotoxicity by distinct non-overlapping mechanisms affecting neuronal function and viability over multiple time courses.

ets-2 promotes the activation of a mitochondrial death pathway in Down's syndrome neurons

Down's syndrome (DS) is characterized by mental retardation and development of Alzheimer's disease (AD). Oxidative stress and mitochondrial dysfunction are both related to neurodegeneration in DS. Several genes in chromosome 21 have been linked to neuronal death, including the transcription factor ets-2. Cortical cultures derived from normal and DS fetal brains were used to study the role of ets-2 in DS neuronal degeneration. ets-2 was expressed in normal human cortical neurons (HCNs) and was markedly upregulated by oxidative stress. When overexpressed in normal HCNs, ets-2 induced a stereotyped sequence of apoptotic changes leading to neuronal death. DS HCNs exhibit intracellular oxidative stress and increased apoptosis after the first week in culture (Busciglio and Yankner, 1995). ets-2 levels were increased in DS HCNs, and, between 7 and 14 d in vitro, DS HCNs showed increased bax, cytoplasmic translocation of cytochrome c and apoptosis inducing factor, and active caspases 3 and 7, consistent with activation of an apoptotic mitochondrial death pathway. Degeneration of DS neurons was reduced by dominant-negative ets-2, suggesting that increased ets-2 expression promotes DS neuronal apoptosis. In the human brain, ets-2 expression was found in neurons and astrocytes. Strong ets-2 immunoreactivity was observed in DS/AD and sporadic AD brains associated with degenerative markers such as bax, intracellular Abeta, and hyperphosphorylated tau. Thus, in DS/AD and sporadic AD brains, converging pathological mechanisms leading to chronic oxidative stress and ets-2 upregulation in susceptible neurons may result in increased vulnerability by promoting the activation of a mitochondrial-dependent proapoptotic pathway of cell death.

Deposition of amyloid fibrils promotes cell-surface accumulation of amyloid β precursor protein

Amyloid beta protein (Abeta) deposition and neuronal degeneration are characteristic pathological features of Alzheimer's disease (AD). In vitro, Abeta fibrils (fAbeta) induce neuronal degeneration reminiscent to AD, but the mechanism of neurotoxicity is unknown. Here we show that amyloid fibrils increase the level of cell-surface full-length amyloid beta precursor protein (h-AbetaPP) and secreted AbetaPP (s-AbetaPP). Pulse-chase analysis indicated that fAbeta selectively inhibited the turnover of cell-surface AbetaPP, without altering its intracellular levels. FAbeta-induced AbetaPP accumulation was not abrogated by cycloheximide, suggesting that increased protein synthesis is not critically required. Abeta fibrils sequester s-AbetaPP from the culture medium and promote its accumulation at the cell surface, indicating that binding of Abeta fibrils mediates AbetaPP accumulation. A time course analysis of Abeta treatment showed that AbetaPP level is elevated before significant cell death can be detected, while other toxic insults do not augment AbetaPP level, suggesting that AbetaPP may be specifically involved in early stages of Abeta-induced neurodegeneration. Finally, Abeta fibrils promote clustering of h-AbetaPP in abnormal focal adhesion-like (FA-like) structures that mediate neuronal dystrophy, increasing its association with the cytoskeleton. These results indicate that the interaction of Abeta fibrils with AbetaPP is an early event in the mechanism of Abeta-induced neurodegeneration that may play a significant role in AD pathogenesis.

Overexpression of the chromosome 21 transcription factor Ets2 induces neuronal apoptosis

Down syndrome (trisomy 21) neurons display an increased rate of apoptosis in vitro. The genes on chromosome 21 that mediate this increased cell death remain to be elucidated. Here we show that the chromosome 21 transcription factor Ets2, a gene that is overexpressed in Down syndrome, is expressed in neurons, and that moderate overexpression of Ets2 leads to increased apoptosis of primary neuronal cultures from Ets2 tg mice that involves activation of caspase-3. Our data therefore suggest that overexpression of ETS2 may contribute to the increased rate of apoptosis of neurons in Down syndrome.

Alzheimer's presenilin 1 mutations impair kinesin-based axonal transport

Several lines of evidence indicate that alterations in axonal transport play a critical role in Alzheimer's disease (AD) neuropathology, but the molecular mechanisms that control this process are not understood fully. Recent work indicates that presenilin 1 (PS1) interacts with glycogen synthase kinase 3beta (GSK3beta). In vivo, GSK3beta phosphorylates kinesin light chains (KLC) and causes the release of kinesin-I from membrane-bound organelles (MBOs), leading to a reduction in kinesin-I driven motility (Morfini et al., 2002b). To characterize a potential role for PS1 in the regulation of kinesin-based axonal transport, we used PS1-/- and PS1 knock-inM146V (KIM146V) mice and cultured cells. We show that relative levels of GSK3beta activity were increased in cells either in the presence of mutant PS1 or in the absence of PS1 (PS1-/-). Concomitant with increased GSK3beta activity, relative levels of KLC phosphorylation were increased, and the amount of kinesin-I bound to MBOs was reduced. Consistent with a deficit in kinesin-I-mediated fast axonal transport, densities of synaptophysin- and syntaxin-I-containing vesicles and mitochondria were reduced in neuritic processes of KIM146V hippocampal neurons. Similarly, we found reduced levels of PS1, amyloid precursor protein, and synaptophysin in sciatic nerves of KIM146V mice. Thus PS1 appears to modulate GSK3beta activity and the release of kinesin-I from MBOs at sites of vesicle delivery and membrane insertion. These findings suggest that mutations in PS1 may compromise neuronal function by affecting GSK-3 activity and kinesin-I-based motility.

GVS-111 prevents oxidative damage and apoptosis in normal and Down's syndrome human cortical neurons

The neuroprotective activity of a novel N-acylprolyl-containing dipeptide analog of the nootropic 2-oxo-1-pyrrolidine acetamide (Piracetam) designated as GVS-111 (DVD-111/Noopept) was tested in two in vitro models of neuronal degeneration mediated by oxidative stress: normal human cortical neurons treated with H(2)O(2), and Down's syndrome (DS) cortical neurons. Incubation of normal cortical neurons with 50 microM H(2)O(2) for 1h resulted in morphological and structural changes consistent with neuronal apoptosis and in the degeneration of more than 60% of the neurons present in the culture. GVS-111 significantly increased neuronal survival after H(2)O(2)-treatment displaying a dose-dependent neuroprotective activity from 10nM to 100 microM, and an IC(50) value of 1.21+/-0.07 microM. GVS-111 inhibited the accumulation of intracellular free radicals and lipid peroxidation damage in neurons treated with H(2)O(2) or FeSO(4), suggesting an antioxidant mechanism of action. GVS-111 exhibited significantly higher neuroprotection compared to the standard cognition enhancer Piracetam, or to the antioxidants Vitamin E, propyl gallate and N-tert-butyl-2-sulpho-phenylnitrone (s-PBN). In DS cortical cultures, chronic treatment with GVS-111 significantly reduced the appearance of degenerative changes and enhanced neuronal survival. The results suggest that the neuroprotective effect of GVS-111 against oxidative damage and its potential nootropic activity may present a valuable therapeutic combination for the treatment of mental retardation and chronic neurodegenerative disorders.

ETS2 overexpression in transgenic models and in Down syndrome predisposes to apoptosis via the p53 pathway

ETS2 is a transcription factor encoded by a gene on human chromosome 21 and alterations in its expression have been implicated in the pathophysiological features of Down syndrome (DS). This study demonstrates that overexpression of ETS2 results in apoptosis. This is shown in a number of circumstances, including ETS2-overexpressing transgenic mice and cell lines and in cells from subjects with DS. Indeed we report for the first time that the ETS2 overexpression transgenic mouse develops a smaller thymus and lymphocyte abnormalities similar to that observed in DS. In all circumstances of ETS2 overexpression, the increased apoptosis correlated with increased p53 and alterations in downstream factors in the p53 pathway. In the human HeLa cancer cell line, transfection with functional p53 enables ETS2 overexpression to induce apoptosis. Furthermore, crossing the ETS2 transgenic mice with p53(-/-) mice genetically rescued the thymic apoptosis phenotype. Therefore, we conclude that overexpression of human chromosome 21-encoded ETS2 induces apoptosis that is dependent on p53. These results have important consequences for understanding DS and oncogenesis and may provide new insights into therapeutic interventions.

Aberrant activation of focal adhesion proteins mediates fibrillar amyloid beta-induced neuronal dystrophy

Neuronal dystrophy is a pathological hallmark of Alzheimer's disease (AD) that is not observed in other neurodegenerative disorders that lack amyloid deposition. Treatment of cortical neurons with fibrillar amyloid beta (Abeta) peptides induces progressive neuritic dystrophy accompanied by a marked loss of synaptophysin immunoreactivity (Grace et al., 2002). Here, we report that fibrillar Abeta-induced neuronal dystrophy is mediated by the activation of focal adhesion (FA) proteins and the formation of aberrant FA structures adjacent to Abeta deposits. In the AD brain, activated FA proteins are observed associated with the majority of senile plaques. Clustered integrin receptors and activated paxillin (phosphorylated at Tyr-31) and focal adhesion kinase (phosphorylated at Tyr-297) are mainly detected in dystrophic neurites surrounding Abeta plaque cores, where they colocalize with hyperphosphorylated tau. Deletion experiments demonstrated that the presence of the LIM domains in the paxillin C terminus and the recruitment of the protein-Tyr phosphatase (PTP)-PEST to the FA complex are required for Abeta-induced neuronal dystrophy. Therefore, both paxillin and PTP-PEST appear to be critical elements in the generation of the dystrophic response. Paxillin is a scaffolding protein to which other FA proteins bind, leading to the formation of the FA contact and initiation of signaling cascades. PTP-PEST plays a key role in the dynamic regulation of focal adhesion contacts in response to extracellular cues. Thus, in the AD brain, fibrillar Abeta may induce neuronal dystrophy by triggering a maladaptive plastic response mediated by FA protein activation and tau hyperphosphorylation.

Fast axonal transport misregulation and Alzheimer's disease

Pathological alterations in the microtubule-associated protein (MAP) tau are well-established in a number of neurodegenerative disorders, including Alzheimer's Disease (AD), frontotemporal dementia (FTD), progressive supranuclear palsy (PSP), and others. Tau protein and in some cases, neurofilament subunits exhibit abnormal phosphorylation on specific serine and threonine residues in these diseases. A large body of biochemical, genetic, and cell biological evidence implicate two major serine-threonine protein kinases, glycogen synthase kinase 3 (GSK-3) and cyclin-dependent kinase 5 (CDK5) as major kinases responsible for both normal and pathological phosphorylation of tau protein in vivo. What remains unclear is whether tau phosphorylation and/or neurofibrillary tangle (NFT) formation are causal or secondary to initiation of neuronal pathology. In fact, many studies have indicated that tau misphosphorylation is not the causal event. Interestingly, some of these kinase and phosphatase activities have recently merged as key regulators of fast axonal transport (FAT). Specifically, CDK5 and GSK-3 have been recently shown to regulate kinesin-driven motility. Given the essential role of FAT in neuronal function, an alternate model for pathogenesis can be proposed. In this model, misregulation of FAT induced by an imbalance in specific kinase-phosphatase activities within neurons represents an early and critical step for the initiation of neuronal pathology. Such a model may explain many of the unique characteristics of late onset of neurological diseases such as AD.

Characterization of neuronal dystrophy induced by fibrillar amyloid beta: implications for Alzheimer's disease

Amyloid deposition, neuronal dystrophy and synaptic loss are characteristic pathological features of Alzheimer's disease (AD). We have used cortical neuronal cultures to assess the dystrophic effect of fibrillar amyloid beta (Abeta) and its relationship with neurotoxicity and synaptic loss. Treatment with fibrillar Abeta led to the development of neuritic dystrophy in the majority of the neurons present in the culture. Morphometric analysis and viability assays showed that neuronal dystrophy appeared significantly earlier and at lower Abeta concentrations than neurotoxicity, suggesting that both effects are generated independently by different cellular mechanisms. The development of dystrophic features required Abeta fibril formation and did not depend on the presence of the RHDS adhesive domain in the sequence of Abeta. Finally, a dramatic reduction in the density of synaptophysin immunoreactivity was closely associated with dystrophic changes in viable neurons. These results suggest that aberrant plastic changes and loss of synaptic integrity induced by fibrillar Abeta may play a significant role in the development of AD pathology.

Altered metabolism of the amyloid beta precursor protein is associated with mitochondrial dysfunction in Down's syndrome

Most Down's syndrome (DS) patients develop Alzheimer's disease (AD) neuropathology. Astrocyte and neuronal cultures derived from fetal DS brain show alterations in the processing of amyloid beta precursor protein (AbetaPP), including increased levels of AbetaPP and C99, reduced levels of secreted AbetaPP (AbetaPPs) and C83, and intracellular accumulation of insoluble Abeta42. This pattern of AbetaPP processing is recapitulated in normal astrocytes by inhibition of mitochondrial metabolism, consistent with impaired mitochondrial function in DS astrocytes. Intracellular Abeta42 and reduced AbetaPPs are also detected in DS and AD brains. The survival of DS neurons is markedly increased by recombinant or astrocyte-produced AbetaPPs, suggesting that AbetaPPs may be a neuronal survival factor. Thus, mitochondrial dysfunction in DS may lead to intracellular deposition of Abeta42, reduced levels of AbetaPPs, and a chronic state of increased neuronal vulnerability.

Presenilin-1 mutations reduce cytoskeletal association, deregulate neurite growth, and potentiate neuronal dystrophy and tau phosphorylation

Mutations in presenilin genes are linked to early onset familial Alzheimer's disease (FAD). Previous work in non-neuronal cells indicates that presenilin-1 (PS1) associates with cytoskeletal elements and that it facilitates Notch1 signaling. Because Notch1 participates in the control of neurite growth, cultured hippocampal neurons were used to investigate the cytoskeletal association of PS1 and its potential role during neuronal development. We found that PS1 associates with microtubules (MT) and microfilaments (MF) and that its cytoskeletal association increases dramatically during neuronal development. PS1 was detected associated with MT in the central region of neuronal growth cones and with MF in MF-rich areas extending into filopodia and lamellipodia. In differentiated neurons, PS1 mutations reduced the interaction of PS1 with cytoskeletal elements, diminished the nuclear translocation of the Notch1 intracellular domain (NICD), and promoted a marked increase in total neurite length. In developing neurons, PS1 overexpression increased the nuclear translocation of NICD and inhibited neurite growth, whereas PS1 mutations M146V, I143T, and deletion of exon 9 (D9) did not facilitate NICD nuclear translocation and had no effect on neurite growth. In cultures that were treated with amyloid beta (Abeta), PS1 mutations significantly increased neuritic dystrophy and AD-like changes in tau such as hyperphosphorylation, release from MT, and increased tau protein levels. We conclude that PS1 participates in the regulation of neurite growth and stabilization in both developing and differentiated neurons. In the Alzheimer's brain PS1 mutations may promote neuritic dystrophy and tangle formation by interfering with Notch1 signaling and enhancing pathological changes in tau.

Fetal human cortical neurons grown in culture: morphological differentiation, biochemical correlates and development of electrical activity

Cultured fetal human cortical neurons derived from second trimester human fetal cortex were analyzed with regard to their morphological differentiation and expression of cell-specific markers. The culture method was adapted from standardized protocols originally developed for the isolation and culture of rodent oligodendrocytes and astrocytes. This technique takes advantage of the different adhesive properties and stratification of central nervous system cells in vitro. Under these culture conditions fetal human cortical neurons underwent morphological differentiation, expressed neuron-specific markers and voltage- and ligand-gated ion channels. Highly enriched cultures of microglia and astrocytes generated from the same starting material also expressed cell-type specific markers. These cultures serve as a valuable tool for the establishment of normative data and as experimental models for neurodevelopmental and neurodegenerative studies.