The Alzheimer-related gene presenilin 1 facilitates notch 1 in primary mammalian neurons

Alzheimer's Disease: Challenges and a Therapeutic Opportunity to Treat It with a Neurotrophic Compound

Alzheimer’s disease (AD) is a progressive neurodegenerative disease with an insidious onset and multifactorial nature. A deficit in neurogenesis and synaptic plasticity are considered the early pathological features associated with neurofibrillary tau and amyloid β pathologies and neuroinflammation. The imbalance of neurotrophic factors with an increase in FGF-2 level and a decrease in brain derived neurotrophic factor (BDNF) and neurotrophin 4 (NT-4) in the hippocampus, frontal cortex and parietal cortex and disruption of the brain micro-environment are other characteristics of AD. Neurotrophic factors are crucial in neuronal differentiation, maturation, and survival. Several attempts to use neurotrophic factors to treat AD were made, but these trials were halted due to their blood-brain barrier (BBB) impermeability, short-half-life, and severe side effects. In the present review we mainly focus on the major etiopathology features of AD and the use of a small neurotrophic and neurogenic peptide mimetic compound; P021 that was discovered in our laboratory and was found to overcome the difficulties faced in the administration of the whole neurotrophic factor proteins. We describe pre-clinical studies on P021 and its potential as a therapeutic drug for AD and related neurodegenerative disorders. Our study is limited because it focuses only on P021 and the relevant literature; a more thorough investigation is required to review studies on various therapeutic approaches and potential drugs that are emerging in the AD field.

Molecular mechanisms of altered adult hippocampal neurogenesis in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia globally. AD is a progressive neurodegenerative disorder, eventually manifesting as severe cognitive impairment. Adult hippocampal neurogenesis (AHN) occurs throughout adulthood and plays an important role in hippocampus-dependent learning and memory. The stages of AHN, predominantly comprising the proliferation, differentiation, survival, and maturation of newborn neurons, are affected to varying degrees in AD. However, the exact molecular mechanisms remain to be elucidated. Recent evidence suggests that the molecules involved in AD pathology contribute to the compromised AHN in AD. Notably, various interventions may have common signaling pathways that, once identified, could be harnessed to enhance adult neurogenesis. This in turn could putatively rescue cognitive deficits associated with impaired neurogenesis as observed in animal models of AD. In this manuscript, we review the current knowledge concerning AHN under normal physiological and AD pathological conditions and highlight the possible role of specific molecules in AHN alteration in AD. In addition, we summarize in vivo experiments with emphasis on the effect of the activation of certain key signalings on AHN in AD rodent models. We propose that these signaling targets and corresponding interventions should be considered when developing novel therapies for AD.

Soluble Gamma-secretase Modulators Attenuate Alzheimer's β-amyloid Pathology and Induce Conformational Changes in Presenilin 1

A central pathogenic event of Alzheimer's disease (AD) is the accumulation of the Aβ42 peptide, which is generated from amyloid-β precursor protein (APP) via cleavages by β- and γ-secretase. We have developed a class of soluble 2-aminothiazole γ-secretase modulators (SGSMs) that preferentially decreases Aβ42 levels. However, the effects of SGSMs in AD animals and cells expressing familial AD mutations, as well as the mechanism of γ-secretase modulation remain largely unknown. Here, a representative of this SGSM scaffold, SGSM-36, was investigated using animals and cells expressing FAD mutations. SGSM-36 preferentially reduced Aβ42 levels without affecting either α- and β-secretase processing of APP nor Notch processing. Furthermore, an allosteric site was identified within the γ-secretase complex that allowed access of SGSM-36 using cell-based, fluorescence lifetime imaging microscopy analysis. Collectively, these studies provide mechanistic insights regarding SGSMs of this class and reinforce their therapeutic potential in AD.

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A novel class soluble 2-aminothiazole γ-secretase modulators (SGSMs) are characterized as potential therapeutics for AD.

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A representative compound, SGSM-36, preferentially decreases Aβ42 levels using animal and cell models of AD.

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An allosteric site was identified within γ-secretase to be accessible by SGSM-36.

Alzheimer's disease (AD) is a devastating neurodegenerative disorder and there is currently no treatment to slow or halt disease progression. Considerable evidence shows that the primary pathological event leading to AD is the production and accumulation of Aβ42 peptide. We have developed a class of soluble 2-aminothiazole γ-secretase modulators (SGSMs) that preferentially decreases Aβ42 levels. The presented studies have primarily elucidated the mechanisms by which our SGSMs decrease Aβ42 levels and attenuate β-amyloid pathology. The results of these experiments will be useful toward the ongoing efforts toward the development of an effective therapy for the treatment and prevention of AD.

Oxidative stress and lipid peroxidation are upstream of amyloid pathology

Oxidative stress is a common feature of the aging process and of many neurodegenerative disorders, including Alzheimer's disease. Understanding the direct causative relationship between oxidative stress and amyloid pathology, and determining the underlying molecular mechanisms is crucial for the development of more effective therapeutics for the disease. By employing microdialysis technique, we report local increase in the amyloid-β42 levels and elevated amyloid-β42/40 ratio in the interstitial fluid within 6h of direct infusion of oxidizing agents into the hippocampus of living and awake wild type mice. The increase in the amyloid-β42/40 ratio correlated with the pathogenic conformational change of the amyloid precursor protein-cleaving enzyme, presenilin1/γ-secretase. Furthermore, we found that the product of lipid peroxidation 4-hydroxynonenal, binds to both nicastrin and BACE, differentially affecting γ- and β-secretase activity, respectively. The present study demonstrates a direct cause-and-effect correlation between oxidative stress and altered amyloid-β production, and provides a molecular mechanism by which naturally occurring product of lipid peroxidation may trigger generation of toxic amyloid-β42 species.

Notch in memories: Points to remember

Memory is a temporally evolving molecular and structural process, which involves changes from local synapses to complex neural networks. There is increasing evidence for an involvement of developmental pathways in regulating synaptic communication in the adult nervous system. Notch signaling has been implicated in memory formation in a variety of species. Nevertheless, the mechanism of Notch underlying memory consolidation remains poorly understood. In this commentary, besides offering an overview of the advances in the field of Notch in memory, we highlight some of the weaknesses of the studies and attempt to cast light on the apparent discrepancies on the role of Notch in memory. We believe that future studies, employing high-throughput technologies and targeted Notch loss and gain of function animal models, will reveal the mechanisms of Notch dependent plasticity and resolve whether this signaling pathway is implicated in the cognitive deficit associated with dementia.

Notch in memories: Points to remember

Memory is a temporally evolving molecular and structural process, which involves changes from local synapses to complex neural networks. There is increasing evidence for an involvement of developmental pathways in regulating synaptic communication in the adult nervous system. Notch signaling has been implicated in memory formation in a variety of species. Nevertheless, the mechanism of Notch in memory consolidation remains poorly understood. In this commentary, besides offering an overview of the advances in the field of Notch in memory, we highlight some of the weaknesses of the studies and attempt to cast light on some of the apparent discrepancies on the role of Notch in memory. We believe that future studies, employing high-throughput technologies and targeted Notch loss and gain of function animal models, will reveal the mechanisms of Notch-dependent plasticity and resolve whether this signaling pathway is implicated in the cognitive deficit associated with dementia.

Role of Notch-1 signaling pathway in PC12 cell apoptosis induced by amyloid beta-peptide (25-35)

Recent studies have demonstrated that Notch-1 expression is increased in the hippocampus of Alzheimer's disease patients. We speculate that Notch-1 signaling may be involved in PC12 cell apoptosis induced by amyloid beta-peptide (25–35) (Aβ_25–35). In the present study, PC12 cells were cultured with different doses (0, 0.1, 1.0, 10 and 100 nmol/L) of N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester, a Notch-1 signaling pathway inhibitor, for 30 minutes. Then cultured cells were induced with Aβ_25–35 for 48 hours. Pretreatment of PC12 cells with high doses of N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (> 10 nmol/L) prolonged the survival of PC12 cells after Aβ_25–35 induction, decreased the expression of apoptosis-related proteins caspase-3, -8, -9, increased the activity of oxidative stress-related superoxide dismutase and catalase, inhibited the production of active oxygen, and reduced nuclear factor kappa B expression. This study indicates that the Notch-1 signaling pathway plays a pivotal role in Aβ_25–35-induced PC12 apoptosis.

Presenilin-1 adopts pathogenic conformation in normal aging and in sporadic Alzheimer's disease

Accumulation of amyloid-β (Aβ) and neurofibrillary tangles in the brain, inflammation and synaptic and neuronal loss are some of the major neuropathological hallmarks of Alzheimer's disease (AD). While genetic mutations in amyloid precursor protein and presenilin-1 and -2 (PS1 and PS2) genes cause early-onset familial AD, the etiology of sporadic AD is not fully understood. Our current study shows that changes in conformation of endogenous wild-type PS1, similar to those found with mutant PS1, occur in sporadic AD brain and during normal aging. Using a mouse model of Alzheimer's disease (Tg2576) that overexpresses the Swedish mutation of amyloid precursor protein but has normal levels of endogenous wild-type presenilin, we report that the percentage of PS1 in a pathogenic conformation increases with age. Importantly, we found that this PS1 conformational shift is associated with amyloid pathology and precedes amyloid-β deposition in the brain. Furthermore, we found that oxidative stress, a common stress characteristic of aging and AD, causes pathogenic PS1 conformational change in neurons in vitro, which is accompanied by increased Aβ42/40 ratio. The results of this study provide important information about the timeline of pathogenic changes in PS1 conformation during aging and suggest that structural changes in PS1/γ-secretase may represent a molecular mechanism by which oxidative stress triggers amyloid-β accumulation in aging and in sporadic AD brain.

Fleshing out the amyloid cascade hypothesis: the molecular biology of Alzheimer's disease

Alzheimer's disease (AD) is a disorder of two pathologies- plaques and tangles. The former have as a key constituent amyloid protein and the latter the microtubule-associaied protein tau. Genetics has demonstrated that changes in either protein are sufficient to cause dementia. The amyloid cascade hypothesis proposes that plaque-related changes precede tangle-related changes and positions amyloid as central to the degeneration of AD. All the evidence suggests this is correct, including evidence that presenil ins alter the processing of the amyloid precursor protein and evidence that disrupting the normal properties of tau underlies the related froniotemporal dementias. The amyloid cascade hypothesis has provided the basis for nearly a decade of intensive basic science - the skeleton of that hypothesis can now be fleshed out, and confidence is growing that this will result in useful disease-modifying therapies in the future.

Hierarchical clustering of gene expression patterns in the Eomes + lineage of excitatory neurons during early neocortical development

BACKGROUND

Cortical neurons display dynamic patterns of gene expression during the coincident processes of differentiation and migration through the developing cerebrum. To identify genes selectively expressed by the Eomes + (Tbr2) lineage of excitatory cortical neurons, GFP-expressing cells from Tg(Eomes::eGFP) Gsat embryos were isolated to > 99% purity and profiled.

RESULTS

We report the identification, validation and spatial grouping of genes selectively expressed within the Eomes + cortical excitatory neuron lineage during early cortical development. In these neurons 475 genes were expressed ≥ 3-fold, and 534 genes ≤ 3-fold, compared to the reference population of neuronal precursors. Of the up-regulated genes, 328 were represented at the Genepaint in situ hybridization database and 317 (97%) were validated as having spatial expression patterns consistent with the lineage of differentiating excitatory neurons. A novel approach for quantifying in situ hybridization patterns (QISP) across the cerebral wall was developed that allowed the hierarchical clustering of genes into putative co-regulated groups. Forty four candidate genes were identified that show spatial expression with Intermediate Precursor Cells, 49 candidate genes show spatial expression with Multipolar Neurons, while the remaining 224 genes achieved peak expression in the developing cortical plate.

CONCLUSIONS

This analysis of differentiating excitatory neurons revealed the expression patterns of 37 transcription factors, many chemotropic signaling molecules (including the Semaphorin, Netrin and Slit signaling pathways), and unexpected evidence for non-canonical neurotransmitter signaling and changes in mechanisms of glucose metabolism. Over half of the 317 identified genes are associated with neuronal disease making these findings a valuable resource for studies of neurological development and disease.

Should Alzheimer's disease be equated with human brain ageing? A maladaptive interaction between brain evolution and senescence

In this review Alzheimer's disease is seen as a maladaptive interaction between human brain evolution and senescence. It is predicted to occur in everyone although does not necessarily lead to dementia. The pathological process is initiated in relation to a senescence mediated functional down-regulation in the posteromedial cortex (Initiation Phase). This leads to a loss of glutamatergic excitatory input to layer II entorhinal cortex neurons. A human specific maladaptive neuroplastic response is initiated in these neurons leading to neuronal dysfunction, NFT formation and death. This leads to further loss of glutamatergic excitatory input and propagation of the maladaptive response along excitatory pathways linking evolutionary progressed vulnerable neurons (Propagation Phase). Eventually neurons are affected in many brain areas resulting in dementia. Possible therapeutic approaches include enhancing glutamatergic transmission. The theory may have implications with regards to how Alzheimer's disease is classified.

Physical exercise increases Notch activity, proliferation and cell cycle exit of type-3 progenitor cells in adult hippocampal neurogenesis

In adult hippocampal neurogenesis of mice, the proliferation of precursor cells can be stimulated by voluntary exercise (wheel-running). Physical activity has an additional effect on late progenitor cells (type-3) by promoting cell survival and further maturation. Notch1 is a key regulator of various steps in neuronal development, including the inhibition of cell cycle exit and neuronal differentiation of neural stem cells, as well as promoting the survival and dendritic branching of newborn neurons. We here report that physical activity increased the proportion and absolute number of doublecortin(+) (DCX) type-2b and type-3 progenitor cells that showed an activated Notch1 pathway. In contrast, the fraction of dividing cells with nuclear Notch intracellular domain expression indicating an activated Notch pathway was not affected by physical exercise. We used double labeling with two halogenated thymidine analogs, iododeoxyuridine and chlorodeoxyuridine, to distinguish between cell cycle exit and continued division at the progenitor cell level. After 7 days of physical exercise, the proliferative activity of precursor cells was increased, whereas the proportion of type-2b/3 cells re-entering S-phase was reduced. Consistent with this observation, the proportion of DCX(+) cells that expressed the marker of postmitotic immature granule cells (calretinin) was enhanced. Running promotes both the proliferation and cell cycle exit of DCX(+) type-3 precursors, possibly by preferentially stimulating a last neurogenic cell division. These pro-proliferative effects are independent of Notch1, whereas the running-induced survival and cell cycle exit of type-3 progenitor cells might by mediated by Notch1 activity.

Transient activation of Notch signaling in the injured adult brain

Brain injury induces various kinds of cellular responses that lead to tissue regeneration and repair. Recent studies have demonstrated that resident progenitors proliferate and then differentiate into mature neuronal cells. We show here that proliferating cells in the cryo-injured cerebral cortex transiently expressed Notch1 immunoreactivity in their cytoplasm. Since activated Notch signaling regulates cellular fate in the developing nervous system, similar regulation may exist in the injured adult brain. To monitor the Notch signaling pathway, we examined whether components of the signaling pathway were co-expressed in Notch1-positive cells. Presenilin-1, a membrane-spanning protease that is required for the release of the Notch intracellular domain, was detected in the Notch1-positive cells and Hes1, a target of the Notch intracellular domain, also co-localized with Notch1 three days after cryo-injury. These results suggest that transient activity of the Notch signaling pathway is involved in the regulation of proliferation and differentiation of progenitors in the injured brain.

Caenorhabditis elegans: an emerging model in biomedical and environmental toxicology

The nematode Caenorhabditis elegans has emerged as an important animal model in various fields including neurobiology, developmental biology, and genetics. Characteristics of this animal model that have contributed to its success include its genetic manipulability, invariant and fully described developmental program, well-characterized genome, ease of maintenance, short and prolific life cycle, and small body size. These same features have led to an increasing use of C. elegans in toxicology, both for mechanistic studies and high-throughput screening approaches. We describe some of the research that has been carried out in the areas of neurotoxicology, genetic toxicology, and environmental toxicology, as well as high-throughput experiments with C. elegans including genome-wide screening for molecular targets of toxicity and rapid toxicity assessment for new chemicals. We argue for an increased role for C. elegans in complementing other model systems in toxicological research.

Localization and cellular distribution of CPNE5 in embryonic mouse brain

CPNE5 is one of the ubiquitous Ca(2+)-dependent, phospholipid-binding proteins that are highly conserved in animals. It was cloned in the fetal human brain with no exact functions identified yet. We have examined the distribution pattern of CPNE5 mRNA and protein in the developing murine brain by using in situ hybridization, western blotting and immunocytochemistry. Expression of CPNE5 mRNA remains high from embryonic day 9.5 (E9.5) to E15.5 in the developing murine brain. Whole-mount in situ hybridization with the E11.5 and E12.5 embryos showed the strong positive signals in the central nervous system. Western-blot analysis showed that CPNE5 protein is expressed in the developing but not in the adult murine brain. In situ hybridization and immunohistochemistry analysis on the embryonic brain sections indicated that both at RNA and protein levels CPNE5 is mainly expressed in frontal cortex, medial nasal prominence, ganglionic eminence and medulla, particularly in the ventricular zones. Further investigation revealed the co-localization of CPNE5 with Tuj1 and Nestin on embryonic brain sections. In addition to the slight expression in primary cultured neural progenitor cells, CPNE5 is found in soma and neurite projections of primary cultured neurons where Tuj1 is co-localized. Our results demonstrate that CPNE5 is expressed in both neural progenitor cells and the differentiated neurons during the neural development, which suggests that CPNE5 might play an important role in the development of murine central nervous system.

Notch-1 immunoexpression is increased in Alzheimer's and Pick's disease

Notch-1 is a protein that influences cell fate decisions, with its expression occurring primarily during embryogenesis and development. However, Notch-1 is also expressed in the adult brain, in regions with high synaptic plasticity, particularly the hippocampus. Its role in adults is unknown; however, it may impact neurite outgrowth or cell differentiation in adult brain regions undergoing neurogenesis. Notch-1 is increased in Alzheimer's disease (AD); however, its expression in other CNS degenerative diseases has not been described. To begin to define the range of degenerative disorders where Notch-1 expression is altered, we examined Notch-1 immunoreactivity in a variety of neurodegenerative diseases to determine whether its increase is selective for AD. We examined sections of hippocampus from 13 AD, 13 classical Pick's disease (PiD; with Pick bodies), 4 dementia lacking distinctive histopathology (DLDH) and 8 control brains, emphasizing hippocampal (dentate gyrus) pathology. We determined that Notch-1 immunoexpression is increased in AD and PiD relative to control cases. DLDH cases were not significantly different than control cases with respect to Notch-1 expression. Given the increase in Notch-1 immunoexpression in AD and PiD, two diseases where abnormal tau aggregates are present, and the lack of Notch-1 immunoexpression in DLDH (where tau aggregates are absent), we cannot rule out the possibility that tau aggregates are associated with Notch-1 expression in neurodegenerative diseases.

Familial Alzheimer's disease presenilin 1 mutations cause alterations in the conformation of presenilin and interactions with amyloid precursor protein

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, an enzymatic activity that cleaves amyloid beta (Abeta) from the amyloid precursor protein (APP). More than 100 mutations spread throughout the PS1 molecule are linked to autosomal dominant familial Alzheimer's disease (FAD). All of these mutations lead to a similar phenotype: an increased ratio of Abeta42 to Abeta40, increased plaque deposition, and early age of onset. We use a recently developed microscopy approach, fluorescence lifetime imaging microscopy, to monitor the relative molecular distance between PS1 N and C termini in intact cells. We show that FAD-linked missense mutations located near the N and C termini, in the mid-region of PS1, and the exon 9 deletion mutation all change the spatial relationship between PS1 N and C termini in a similar way, increasing proximity of the two epitopes. This effect is opposite of that observed by treatment with Abeta42-lowering nonsteroidal anti-inflammatory drugs (NSAIDs) (Lleo et al., 2004b). Accordingly, treatment of M146L PS1-overexpressing neurons with high-dose NSAIDs somewhat offsets the conformational change associated with the mutation. Moreover, by monitoring the relative distance between a PS1 loop epitope and the APP C terminus, we demonstrate that the FAD PS1 mutations are also associated with a consistent change in the configuration of the PS1-APP complex. The nonpathogenic E318G PS1 polymorphism had no effect on PS1 N terminus-C terminus proximity or PS1-APP interactions. We propose that the conformational change we observed may therefore provide a shared molecular mechanism for FAD pathogenesis caused by a wide range of PS1 mutations.

BACE is degraded via the lysosomal pathway

Amyloid plaques are formed by aggregates of amyloid-beta-peptide, a 37-43-amino acid fragment (primarily Abeta(40) and Abeta(42)) generated by proteolytic processing of the amyloid precursor protein (APP) by beta- and gamma-secretases. A type I transmembrane aspartyl protease, BACE (beta-site APP cleaving enzyme), has been identified to be the beta-secretase. BACE is targeted through the secretory pathway to the plasma membrane where it can be internalized to endosomes. The carboxyl terminus of BACE contains a di-leucine-based signal for sorting of transmembrane proteins to endosomes and lysosomes. In this study, we set out to determine whether BACE is degraded by the lysosomal pathway and whether the di-leucine motif is necessary for targeting BACE to the lysosomes. Here we show that lysosomal inhibitors, chloroquine and NH(4)Cl, lead to accumulation of endogenous and ectopically expressed BACE in a variety of cell types, including primary neurons. Furthermore, the inhibition of lysosomal hydrolases results in the redistribution and accumulation of BACE in the late endosomal/lysosomal compartments (lysosome-associated membrane protein 2 (LAMP2)-positive). In contrast, the BACE-LL/AA mutant, in which Leu(499) and Leu(500) in the COOH-terminal sequence (DDISLLK) were replaced by alanines, only partially co-localized with LAMP2-positive compartments following inhibition of lysosomal hydrolases. Collectively, our data indicate that BACE is transported to the late endosomal/lysosomal compartments where it is degraded via the lysosomal pathway and that the di-leucine motif plays a role in sorting BACE to lysosomes.

Dendritic spine abnormalities in amyloid precursor protein transgenic mice demonstrated by gene transfer and intravital multiphoton microscopy

Accumulation of amyloid-beta (Abeta) into senile plaques in Alzheimer's disease (AD) is a hallmark neuropathological feature of the disorder, which likely contributes to alterations in neuronal structure and function. Recent work has revealed changes in neurite architecture associated with plaques and functional changes in cortical signaling in amyloid precursor protein (APP) expressing mouse models of AD. Here we developed a method using gene transfer techniques to introduce green fluorescent protein (GFP) into neurons, allowing the investigation of neuronal processes in the vicinity of plaques. Multiphoton imaging of GFP-labeled neurons in living Tg2576 APP mice revealed disrupted neurite trajectories and reductions in dendritic spine density compared with age-matched control mice. A profound deficit in spine density (approximately 50%) extends approximately 20 mum from plaque edges. Importantly, a robust decrement (approximately 25%) also occurs on dendrites not associated with plaques, suggesting widespread loss of postsynaptic apparatus. Plaques and dendrites remained stable over the course of weeks of imaging. Postmortem analysis of axonal immunostaining and colocalization of synaptophysin and postsynaptic density 95 protein staining around plaques indicate a parallel loss of presynaptic and postsynaptic partners. These results show considerable changes in dendrites and dendritic spines in APP transgenic mice, demonstrating a dramatic synaptotoxic effect of dense-cored plaques. Decreased spine density will likely contribute to altered neural system function and behavioral impairments observed in Tg2576 mice.

Presenilin function and gamma-secretase activity

Alzheimer's disease (AD) is the most common form of dementia and is characterized pathologically by the accumulation of beta-amyloid (Abeta) plaques and neurofibrillary tangles in the brain. Genetic studies of AD first highlighted the importance of the presenilins (PS). Subsequent functional studies have demonstrated that PS form the catalytic subunit of the gamma-secretase complex that produces the Abeta peptide, confirming the central role of PS in AD biology. Here, we review the studies that have characterized PS function in the gamma-secretase complex in Caenorhabditis elegans, mice and in in vitro cell culture systems, including studies of PS structure, PS interactions with substrates and other gamma-secretase complex members, and the evidence supporting the hypothesis that PS are aspartyl proteases that are active in intramembranous proteolysis. A thorough knowledge of the mechanism of PS cleavage in the context of the gamma-secretase complex will further our understanding of the molecular mechanisms that cause AD, and may allow the development of therapeutics that can alter Abeta production and modify the risk for AD.

Progression of Cerebral Amyloid Angiopathy in Transgenic Mouse Models of Alzheimer Disease

Cerebral amyloid angiopathy (CAA), the deposition of beta-amyloid (Abeta3) in cerebral vessels, has been implicated as a common cause of hemorrhagic stroke and other forms of vascular disease. CAA is also a frequent concomitant of Alzheimer disease (AD). While the longterm consequences of CAA are well recognized from clinical and pathologic studies, numerous questions remain unanswered regarding the progression of the disease. Examination of CAA in traditional histologic sections does not easily allow for characterization of CAA, particularly in leptomeningeal vessels. In order to approach this topic, we used low magnification imaging of intact, postmortem brains from transgenic mouse models of AD-like pathology to define the spatial and temporal characteristics of CAA in leptomeningeal vessels. Imaging of brains from 10- to 26-month-old animals demonstrated a stereotypical pattern to the development of CAA, with vessels over the dorsal surface of the brain showing an anterior-to-posterior and large-to-small vessel gradient of involvement. High magnification imaging revealed that CAA deposition began with a banding pattern determined by the organization of the vascular smooth muscle cells. Further analysis of the pattern of amyloid deposits showed shrinkage and disappearance of the gaps between clusters of amyloid bands, gradually reaching a confluent pattern. These data led to a classification system to describe the severity of CAA deposition and demonstrate the potential of using intact brains to generate maps defining the progression and kinetics of CAA. This approach should lead to more informed analysis of the consequences of evolving therapeutic options for AD on this related form of vascular pathology.

The low density lipoprotein receptor-related protein (LRP) is a novel beta-secretase (BACE1) substrate

BACE is a transmembrane protease with beta-secretase activity that cleaves the amyloid precursor protein (APP). After BACE cleavage, APP becomes a substrate for gamma-secretase, leading to release of amyloid-beta peptide (Abeta), which accumulates in senile plaques in Alzheimer disease. APP and BACE are co-internalized from the cell surface to early endosomes. APP is also known to interact at the cell surface and be internalized by the low density lipoprotein receptor-related protein (LRP), a multifunctional endocytic and signaling receptor. Using a new fluorescence resonance energy transfer (FRET)-based assay of protein proximity, fluorescence lifetime imaging (FLIM), and co-immunoprecipitation we demonstrate that the light chain of LRP interacts with BACE on the cell surface in association with lipid rafts. Surprisingly, the BACE-LRP interaction leads to an increase in LRP C-terminal fragment, release of secreted LRP in the media and subsequent release of the LRP intracellular domain from the membrane. Taken together, these data suggest that there is a close interaction between BACE and LRP on the cell surface, and that LRP is a novel BACE substrate.

Nonsteroidal anti-inflammatory drugs lower Abeta42 and change presenilin 1 conformation

Recent reports suggest that some commonly used nonsteroidal anti-inflammatory drugs (NSAIDs) unexpectedly shift the cleavage products of amyloid precursor protein (APP) to shorter, less fibrillogenic forms, although the underlying mechanism remains unknown. We now demonstrate, using a fluorescence resonance energy transfer method, that Abeta(42)-lowering NSAIDs specifically affect the proximity between APP and presenilin 1 and alter presenilin 1 conformation both in vitro and in vivo, suggesting a novel allosteric mechanism of action.

Notch is required for long-term memory in Drosophila

A role for Notch in the elaboration of existing neural processes is emerging that is distinct from the increasingly well understood function of this gene in binary cell-fate decisions. Several research groups, by using a variety of organisms, have shown that Notch is important in the development of neural ultrastructure. Simultaneously, Presenilin (Psn) was identified both as a key mediator of Notch signaling and as a site of genetic lesions that cause early-onset Alzheimer's disease. Here we demonstrate that Notch loss of function produces memory deficits in Drosophila melanogaster. The effects are specific to long-term memory, which is thought to depend on ultrastructural remodeling. We propose that Notch plays an important role in the neural plasticity underlying consolidated memory.

Activated Notch1 associates with a presenilin-1/gamma-secretase docking site

Presenilin-1 (PS1), implicated as the active component of the gamma-secretase enzymatic complex, is known to cleave the cell surface receptor Notch1 after ligand binding. Here we directly visualize Notch1-PS1 interactions using a novel fluorescence lifetime imaging microscopy assay to monitor fluorescence resonance energy transfer. We demonstrate that endogenous Notch1 and PS1 move into close proximity at the cell surface after activation of Notch1 by the Delta1 ligand. A constitutively active N-terminally truncated form of Notch1, an immediate substrate of the gamma-secretase complex, similarly is found in close proximity to PS1. Interestingly, this interaction remains in the presence of a potent gamma-secretase active site inhibitor. Thus ligand binding to Notch1 appears to result in access of truncated Notch1 to a putative docking site on the PS1-gamma-secretase complex. These results suggest a novel mechanism of ligand binding-mediated signal transduction of Notch1.

Notch1 competes with the amyloid precursor protein for gamma-secretase and down-regulates presenilin-1 gene expression

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, which is involved in the cleavage of several substrates including the amyloid precursor protein (APP) and Notch1. Based on the fact that APP and Notch are processed by the same gamma-secretase, we postulated that APP and Notch compete for the enzyme activity. In this report, we examined the interactions between APP, Notch, and PS1 using the direct gamma-secretase substrates, Notch 1 Delta extracellular domain (N1DeltaEC) and APP carboxyl-terminal fragment of 99 amino acids, and measured the effects on amyloid-beta protein production and Notch signaling, respectively. Additionally, we tested the hypothesis that downstream effects on PS1 expression may coexist with the competition phenomenon. We observed significant competition between Notch and APP for gamma-secretase activity; transfection with either of two direct substrates of gamma-secretase led to a reduction in the gamma-cleaved products, Notch intracellular domain or amyloid-beta protein. In addition, however, we found that activation of the Notch signaling pathway, by either N1 Delta EC or Notch intracellular domain, induced down-regulation of PS1 gene expression. This finding suggests that Notch activation directly engages gamma-secretase and subsequently leads to diminished PS1 expression, suggesting a complex set of feedback interactions following Notch activation.

Notch and Presenilin: regulated intramembrane proteolysis links development and degeneration

Intensive studies of three proteins--Presenilin, Notch, and the amyloid precursor protein (APP)--have led to the recognition of a direct intersection between early development and late-life neurodegeneration. Notch signaling mediates many different intercellular communication events that are essential for determining the fates of neural and nonneural cells during development and in the adult. The Notch receptor acts in a core pathway as a membrane-bound transcription factor that is released to the nucleus by a two-step cleavage mechanism called regulated intramembrane proteolysis (RIP). The second cleavage is effected by Presenilin, an unusual polytopic aspartyl protease that apparently cleaves Notch and numerous other single-transmembrane substrates within the lipid bilayer. Another Presenilin substrate, APP, releases the amyloid ss-protein that can accumulate over time in limbic and association cortices and help initiate Alzheimer's disease. Elucidating the detailed mechanism of Presenilin processing of membrane proteins is important for understanding diverse signal transduction pathways and potentially for treating and preventing Alzheimer's disease.

Learning and memory deficits in Notch mutant mice

Notch is a critical component of evolutionarily conserved signaling mechanisms that regulate development and may contribute to plasticity-related processes, including changes in neurite structure and maintenance of neural stem cells. Deficits in the Notch pathway are responsible for Alagille and Cadasil syndromes, which are associated with mental retardation and dementia. Additionally, in postmitotic neurons, Notch proteins interact with presenilins and with beta-amyloid precursor protein and could therefore have a role in the memory deficits associated with familial and sporadic Alzheimer's disease. To test if alterations in Notch signaling can lead to learning and memory deficits, we studied mice with mutations in this pathway. Here, we show that null heterozygous mutations in Notch1 result in deficits in spatial learning and memory without affecting other forms of learning, motor control, or exploratory activity. We also show that null heterozygous mutations in the downstream cofactor RBP-J result in similarly specific spatial learning and memory deficits. These data indicate that a constitutive decrease in Notch signaling can result in specific learning and memory deficits and suggest that abnormalities in Notch-dependent transcription may contribute to the cognitive deficits associated with Alzheimer's disease and Alagille and Cadasil syndromes.

APH1, PEN2, and Nicastrin increase Abeta levels and gamma-secretase activity

A major component of the amyloid plaque core in Alzheimer's disease (AD) is the 40-42-residue amyloid beta peptide (Abeta). Mutations linked to AD such as those in presenilins 1 (PS1) and 2 (PS2) invariably increase the longer Abeta42 species that forms neurotoxic oligomers. It is believed that PS1/2 constitute the catalytic subunit of the gamma-secretase responsible for the final step in Abeta biogenesis. Recent genetic studies have identified a number of additional genes encoding APH1a, APH1b, PEN2, and Nicastrin proteins, which are part of the gamma-secretase complex with PS1. Further, knockout studies using RNAi showed that these components are essential for gamma-secretase activity. However, the nature of gamma-secretase and how the aforementioned proteins regulate its activity are still incompletely understood. Here we present evidence that unlike PS1, overexpression of these proteins can increase the levels of Abeta, suggesting that these proteins are limiting for gamma-secretase activity. In addition, our studies also suggest that the presenilin partners regulate the relative levels of Abeta40 and Abeta42.

Biochemical and immunocytochemical characterization of calsenilin in mouse brain

Mutations in the presenilin 1 and 2 genes cause the majority of early onset familial forms of Alzheimer's disease. Here we describe the biochemical and immunohistochemical characterization of calsenilin, a novel calcium binding protein that we have previously shown to interact with presenilins 1 and 2, in mouse brain. The co-immunoprecipitation of endogenous calsenilin and presenilin 1 demonstrates that these proteins are physiologic binding partners. Although calsenilin has been predicted to be a soluble protein, we have found that the majority of it is tightly associated with the cytoplasmic face of intracellular membranes and that it can only be dissociated using harsh treatments such as urea. In addition, we have demonstrated that calsenilin is a developmentally regulated protein that is mainly present in the brain, where it localizes to both the hippocampus and cerebellum. Calsenilin staining co-localized with the somatodendritic marker microtubule-associated protein-2 primarily in the granular cell layer of the cerebellum, indicating that calsenilin expression is primarily neuronal. In primary cultured neurons, calsenilin immunoreactivity was observed in cell bodies as well as in some neuronal processes. Co-localization experiments using specific axonal and dendritic markers indicate that these processes were mainly axonal in nature, although a smaller subset of dendrites also appears to contain calsenilin. In summary, we have established that calsenilin and presenilin 1 can interact at physiologic levels, and that calsenilin is a developmentally regulated protein that is expressed primarily in the cerebellum and hippocampus. Although calsenilin is a soluble protein, it is tightly associated with the membrane. Finally, the expression pattern of calsenilin, which is similar to that of the presenilin(s), suggests that the common locations of these two proteins provide an opportunity for physical interaction in vivo.

Imaging Abeta plaques in living transgenic mice with multiphoton microscopy and methoxy-X04, a systemically administered Congo red derivative

The identification of amyloid deposits in living Alzheimer disease (AD) patients is important for both early diagnosis and for monitoring the efficacy of newly developed anti-amyloid therapies. Methoxy-X04 is a derivative of Congo red and Chrysamine-G that contains no acid groups and is therefore smaller and much more lipophilic than Congo red or Chrysamine-G. Methoxy-X04 retains in vitro binding affinity for amyloid beta (Abeta) fibrils (Ki = 26.8 nM) very similar to that of Chrysamine-G (Ki = 25.3 nM). Methoxy-X04 is fluorescent and stains plaques, tangles, and cerebrovascular amyloid in postmortem sections of AD brain with good specificity. Using multiphoton microscopy to obtain high-resolution (1 microm) fluorescent images from the brains of living PSI/APP mice, individual plaques could be distinguished within 30 to 60 min after a single i.v. injection of 5 to 10 mg/kg methoxy-X04. A single i.p. injection of 10 mg/kg methoxy-X04 also produced high contrast images of plaques and cerebrovascular amyloid in PSI/APP mouse brain. Complementary quantitative studies using tracer doses of carbon- 11-labeled methoxy-X04 show that it enters rat brain in amounts that suggest it is a viable candidate as a positron emission tomography (PET) amyloid-imaging agent for in vivo human studies.

A lipophilic thioflavin-T derivative for positron emission tomography (PET) imaging of amyloid in brain

The synthesis of a new lipophilic thioflavin-T analogue (2-[4' -(methylamino)phenyl]benzothiazole, 6) with high affinity for amyloid is reported. Intravenous injection of [(11)C]-labeled 6 in control mice resulted in high brain uptake. Amyloid deposits were imaged with multiphoton microscopy in the brains of living transgenic mice following the systemic injection of unlabeled 6. [(11)C]6 is a promising amyloid imaging agent for Alzheimer's disease.

Notch1 and amyloid precursor protein are competitive substrates for presenilin1-dependent gamma-secretase cleavage

Proteolytic processing of the amyloid precursor protein (APP) by beta- and gamma-secretases results in the production of a highly amyloidogenic Abeta peptide, which deposits in the brains of Alzheimer's disease patients. Similar gamma-secretase processing occurs in another transmembrane protein, Notch1, releasing a potent signaling molecule, the Notch C-terminal domain. It has been shown that both events are dependent on a presenilin-dependent protease. We now test the hypothesis that activated Notch1 and APP are competitive substrates for the same proteolytic activity in neurons. Treatment of neurons with the native Notch ligand, Delta, induces endogenous Notch1 intramembraneous cleavage and diminishes Abeta production in a dose-dependent manner. Complementary experiments showed that the converse was also true. Overexpressing human APP (APP(695Sw)) in neurons leads to a decrease in endogenous Notch1 signal transduction, as assessed by a CBF1 luciferase transcription assay, by Notch C-terminal domain nuclear translocation in vitro and by analysis of Notch C-terminal domain generation and Notch1 staining in vivo. In summary, two complementary approaches suggest that APP and Notch1 are physiologically relevant competitive substrates for gamma-secretase activity.

The pharmacology of novel acetylcholinesterase inhibitors, (+/-)-huprines Y and X, on the Torpedo electric organ

The effects of the tacrine-huperzine A hybrid acetylcholinesterase inhibitors, (+/-)-12-amino-3-chloro-9-methyl-6,7,10,11-tetrahydro-7,11-methanocycloocta[b]quinoline hydrochloride ((+/-)-huprine Y) and (+/-)-12-amino-3-chloro-9-ethyl-6,7,10,11-tetrahydro-7,11-methanocycloocta[b]quinoline hydrochloride ((+/-)-huprine X), were tested on spontaneous synaptic activity by measuring the amplitude, the rise time, the rate of rise, the half-width and the area or the electrical charge of the miniature endplate potentials (m.e.p.ps) recorded extracellularly on Torpedo electric organ fragments. (+/-)-Huprine Y and (+/-)-huprine X at a concentration of 500 nM increased all the m.e.p.p. variables analyzed. The effect of (+/-)-huprine Y was smaller than that of (+/-)-huprine X for all the variables except for the rate of rise where there was no significant difference. The effects of these drugs were also tested on nicotinic receptors by analyzing the currents elicited by acetylcholine (100 microM) in Xenopus laevis oocytes, transplanted with membranes from Torpedo electric organ. Both drugs inhibited the currents in a reversible manner, (+/-)-huprine Y (IC(50)=452 nM) being more effective than (+/-)-huprine X (IC(50)=4865 nM). The Hill coefficient was 0.5 for both drugs. The inhibition of the nicotinic receptor was voltage-dependent and decreased at depolarizing potentials, and there was no significant difference in the effects between (+/-)-huprine Y and (+/-)-huprine X at concentrations near to their IC(50) values. At depolarizing potentials between -20 and +15 mV, these drugs did not have any detectable effect on the blockade of the nicotinic receptor. Both huprines increased the desensitization of the nicotinic receptors since the current closed quickly in the presence of the drugs, and there was no significant difference in this effect between (+/-)-huprine Y (500 nM) and (+/-)-huprine X (5 microM). We conclude that (+/-)-huprine Y and (+/-)-huprine X increase the level of acetylcholine in the synaptic cleft more effectively than tacrine. The interaction of (+/-)-huprine X with nicotinic receptors is weaker than that of (+/-)-huprine Y, suggesting that (+/-)-huprine X would be more specific to maintain the extracellular acetylcholine concentration.

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.

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".

Effect of PS1 deficiency and an APP gamma-secretase inhibitor on Notch1 signaling in primary mammalian neurons

Presenilin1 (PS1) has been implicated in normal Notch1 processing and signaling in several experimental systems. In the present study, the relationship between PS1 and Notch1 in mammalian neurons is studied by analyzing Notch1 cleavage and C-terminal nuclear translocation as well as Notch1 signaling via the transactivation of a CBF1-luciferase reporter construct. We show that full-length Notch1 [N1(FL)] transfected into wild type (WT) primary neurons is cleaved in the presence of its biological ligand Delta (Dl) and translocated to the nucleus within 1--3 min of ligand addition. PS1 deficient neurons show normal Notch1 insertion into the cellular membrane, yet lack Notch1 activation resulting in markedly inhibited nuclear translocation of the C-terminal Notch fragment (NICD). PS1 deficient neurons also have impaired Notch1 signaling which can be restored fully or partially to levels seen in WT littermates by transfection with WT or familial Alzheimer's disease-associated M146L mutant PS1, respectively. We also show that pharmacological inhibition of PS1-associated gamma-secretase activity parallels the effects of genetic PS1 deficiency in these assays. These results support the hypothesis that PS1 deficiency blocks neuronal Notch1 processing and signaling.

Presenilin function in APP processing

Familial Alzheimer's disease (FAD) is now linked to at least three genes encoding the amyloid precursor protein (APP) on chromosome 21, and presenilin 1 and 2 on chromosome 14 and 1, respectively. FAD cases in whom presenilin mutations occur are more frequent than those with APP mutations. However, altogether they only account for approximately 0.1% of all the people suffering from Alzheimer's disease (AD), and the causes of the remaining 99.9% of the sporadic form of AD or senile dementia remain unknown. Since FAD presents with the same neuropathological features as sporadic AD, i.e., cognitive impairments and the amyloid plaques and tangles in the brain, our working hypothesis is that similar molecular pathogenic mechanisms underly both sporadic and familial AD. It follows that APP and the presenilins must be key players in the disease. Detailed knowledge about the cell biology of these proteins will be a rich source of insight into the pathology of AD, but will also shed light on the fundamental neurobiology of these proteins.

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

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.

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.

Aspartate mutations in presenilin and gamma-secretase inhibitors both impair notch1 proteolysis and nuclear translocation with relative preservation of notch1 signaling

It has been hypothesized that a presenilin 1 (PS1)-related enzymatic activity is responsible for proteolytic cleavage of the C-terminal intracellular protein of Notch1, in addition to its role in beta-amyloid protein (Abeta) formation from the amyloid precursor protein (APP). We developed an assay to monitor ligand-induced Notch1 proteolysis and nuclear translocation in individual cells : Treatment of full-length Notch1-enhanced green fluorescent protein-transfected Chinese hamster ovary (CHO) cells with a soluble preclustered form of the physiologic ligand Delta leads to rapid accumulation of the C terminus of Notch1 in the nucleus and to transcriptional activation of a C-promoter binding factor 1 (CBF1) reporter construct. Nuclear translocation was blocked by cotransfection with Notch's physiologic inhibitor Numb. Using this assay, we now confirm and extend the observation that PS1 is involved in Notch1 nuclear translocation and signaling in mammalian cells. We demonstrate that the D257A and the D385A PS1 mutations, which had been shown previously to block APP gamma-secretase activity, also prevent Notch1 cleavage and translocation to the nucleus but do not alter Notch1 trafficking to the cell surface. We also show that two APP gamma-secretase inhibitors block Notch1 nuclear translocation with an IC(50) similar to that reported for APP gamma-secretase. Notch1 signaling, assessed by measuring the activity of CBF1, a downstream transcription factor, was impaired but not abolished by the PS1 aspartate mutations or gamma-secretase inhibitors. Our results support the hypotheses that (a) PS1-dependent APP gamma-secretase-like enzymatic activity is critical for both APP and Notch processing and (b) the Notch1 signaling pathway remains partially activated even when Notch1 proteolytic processing and nuclear translocation are markedly inhibited. The latter is an important finding from the perspective of therapeutic treatment of Alzheimer's disease by targeting gamma-secretase processing of APP to reduce Abeta production.

Presenilin is required for proper morphology and function of neurons in C. elegans

Mutations in the human presenilin genes cause the most frequent and aggressive forms of familial Alzheimer's disease (FAD). Here we show that in addition to its role in cell fate decisions in non-neuronal tissues, presenilin activity is required in terminally differentiated neurons in vivo. Mutations in the Caenorhabditis elegans presenilin genes sel-12 and hop-1 result in a defect in the temperature memory of the animals. This defect is caused by the loss of presenilin function in two cholinergic interneurons that display neurite morphology defects in presenilin mutants. The morphology and function of the affected neurons in sel-12 mutant animals can be restored by expressing sel-12 only in these cells. The wild-type human presenilin PS1, but not the FAD mutant PS1 A246E, can also rescue these morphological defects. As lin-12 mutant animals display similar morphological and functional defects to presenilin mutants, we suggest that presenilins mediate their activity in postmitotic neurons by facilitating Notch signalling. These data indicate cell-autonomous and evolutionarily conserved control of neural morphology and function by presenilins.

Lack of requirement for presenilin1 in Notch1 signaling

Studies in invertebrates have indicated a functional requirement for presenilin (PS) genes in the Notch pathway [1-5]. One model of Notch signal transduction suggests that proteolysis releases an activated Notch fragment that migrates to the nucleus and regulates gene transcription in concert with CBF1/Su(H)/lag1 (CSL) proteins [6-9]. Recent studies suggest that PS genes control the proteolysis and nuclear access of the Notch intracellular domain [3,4,10,11], offering a basis for the functional interaction of PS and Notch genes [12]. Here, we report that Notch1 signaling elicited by the ligand Delta1 was quantitatively unchanged in PS1-deficient primary embryonic fibroblasts (PEFs). Notch1 signals were measured by both the activation of the hairy/enhancer of split (HES1) promoter and by the antagonism of MyoD-induced muscle creatine kinase (MCK) promoter activity. A membrane-tethered ligand-independent Notch1 construct also showed full efficacy in both assays, despite its presumed requirement for cleavage. Although signaling through Notch1 persisted in PS1-deficient cells, we found a marked reduction in the appearance of a complex of a cleaved, intracellular Notch fragment (NICD) and a CSL protein, as previously reported [6] [10]. These studies reveal that PS1 is not required for ligand-dependent Notch signaling, and that PS1 and PS2 may be redundant. Our data also suggest that the identified NICD fragment may not be necessary for Notch signal transduction [9].

Contact-dependent inhibition of cortical neurite growth mediated by notch signaling

The exuberant growth of neurites during development becomes markedly reduced as cortical neurons mature. In vitro studies of neurons from mouse cerebral cortex revealed that contact-mediated Notch signaling regulates the capacity of neurons to extend and elaborate neurites. Up-regulation of Notch activity was concomitant with an increase in the number of interneuronal contacts and cessation of neurite growth. In neurons with low Notch activity, which readily extend neurites, up-regulation of Notch activity either inhibited extension or caused retraction of neurites. Conversely, in more mature neurons that had ceased their growth after establishing numerous connections and displayed high Notch activity, inhibition of Notch signaling promoted neurite extension. Thus, the formation of neuronal contacts results in activation of Notch receptors, leading to restriction of neuronal growth and a subsequent arrest in maturity.

Notch1 inhibits neurite outgrowth in postmitotic primary neurons

Notch plays an important role in cell fate decisions in uncommitted proliferative cells, including neurogenesis, but is believed to not have a role in postmitotic cells. We have shown previously that Notch1 is highly expressed in embryonal mouse and human brain, but surprisingly it continues to be expressed at low levels in the adult brain. The function of Notch1 in postmitotic neurons in mammals is unknown. To better understand the potential role of Notch1 in mature central nervous system neurons we studied the effect of Notch1 transfection on neurite outgrowth in primary neocortex hippocampal neurons. Transfection at two days in vitro with full length Notch1 inhibited neurite outgrowth. Transfection at five to six days in vitro, after neurite outgrowth was established, led to apparent regression of neurites. These effects were enhanced when truncated constitutively active forms of Notch1 were introduced. Co-transfection with Numb, a physiological inhibitor of Notch, blocked Notch's effect on neurite outgrowth. We also examined whether Notch1 could activate C-promoter binding factor (CBF1) transcription factor using C-promoter binding factor-luciferase constructs, and demonstrated that this signal transduction pathway is present and can be activated in postmitotic neurons. Our results show that in postmitotic neurons Notch1 influences neurite morphology, and can activate its native signal transduction pathway. These data strongly suggest that Notch1 may play a physiologically important role in the central nervous system beyond neurogenesis.