Interaction between amyloid precursor protein and presenilins in mammalian cells: implications for the pathogenesis of Alzheimer disease

Physiology and pharmacology of amyloid precursor protein

Amyloid precursor protein (APP) is an evolutionarily conserved transmembrane protein and a well-characterized precursor protein of amyloid-beta (Aβ) peptides, which accumulate in the brains of individuals with Alzheimer's disease (AD)-related pathologies. Aβ has been extensively investigated since the amyloid hypothesis in AD was proposed. Besides Aβ, previous studies on APP and its proteolytic cleavage products have suggested their diverse pathological and physiological functions. However, their roles still have not been thoroughly understood. In this review, we extensively discuss the evolutionarily-conserved biology of APP, including its structure and processing pathway, as well as recent findings on the physiological roles of APP and its fragments in the central nervous system and peripheral nervous system. We have also elaborated upon the current status of APP-targeted therapeutic approaches for AD treatment by discussing inhibitors of several proteases participating in APP processing, including α-, β-, and γ-secretases. Finally, we have highlighted the future perspectives pertaining to further research and the potential clinical role of APP.

PSEN2 Mutation Spectrum and Novel Functionally Validated Mutations in Alzheimer’s Disease: Data from PUMCH Dementia Cohort

Background: The established causative mutations in the APP, PSEN1, and PSEN2 can explain less than 1%,Alzheimer’s disease (AD) patients. Of the identified variants, the PSEN2 mutations are even less common. Objective: With the genetic study from the dementia cohort of Peking Union Medical College Hospital (PUMCH), we aim to illustrate the PSEN2 mutation spectrum and novel functionally validated mutations in Chinese AD patients. Methods: 702 AD participants, aged 30–85, were identified in PUMCH dementia cohort. They all received history inquiry, physical examination, biochemical test, cognitive evaluation, brain CT/MRI, and next-generation DNA sequencing. Functional analysis was achieved by transfection of the HEK293 cells with plasmids harboring the wild-type PSEN2 or candidate mutations. Results: Nine PSEN2 rare variants were found, including two reported (M239T, R62C) and seven novel variants (N141S, I368F, L396I, G117X, I146T, S147N, H220Y). The HEK293 cells transfected with the PSEN2 N141S, M239T, I368F plasmids showed higher Aβ 42 and Aβ 42/Aβ 40 levels relative to the wild-type PSEN2. The PSEN2 L396I, G117X, S147N, H220Y, and R62C did not alter Aβ 42, Aβ 40 levels, or Aβ 42/Aβ 40 ratio. 1.9%,(13/702) subjects harbored rare PSEN2 variants. 0.4%,(3/702) subjects carried pathogenic/likely pathogenic PSEN2 mutations. The three subjects with the functionally validated PSEN2 mutations were all familial early-onset AD patients. The common symptoms included amnesia and mental symptom. Additionally, the M239T mutation carrier presented with dressing apraxia, visuospatial agraphia, dyscalculia and visual mislocalization. Conclusion: The PSEN2 N141S, M239T, and I368F are functionally validated mutations.

β-amyloid: The known unknowns

Alzheimer's disease (AD) stands out as a major disease without any form of preventative or disease modifying therapy. This is not for lack of trying. 33 phase 3 clinical trials of drugs targeting amyloid beta (Aβ) have failed to slow cognitive decline in AD. The field is at a cross-roads about whether to continue anti-Aβ therapy or more actively pursue alternative targets. With the burden of this disease to patients, families, and healthcare budgets growing yearly, the need for disease modifying AD therapies has become one of the highest priorities in all of medicine. While pathology, genetic and biochemical data offer a popular narrative for the causative role of Aβ, there are alternative explanations, and dissenting findings that, now more than ever, warrant thorough reanalysis. This review questions the major assumptions about Aβ on which therapies for AD were premised, and invites renewed interrogation into AD pathogenesis.

■ REVIEW : The Presenilins

Presenilin-1 and presenilin-2 are highly homologous genes located on chromosomes 14 and 1, respectively, that have recently been linked to some cases of early-onset autosomal dominant inherited forms of Alzhei mer's disease (AD). Presenilins are integral membrane proteins localized in the endoplasmic reticulum of neurons throughout the nervous system. Studies of presenilin-1 knockout mice, and of invertebrate homo logues of presenilins and their interacting proteins, suggest major roles for presenilins in normal develop ment. Presenilin-1 mutant knockin mice do not exhibit developmental abnormalities, which indicates that the pathogenic mechanism of presenilin mutations involves gain of an adverse property of the mutant protein. Expression of presenilin mutations in cultured neurons and transgenic mice results in increased sensitivity to apoptosis induced by trophic factor withdrawal and exposure to oxidative and metabolic insults, and also alters gene expression. The pathogenic mechanism of presenilin mutations may involve perturbed endo plasmic reticulum calcium homeostasis resulting in enhanced oxidative stress, altered proteolytic processing of the amyloid precursor protein (APP), and increased neuronal vulnerability to excitotoxicity. Studies of presenilins are rapidly increasing our understanding the molecular and cellular underpinnings of AD and are also elucidating novel roles of the endoplasmic reticulum in neuronal plasticity and cell death. NEURO SCIENTIST 5:112-124, 1999

Physical and functional interaction between the α- and γ-secretases: A new model of regulated intramembrane proteolysis

Many single-transmembrane proteins are sequentially cleaved by ectodomain-shedding α-secretases and the γ-secretase complex, a process called regulated intramembrane proteolysis (RIP). These cleavages are thought to be spatially and temporally separate. In contrast, we provide evidence for a hitherto unrecognized multiprotease complex containing both α- and γ-secretase. ADAM10 (A10), the principal neuronal α-secretase, interacted and cofractionated with γ-secretase endogenously in cells and mouse brain. A10 immunoprecipitation yielded γ-secretase proteolytic activity and vice versa. In agreement, superresolution microscopy showed that portions of A10 and γ-secretase colocalize. Moreover, multiple γ-secretase inhibitors significantly increased α-secretase processing (r = -0.86) and decreased β-secretase processing of β-amyloid precursor protein. Select members of the tetraspanin web were important both in the association between A10 and γ-secretase and the γ → α feedback mechanism. Portions of endogenous BACE1 coimmunoprecipitated with γ-secretase but not A10, suggesting that β- and α-secretases can form distinct complexes with γ-secretase. Thus, cells possess large multiprotease complexes capable of sequentially and efficiently processing transmembrane substrates through a spatially coordinated RIP mechanism.

Twenty Years of Presenilins--Important Proteins in Health and Disease

Alzheimer's disease (AD) is characterized by progressive decline in cognitive functions associated with depositions of aggregated proteins in the form of extracellular plaques and neurofibrillary tangles in the brain. Extracellular plaques contain characteristic fibrils of amyloid β peptides (Aβ); tangles consist of paired helical filaments of the microtubuli-associated protein tau. Although AD manifests predominantly at ages above 65 years, rare cases show a much earlier onset of disease symptoms with very similar neuropathological characteristics. In 1995, two homologous genes were identified, in which mutations are associated with dominantly inherited familial forms of early onset AD. The genes therefore were dubbed presenilins (PS) and encode polytopic transmembrane proteins. At this time the role of these proteins in the pathogenesis of AD and their biological function in general were completely unknown. However, individuals carrying PS mutations showed alterations in the composition of different length variants of Aβ peptides in blood and cerebrospinal fluid, which indicated the potential involvement of presenilins in the metabolism of Aβ. After 20 years of intense research, the roles of presenilins in Aβ generation as well as important functions in biological processes have been identified. Presenilins represent the catalytic components of protease complexes that directly cleave the amyloid precursor protein (APP) but also many other proteins with important physiological functions. Here, the progress in presenilin research from basic characterization of their cellular functions to the targeting in clinical trials for AD therapy, and potential future directions, will be discussed.

Curcumin Targeted, Polymalic Acid-Based MRI Contrast Agent for the Detection of Aβ Plaques in Alzheimer's Disease

Currently, there is no gadolinium-based contrast agent available for conventional magnetic resonance imaging (MRI) detection of amyloidal beta (Aβ) plaques in Alzheimer's disease (AD). Its timely finding would be vital for patient survival and quality of life. Curcumin (CUR), a common Indian spice effectively binds to Aβ plaques which is a hallmark of AD. To address this binding, we have designed a novel nanoimaging agent (NIA) based on nature-derived poly(β-l-malic acid) (PMLA) containing covalently attached gadolinium-DOTA(Gd-DOTA) and nature-derived CUR. The all-in-one agent recognizes and selectively binds to Aβ plaques and is detected by MRI. It efficiently detected Aβ plaques in human and mouse samples by an ex vivo staining. The method can be useful in clinic for safe and noninvasive diagnosis of AD.

Presenilin transmembrane domain 8 conserved AXXXAXXXG motifs are required for the activity of the γ-secretase complex

Understanding the molecular mechanisms controlling the physiological and pathological activity of γ-secretase represents a challenging task in Alzheimer disease research. The assembly and proteolytic activity of this enzyme require the correct interaction of the 19 transmembrane domains (TMDs) present in its four subunits, including presenilin (PS1 or PS2), the γ-secretase catalytic core. GXXXG and GXXXG-like motifs are critical for TMDs interactions as well as for protein folding and assembly. The GXXXG motifs on γ-secretase subunits (e.g. APH-1) or on γ-secretase substrates (e.g. APP) are known to be involved in γ-secretase assembly and in Aβ peptide production, respectively. We identified on PS1 and PS2 TMD8 two highly conserved AXXXAXXXG motifs. The presence of a mutation causing an inherited form of Alzheimer disease (familial Alzheimer disease) in the PS1 motif suggested their involvement in the physiopathological configuration of the γ-secretase complex. In this study, we targeted the role of these motifs on TMD8 of PSs, focusing on their role in PS assembly and catalytic activity. Each motif was mutated, and the impact on complex assembly, activity, and substrate docking was monitored. Different amino acid substitutions on the same motif resulted in opposite effects on γ-secretase activity, without affecting the assembly or significantly impairing the maturation of the complex. Our data suggest that AXXXAXXXG motifs in PS TMD8 are key determinants for the conformation of the mature γ-secretase complex, participating in the switch between the physiological and pathological functional conformations of the γ-secretase.

Native top-down mass spectrometry for the structural characterization of human hemoglobin

Native mass spectrometry (MS) has become an invaluable tool for the characterization of proteins and noncovalent protein complexes under near physiological solution conditions. Here we report the structural characterization of human hemoglobin (Hb), a 64 kDa oxygen-transporting protein complex, by high resolution native top-down MS using electrospray ionization and a 15-Tesla Fourier transform ion cyclotron resonance mass spectrometer. Native MS preserves the noncovalent interactions between the globin subunits, and electron capture dissociation (ECD) produces fragments directly from the intact Hb complex without dissociating the subunits. Using activated ion ECD, we observe the gradual unfolding process of the Hb complex in the gas phase. Without protein ion activation, the native Hb shows very limited ECD fragmentation from the N-termini, suggesting a tightly packed structure of the native complex and therefore a low fragmentation efficiency. Precursor ion activation allows a steady increase in N-terminal fragment ions, while the C-terminal fragments remain limited (38 c ions and four z ions on the α chain; 36 c ions and two z ions on the β chain). This ECD fragmentation pattern suggests that upon activation, the Hb complex starts to unfold from the N-termini of both subunits, whereas the C-terminal regions and therefore the potential regions involved in the subunit binding interactions remain intact. ECD-MS of the Hb dimer shows similar fragmentation patterns as the Hb tetramer, providing further evidence for the hypothesized unfolding process of the Hb complex in the gas phase. Native top-down ECD-MS allows efficient probing of the Hb complex structure and the subunit binding interactions in the gas phase. It may provide a fast and effective means to probe the structure of novel protein complexes that are intractable to traditional structural characterization tools.

Regular and moderate exercise initiated in middle age prevents age-related amyloidogenesis and preserves synaptic and neuroprotective signaling in mouse brain cortex

Although the beneficial responses induced in the central nervous system by early-initiated exercise have been broadly investigated, the effects of a chronic and moderate lately-initiated exercise on biochemical hallmarks of very early brain senescence have not been extensively studied. We previously reported that a midlife-initiated regimen of moderate running was able not only to prevent the age-related decay of antioxidative and detoxification functions in mouse brain cortex, but also to preserve neurotrophic support and molecular integrity. On this basis, this work investigated whether and how a 2-mo or 4-mo midlife-initiated running protocol could affect the activity of those systems involved in maintaining neuronal function and in preventing the onset of neurodegeneration within the brain cortex of middle-aged CD-1 mice. In particular, we analyzed the production of the peptide amyloid-β and the expression of synapsin Ia, which is known to play a key role in neurotransmission and synaptic plasticity. In addition, we studied the expression of sirtuin 3, as a protein marker of neuroprotection against age-dependent mitochondrial dysfunction, as well as the pro-death pathway induced by proBDNF through the interaction with p75NTR and the co-receptor sortilin. The midlife-initiated 4-mo running program triggered multiple responses within the mouse brain cortex, through the activation of anti-amyloidogenic, pro-survival, synaptogenic and neuroprotective pathways. However, most of the beneficial actions of the exercise regimen appeared only after 4months, since 2-mo-exercised mice showed marked impairments of the endpoints we considered. This could imply that a midlife-initiated regimen of moderate treadmill running may require an adequate time lag to activate beneficial compensative mechanisms within the mouse brain cortex.

The proton-pump inhibitor lansoprazole enhances amyloid beta production

A key event in the pathogenesis of Alzheimer’s disease (AD) is the accumulation of amyloid-β (Aβ) species in the brain, derived from the sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. Based on a systems biology study to repurpose drugs for AD, we explore the effect of lansoprazole, and other proton-pump inhibitors (PPIs), on Aβ production in AD cellular and animal models. We found that lansoprazole enhances Aβ37, Aβ40 and Aβ42 production and lowers Aβ38 levels on amyloid cell models. Interestingly, acute lansoprazole treatment in wild type and AD transgenic mice promoted higher Aβ40 levels in brain, indicating that lansoprazole may also exacerbate Aβ production in vivo. Overall, our data presents for the first time that PPIs can affect amyloid metabolism, both in vitro and in vivo.

Alzheimer's disease

Over the last three decades, advances in biochemical pathology and human genetics have illuminated one of the most enigmatic subjects in biomedicine--neurodegeneration. Eponymic diseases of the nervous system such as Alzheimer's, Parkinson's, and Huntington's diseases that were long characterized by mechanistic ignorance have yielded striking progress in our understanding of their molecular underpinnings. A central theme in these and related disorders is the concept that certain normally soluble neuronal proteins can misfold and aggregate into oligomers and amyloid fibrils which can confer profound cytotoxicity. Perhaps the foremost example, both in terms of its societal impact and how far knowledge has moved toward the clinic, is that of Alzheimer's disease (AD). Here, we will review the classical protein lesions of the disorder that have provided a road map to etiology and pathogenesis. We will discuss how elucidating the genotype-to-phenotype relationships of familial forms of Alzheimer's disease has highlighted the importance of the misfolding and altered proteostasis of two otherwise soluble proteins, amyloid β-protein and tau, suggesting mechanism-based therapeutic targets that have led to clinical trials.

Enhanced ROS generation mediated by Alzheimer's disease presenilin regulation of InsP3R Ca2+ signaling

Familial Alzheimer's disease (FAD) is caused by mutations in amyloid precursor protein and presenilins (PS1, PS2). Many FAD-linked PS mutations affect intracellular calcium (Ca(2+)) homeostasis by proximal mechanisms independent of amyloid production by dramatically enhancing gating of the inositol trisphosphate receptor (InsP(3)R) intracellular Ca(2+) release channel by a gain-of-function effect that mirrors genetics of FAD and is independent of secretase activity. Electrophysiological recordings of InsP(3)R in FAD patient B cells, cortical neurons of asymptomatic PS1-AD mice, and other cells revealed they have higher occupancy in a high open probability burst mode, resulting in enhanced Ca(2+) signaling. Exaggerated Ca(2+) signaling through this mechanism results in enhanced generation of reactive oxygen species, believed to be an important component in AD pathogenesis. Exaggerated Ca(2+) signaling through InsP(3)R-PS interaction is a disease specific and robust proximal mechanism in AD that may contribute to the pathology of AD by enhanced generation of reactive oxygen species.

Detecting and analyzing differentially activated pathways in brain regions of Alzheimer's disease patients

Alzheimer's disease (AD) generally results in neuronal loss due to protein dysfunction in various brain regions. Genome-wide data have provided new opportunities to analyze the underlying mechanisms of AD. Here, we present a novel network-based systems biology framework to identify and analyze differentially activated pathways by integrating human protein-protein interaction data and gene expression profile data in six brain regions. Specifically, we propose a new scoring system by ranking the edges associated with AD. Then, an edge expansion algorithm is designed to identify the dysfunctional pathways implicated in AD pathogenesis in six brain regions respectively. The analyses of the similarities and differences of these dysfunctional pathways provide insights into understanding the dynamics of AD progression in six brain regions from a network perspective, which will further shed light on the pathogenesis of AD.

Pharmacogenetic regulation of acetylcholinesterase activity in Drosophila reveals the regulatory mechanisms of AChE inhibitors in synaptic plasticity

We conducted experiments in Drosophila to investigate the consequences of altered acetylcholinesterase (AChE) activity in the nervous system. In ace hypomorphic mutant larvae, the amount of ace mRNA and the activity of AChE both in vivo and in vitro were significantly reduced compared with those of controls. Reduced Ace in Drosophila larvae resulted in significant down-regulation of branch length and the number of boutons in Type 1 glutamatergic neuromuscular junctions (NMJs). These defects in ace hypomorphic mutant larvae were suppressed when Musca domestica AChE was transgenically expressed. Because AChE inhibitors are utilized for medications for Alzheimer's disease, we investigated whether pharmacological inhibition of AChE activity induced any synaptic defects. We found that controls exposed to a sublethal dose of DDVP phenocopied the synaptic structural defects of the ace hypomorphic mutant. These results suggest that down-regulation of AChE activity, regardless of whether it is due to genetic or pharmacological manipulations, results in altered synaptic architecture. Our study suggests that exposure to AChE inhibitors for 6-12 months may induce altered synaptic architectures in human brains with Alzheimer's diseases, similar to those reported here. These changes may underlie or contribute to the loss of efficacy of AChE inhibitors after prolonged treatment.

Interactome mapping suggests new mechanistic details underlying Alzheimer's disease

Recent advances toward the characterization of Alzheimer's disease (AD) have permitted the identification of a dozen of genetic risk factors, although many more remain undiscovered. In parallel, works in the field of network biology have shown a strong link between protein connectivity and disease. In this manuscript, we demonstrate that AD-related genes are indeed highly interconnected and, based on this observation, we set up an interaction discovery strategy to unveil novel AD causative and susceptibility genes. In total, we report 200 high-confidence protein-protein interactions between eight confirmed AD-related genes and 66 candidates. Of these, 31 are located in chromosomal regions containing susceptibility loci related to the etiology of late-onset AD, and 17 show dysregulated expression patterns in AD patients, which makes them very good candidates for further functional studies. Interestingly, we also identified four novel direct interactions among well-characterized AD causative/susceptibility genes (i.e., APP, A2M, APOE, PSEN1, and PSEN2), which support the suggested link between plaque formation and inflammatory processes and provide insights into the intracellular regulation of APP cleavage. Finally, we contextualize the discovered relationships, integrating them with all the interaction data reported in the literature, building the most complete interactome associated to AD. This general view facilitates the analyses of global properties of the network, such as its functional modularity, and triggers many hypotheses on the molecular mechanisms implicated in AD. For instance, our analyses suggest a putative role for PDCD4 as a neuronal death regulator and ECSIT as a molecular link between oxidative stress, inflammation, and mitochondrial dysfunction in AD.

Endothelial nitric oxide modulates expression and processing of amyloid precursor protein

RATIONALE

the exact etiology of sporadic Alzheimer disease (AD) is unclear, but it is interesting that several cardiovascular risk factors are associated with higher incidence of AD. The link between these risk factors and AD has yet to be identified; however, a common feature is endothelial dysfunction, specifically, decreased bioavailability of nitric oxide (NO).

OBJECTIVE

to determine the relationship between endothelial derived NO and the expression and processing of amyloid precursor protein (APP).

METHODS AND RESULTS

we used human brain microvascular endothelial cells to examine the role of NO in modulating APP expression and processing in vitro. Inhibition of endothelial nitric oxide synthase (eNOS) with the specific NOS inhibitor L-NAME (N(G)-nitro-l-arginine methyl ester) led to increased APP and BACE1 (β-site APP-cleaving enzyme1) protein levels, as well as increased secretion of the amyloidogenic peptide amyloid β (Aβ) (control 10.93 ± 0.70 pg/mL; L-NAME 168.21 ± 27.38 pg/mL; P<0.001). To examine the role of NO in modulation of APP expression and processing in vivo, we used brain and cerebral microvessels from eNOS-deficient (eNOS(-/-)) mice. Brain tissue from eNOS(-/-) mice had statistically higher APP and BACE1 protein levels, as well as increased BACE1 enzyme activity and Aβ (Aβ(1)(-)(42) wild-type control, 0.737 pg/mg; eNOS(-/-), 1.475 pg/mg; P<0.05), compared with wild-type controls (n=6 to 8 animals per background). Brain microvessels from eNOS(-/-) mice also showed statistically higher BACE1 protein levels as compared with wild-type control.

CONCLUSIONS

our data suggest that endothelial NO plays an important role in modulating APP expression and processing within the brain and cerebrovasculature. The NO/cGMP pathway may be an important therapeutic target in preventing and treating mild cognitive impairment, as well as AD.

Zebrafish as a tool in Alzheimer's disease research

Alzheimer's disease is the most prevalent form of neurodegenerative disease. Despite many years of intensive research our understanding of the molecular events leading to this pathology is far from complete. No effective treatments have been defined and questions surround the validity and utility of existing animal models. The zebrafish (and, in particular, its embryos) is a malleable and accessible model possessing a vertebrate neural structure and genome. Zebrafish genes orthologous to those mutated in human familial Alzheimer's disease have been defined. Work in zebrafish has permitted discovery of unique characteristics of these genes that would have been difficult to observe with other models. In this brief review we give an overview of Alzheimer's disease and transgenic animal models before examining the current contribution of zebrafish to this research area. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

CHOLESTEROL AND NEURONAL SUSCEPTIBILITY TO BETA-AMYLOID TOXICITY

Alzheimer's disease (AD) is a devastating neurocognitive disorder rapidly growing across the elderly population. Although few cases arise due to genetic mutations, sporadic AD is the most common form of this disease. Therefore, there is a continuing research effort to discover a unifying cause of this form of AD. To date, the only strong genetic correlate to the sporadic AD is inheritance of the apolipoprotein E4 (ApoE4) allele, whose encoded protein is involved in cholesterol transport in the central nervous system. This genetic link has prompted a series of studies on the potential molecular mechanisms by which cholesterol could modulate neuronal degeneration in the context of AD. In this review, we discussed the involvement of cholesterol in the production of the pathological hallmarks of the disease and how it might alter the susceptibility of cells to AD-related insult. Finally, we discussed the use of cholesterol-lowering drugs as a potential preventative approach in AD.

Aph-1 associates directly with full-length and C-terminal fragments of gamma-secretase substrates

γ-Secretase is a ubiquitous, multiprotein enzyme composed of presenilin, nicastrin, Aph-1, and Pen-2. It mediates the intramembrane proteolysis of many type 1 proteins, plays an essential role in numerous signaling pathways, and helps drive the pathogenesis of Alzheimer disease by excising the hydrophobic, aggregation-prone amyloid β-peptide from the β-amyloid precursor protein. A central unresolved question is how its many substrates bind and enter the γ-secretase complex. Here, we provide evidence that both the β-amyloid precursor protein holoprotein and its C-terminal fragments, the immediate substrates of γ-secretase, can associate with Aph-1 at overexpressed as well as endogenous protein levels. This association was observed using bi-directional co-immunoprecipitation in multiple systems and detergent conditions, and an β-amyloid precursor protein-Aph-1 complex was specifically isolated following in situ cross-linking in living cells. In addition, another endogenous canonical γ-substrate, Jagged, showed association of both its full-length and C-terminal fragment forms with Aph-1. We were also able to demonstrate that this interaction with substrates was conserved across the multiple isoforms of Aph-1 (β, αS, and αL), as they were all able to bind β-amyloid precursor protein with similar affinity. Finally, two highly conserved intramembrane histidines (His-171 and His-197) within Aph-1, which were recently shown to be important for γ-secretase activity, are required for efficient binding of substrates. Taken together, our data suggest a dominant role for Aph-1 in interacting with γ-secretase substrates prior to their processing by the proteolytic complex.

Bioluminescence imaging reveals inhibition of tumor cell proliferation by Alzheimer's amyloid beta protein

Background

Cancer and Alzheimer's disease (AD) are two seemingly distinct diseases and rarely occur simultaneously in patients. To explore molecular determinants differentiating pathogenic routes towards AD or cancer, we investigate the role of amyloid β protein (Aβ) on multiple tumor cell lines that are stably expressing luciferase (human glioblastoma U87; human breast adenocarcinoma MDA-MB231; and mouse melanoma B16F).

Results

Quantification of the photons emitted from the MDA-MB231 or B16F cells revealed a significant inhibition of cell proliferation by the conditioning media (CM) derived from amyloid precursor protein (APP) over-expressing cells. The inhibition of U87 cells was observed only after the media was conditioned for longer than 2 days with APP over-expressing cells.

Conclusion

Our results suggest that Aβ plays an inhibitory role in tumor cell proliferation; this effect could depend on the type of tumor cells and amount of Aβ.

Intracellular trafficking of presenilin 1 is regulated by beta-amyloid precursor protein and phospholipase D1

Excessive accumulation of beta-amyloid peptides in the brain is a major cause for the pathogenesis of Alzheimer disease. beta-Amyloid is derived from beta-amyloid precursor protein (APP) through sequential cleavages by beta- and gamma-secretases, whose enzymatic activities are tightly controlled by subcellular localization. Delineation of how intracellular trafficking of these secretases and APP is regulated is important for understanding Alzheimer disease pathogenesis. Although APP trafficking is regulated by multiple factors including presenilin 1 (PS1), a major component of the gamma-secretase complex, and phospholipase D1 (PLD1), a phospholipid-modifying enzyme, regulation of intracellular trafficking of PS1/gamma-secretase and beta-secretase is less clear. Here we demonstrate that APP can reciprocally regulate PS1 trafficking; APP deficiency results in faster transport of PS1 from the trans-Golgi network to the cell surface and increased steady state levels of PS1 at the cell surface, which can be reversed by restoring APP levels. Restoration of APP in APP-deficient cells also reduces steady state levels of other gamma-secretase components (nicastrin, APH-1, and PEN-2) and the cleavage of Notch by PS1/gamma-secretase that is more highly correlated with cell surface levels of PS1 than with APP overexpression levels, supporting the notion that Notch is mainly cleaved at the cell surface. In contrast, intracellular trafficking of beta-secretase (BACE1) is not regulated by APP. Moreover, we find that PLD1 also regulates PS1 trafficking and that PLD1 overexpression promotes cell surface accumulation of PS1 in an APP-independent manner. Our results clearly elucidate a physiological function of APP in regulating protein trafficking and suggest that intracellular trafficking of PS1/gamma-secretase is regulated by multiple factors, including APP and PLD1.

Amyloidogenic metal-binding proteins: new investigative pathways

Neurodegenerative diseases remain perplexing and problematic for modern research. Those associated with amyloidogenic proteins have often been lumped together simply because those proteins aggregate. However, research has identified a more logical reason to group some of these diseases together. The associated proteins not only aggregate, but also bind copper. The APP (amyloid precursor protein) binds copper in an N-terminal region. Binding of copper has been suggested to influence generation of beta-amyloid from the protein. PrP (prion protein) binds copper, and this appears to be necessary for its normal function and might also reduce its probability of conversion into an infectious prion. alpha-Synuclein, a protein associated with Parkinson's disease, also binds copper, but, in this case, it potentially increases the rate at which the protein aggregates. The similarities between these proteins, in terms of metal binding, has allowed us to investigate them using similar approaches. In the present review, we discuss some of these approaches.

Quantification of gamma-secretase modulation differentiates inhibitor compound selectivity between two substrates Notch and amyloid precursor protein

Background

Deposition of amyloid-β protein (Aβ) is a major pathological hallmark of Alzheimer's disease (AD). Aβ is generated from γ-secretase cleavage of amyloid precursor protein (APP). In addition to APP, γ-secretase also cleaves other type I integral membrane proteins, including the Notch receptor, a key molecule involved in embryonic development.

Results

To explore selective γ-secretase inhibitors, a combination of five methods was used to systematically determine these inhibitors' profiles on the γ-secretase cleavage of APP and Notch. When two potent γ-secretase inhibitors, compound E (cpd E) and DAPT, were used in a conventional in vitro γ-secretase activity assay, cpd E completely blocked Aβ generation from the cleavage of substrate APP C100, but only had a minor effect on Notch cleavage and NICD generation. Next, cpd E and DAPT were applied to HEK293 cells expressing a truncated Notch substrate NotchΔE. Both cpd E and DAPT were more potent in blocking Aβ generation than NICD generation. Third, a reporter construct was created that carried the NICD targeting promoter with three Su(H) binding sequences followed by the luciferase gene. We found that the inhibition of NICD generation by cpd E and DAPT was consistent with the reduced expression of luciferase gene driven by this Notch targeting promoter. Fourth, levels of "Notch-Aβ-like" (Nβ*) peptide derived from two previously reported chimeric APP with its transmembrane domain or the juxtamembrane portion replaced by the Notch sequence were quantified. Measurement of Nβ* peptides by ELISA confirmed that EC₅₀'s of cpd E were much higher for Nβ* than Aβ. Finally, the expression levels of Notch target gene her6 in cpd E or DAPT-treated zebrafish were correlated with the degree of tail curvature due to defective somitogenesis, a well characterized Notch phenotype in zebrafish.

Conclusion

Our ELISA-based quantification of Aβ and Nβ* in combination with the test in zebrafish provides a novel approach for efficient cell-based screening and in vivo validation of APP selective γ-secretase inhibitors.

Molecular genetics of Alzheimer's disease: an update

Alzheimer's disease (AD) is a complex disorder of the central nervous system (CNS). Molecular genetic research has provided a wealth of information regarding the genetic etiology of this devastating disease. Identification and functional characterization of autosomal dominant mutations in the amyloid precursor protein gene (APP) and the presenilin genes 1 and 2 (PSEN1 and PSEN2) have contributed substantially to our understanding of the biological mechanisms leading towards CNS neurodegeneration in AD. Nonetheless, a large part of the genetic etiology remains unresolved, especially that of more common, sporadic forms of AD. While substantial efforts were invested in the identification of genetic risk factors underlying sporadic AD, using carefully designed genetic association studies in large patient-control groups, the only firmly established risk factor remains the epsilon4 allele of the apolipoprotein E gene (APOE). Nevertheless, one can expect that with the current availability of high-throughput genotyping platforms and dense maps of single-nucleotide polymorphisms (SNPs), large-scale genetic studies will eventually generate additional knowledge about the genetic risk profile for AD. This review provides an overview of the current understanding in the field of AD genetics, covering both the rare monogenic forms as well as recent developments in the search for novel AD susceptibility genes.

An update on the amyloid hypothesis

Alzheimer's disease (AD) is a devastating neurodegenerative disease. To rationally develop novel therapeutic and/or preventative agents for AD, an understanding of the etiology and pathogenesis of this complex disease is necessary. This article examines the evidence for the amyloid hypothesis of AD pathogenesis and discusses how it relates to the neurological and neuropathological features of AD, the known genetic risk factors and causative mutations, and the heightened risk associated with advanced age.

The genetics of Alzheimer's disease

The genetics of Alzheimer's disease is produced by 3 essentially interactive gene groups: (1) APP and presenilin 1 and 2; (2) APOE E2, E3, and E4; (3) genes on chromosomes 9, 10, 12, etc. If any gene in (1) mutates, beta amyloid (Abeta) increases sharply beyond what the genes of (3) can remove, with early-onset Alzheimer's disease the result. With retention of Abeta by E4 in (2), Alzheimer's disease might result even though (1) and (3) are normal. If any gene in (3) mutates, the level of Abeta will rise, but because many genes are involved in Abeta removal, late-onset Alzheimer's disease would be detected only eventually.

N-glycosylation of human nicastrin is required for interaction with the lectins from the secretory pathway calnexin and ERGIC-53

The gamma-secretase complex, composed of four non-covalently bound transmembrane proteins Presenilin, Nicastrin (NCT), APH-1 and PEN-2, is responsible for the intramembranous cleavage of amyloid precursor protein (APP), Notch and several other type I transmembrane proteins. gamma-Secretase cleavage of APP releases the Abeta peptides, which form the amyloid plaques characteristic of Alzheimer's disease brains, and cleavage of Notch releases an intracellular signalling peptide that is critical for numerous developmental processes. NCT, a type I membrane protein, is the only protein within the complex that is glycosylated. The importance of these glycosylation sites is not fully understood. Here, we have observed that NCT N-linked oligosaccharides mediated specific interactions with the secretory pathway lectins calnexin and ERGIC-53. In order to investigate the role played by N-glycosylation, mutation of each site was performed. All hNCT mutants interacted with calnexin and ERGIC-53, indicating that the association was not mediated by any single N-glycosylation site. Moreover, the interaction with ERGIC-53 still occurred in PS1/2 double knockout cells as detected in immunoprecipitation as well as confocal immunofluorescence microscopy studies, which indicated that NCT interacted with ERGIC-53 prior to its association with the active gamma-secretase complex.

Axonal Transport and Alzheimer's Disease

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

Reduction of iron-regulated amyloid precursor protein and beta-amyloid peptide by (-)-epigallocatechin-3-gallate in cell cultures: implications for iron chelation in Alzheimer's disease

Brain iron dysregulation and its association with amyloid precursor protein (APP) plaque formation are implicated in Alzheimer's disease (AD) pathology and so iron chelation could be considered a rational therapeutic strategy for AD. Here we analyzed the effect of the main polyphenol constituent of green tea, (-)-epigallocatechin-3-gallate (EGCG), which possesses metal-chelating and radical-scavenging properties, on the regulation of the iron metabolism-related proteins APP and transferrin receptor (TfR). EGCG exhibited potent iron-chelating activity comparable to that of the prototype iron chelator desferrioxamine, and dose dependently (1-10 microm) increased TfR protein and mRNA levels in human SH-SY5Y neuroblastoma cells. Both the immature and full-length cellular holo-APP were significantly reduced by EGCG, as shown by two-dimensional gel electrophoresis, without altering APP mRNA levels, suggesting a post-transcriptional action. Indeed, EGCG suppressed the translation of a luciferase reporter gene fused to the APP mRNA 5'-untranslated region, encompassing the APP iron-responsive element. The finding that Fe(2)SO(4) reversed the action of EGCG on APP and TfR proteins reinforces the likelihood that these effects are mediated through modulation of the intracellular iron pool. Furthermore, EGCG reduced toxic beta-amyloid peptide generation in Chinese hamster ovary cells overexpressing the APP 'Swedish' mutation. Thus, the natural non-toxic brain-permeable EGCG may provide a potential therapeutic approach for AD and other iron-associated disorders.

Presenilin-1-mediated Retention of APP Derivatives in Early Biosynthetic Compartments

Processing of the amyloid precursor protein (APP) leads to the production of amyloid-beta (Abeta), the major component of extracellular plaques in the brains of Alzheimer's disease (AD) patients. Presenilin-1 (PS-1) plays a key role in the final step of Abeta formation, the gamma-secretase cleavage. Previously, we showed that PS-1 is retained in pre-Golgi compartments by incorporation into COPI-coated membranes of the vesicular tubular clusters (VTCs) between endoplasmic reticulum (ER) and Golgi complex. Here, we show that PS-1 also mediates the retention of the beta-cleavage-derived APP-C-terminal fragment (CTFbeta) and/or Abeta in pre-Golgi membranes. Overexpression of PS-1 increased the percentage of CTFbeta and/or Abeta in VTCs as well as their distribution to COPI-coated VTC membranes. By contrast, overexpression of the dominant-negative aspartate mutant PS-1(D257A) or PS-knockout decreased incorporation of these APP derivatives into COPI-coated membranes. Sorting of APP derivatives to COPI-coated VTC membranes was not depending on the APP cytosolic tail. In post-Golgi compartments, PS-1 expression enhanced the association of full-length APP/APPs with endosomal compartments at the expense of plasma membrane-bound APP. We conclude that PS-1, in addition to its role in gamma-secretase cleavage, is also required for the subcellular routing of APP and its derivatives. Malfunctioning of PS-1 in this role may have important consequences for the progress of AD.

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.

Age-related changes of intracellular Abeta in cynomolgus monkey brains

To confirm the intracellular accumulation of amyloid beta-protein (Abeta), we carefully performed immunohistochemistry using brains of cynomolgus monkeys of various ages. Cortical neurones and their large neurites were immunostained with antibodies against Abeta in young monkey brains. In aged monkey brains, intracellular Abeta localized within cortical neurones; no clear association was found between the presence of intracellular Abeta and senile plaques (SPs). Interestingly, we did not observe Abeta-immunoreactive cortical neurones in brains fixed with neutral buffered formalin. Western blot analyses of microsomal and nerve ending fractions derived from the brains of young to aged monkeys revealed that intracellular Abeta generation changed with age. In the microsomal fraction, the amount of Abeta42 significantly increased in brains from older monkeys (>30 years of age), and the amount of Abeta43 significantly decreased with age in the microsomal fraction. The amount of Abeta40 remained the same regardless of age. Biochemical analyses also showed that intracellular levels of each of these Abeta molecules significantly increased with age in nerve ending fractions. As we previously observed that a similar accumulation of presenilin1, beta-amyloid precursor protein (APP) and APP C-terminal fragment cleaved by beta-secretase in the nerve ending fractions obtained from brains with SPs, the accumulation of intracellular Abeta in this fraction may be closely related to formation of spontaneous SPs with age. Taken together, these results suggest that intensive investigation of age-related changes in the nerve ending will contribute to a better understanding of the pathogenesis of age-related neurodegenerative disorders such as sporadic Alzheimer's disease.

Presenilin-dependent gamma-secretase processing regulates multiple ERBB4/HER4 activities

Transmembrane receptors typically transmit cellular signals following growth factor stimulation by coupling to and activating downstream signaling cascades. Reports of proteolytic processing of cell surface receptors to release an intracellular domain (ICD) has raised the possibility of novel signaling mechanisms directly mediated by the receptor ICD. The receptor tyrosine kinase ERBB4/HER4 (referred to here as ERBB4) undergoes sequential processing by tumor necrosis factor-alpha converting enzyme and presenilin-dependent gamma-secretase to release the ERBB4 ICD (4ICD). Our recent data suggests that regulation of gene expression by the ERBB4 nuclear protein and the proapoptotic activity of ERBB4 involves the gamma-secretase release of 4ICD. To determine the role gamma-secretase processing plays in ERBB4 signaling, we generated an ERBB4 allele with the transmembrane residue substitution V673I (ERBB4-V673I). We demonstrate that ERBB4-V673I fails to undergo processing by gamma-secretase but retains normal cell surface signaling activity. In contrast to wild-type ERBB4, however, ERBB4-V673I was excluded from the nuclei of transfected cells and failed to activate STAT5A stimulation of the beta-casein promoter. These results support the contention that gamma-secretase processing of ERBB4 is necessary to release a functional 4ICD nuclear protein which directly regulates gene expression. We also demonstrate that 4ICD failed to accumulate within mitochondria of ERBB4-V673I transfected cells and the potent proapoptotic activity of ERBB4 was completely abolished in cells expressing ERBB4-V673I. Our results provide the first formal demonstration that proteolytic processing of ERBB4 is a critical event regulating multiple receptor signaling activities.

PAR-4 is involved in regulation of beta-secretase cleavage of the Alzheimer amyloid precursor protein

Mounting evidence indicates that aberrant production and aggregation of amyloid beta-peptide (Abeta)-(1-42) play a central role in the pathogenesis of Alzheimer disease (AD). Abeta is produced when amyloid precursor protein (APP) is cleaved by beta- and gamma-secretases at the N and C termini of the Abeta domain, respectively. The beta-secretase is membrane-bound aspartyl protease, most commonly known as BACE1. Because BACE1 cleaves APP at the N terminus of the Abeta domain, it catalyzes the first step in Abeta generation. PAR-4 (prostate apoptosis response-4) is a leucine zipper protein that was initially identified to be associated with neuronal degeneration and aberrant Abeta production in models of AD. We now report that the C-terminal domain of PAR-4 is necessary for forming a complex with the cytosolic tail of BACE1 in co-immunoprecipitation assays and in vitro pull-down experiments. Overexpression of PAR-4 significantly increased, whereas silencing of PAR-4 expression by RNA interference significantly decreased, beta-secretase cleavage of APP. These results suggest that PAR-4 may be directly involved in regulating the APP cleavage activity of BACE1. Because the increased BACE1 activity observed in AD patients does not seem to arise from genetic mutations or polymorphisms in BACE1, the identification of PAR-4 as an endogenous regulator of BACE1 activity may have significant implications for developing novel therapeutic strategies for AD.

Uncoupled packaging of amyloid precursor protein and presenilin 1 into coat protein complex II vesicles

Mutant forms of presenilin (PS) 1 and 2 and amyloid precursor protein (APP) lead to familial Alzheimer's disease. Several reports indicate that PS may modulate APP export from the endoplasmic reticulum (ER). To develop a test of this possibility, we reconstituted the capture of APP and PS1 in COPII (coat protein complex II) vesicles formed from ER membranes in permeabilized cultured cells. The recombinant forms of mammalian COPII proteins were active in a reaction that measures coat subunit assembly and coated vesicle budding on chemically defined synthetic liposomes. However, the recombinant COPII proteins were not active in cargo capture and vesicle budding from microsomal membranes. In contrast, rat liver cytosol was active in stimulating the sorting and packaging of APP, PS1, and p58 (an itinerant ER to Golgi marker protein) into transport vesicles from donor ER membranes. Budding was stimulated in dilute cytosol by the addition of recombinant COPII proteins. Fractionation of the cytosol suggested one or more additional proteins other than the COPII subunits may be essential for cargo selection or vesicle formation from the mammalian ER membrane. The recombinant Sec24C specifically recognized the APP C-terminal region for packaging. Titration of Sarla distinguished the packaging requirements of APP and PS1. Furthermore, APP packaging was not affected by deletion of PS1 or PS1 and 2, suggesting APP and PS1 trafficking from the ER are normally uncoupled.

The Enantiomer of Cholesterol

Cholesterol plays a variety of significant roles in biological systems. However, the mechanisms by which cholesterol functions remain largely unclear. The enantiomer of cholesterol (ent-cholesterol)--which has identical physical properties, but opposite three-dimensional configuration compared to cholesterol--is a unique tool that can be used to better understand the mechanisms of cholesterol function. We review the literature pertaining to ent-cholesterol, focusing in particular on its use in biological studies.

Subcellular analysis of aberrant protein structure in age-related neurodegenerative disorders

Subcellular interactions of neurodegenerative disease proteins may provide a basic molecular mechanism underlying neuronal disorders. Each protein may also exhibit activities related to normal cell structure and function. It may be necessary to develop methodologies to distinguish between normal and abnormal intracellular interactions of such proteins in human cells. The latter would result in distinct perturbations in cell function depending both on the specific protein or nucleic acid interactions as well as its subcellular localization. Individual neurodegenerative disorders may be characterized by distinct alterations in subcellular neuronal protein structure and function. We developed as a basic experimental paradigm a novel human cell system to identify and examine such abnormal neuronal protein structures. The basic rationale is that neurodegenerative protein interactions would result in the formation of intracellular high molecular weight (HMW) complexes in cells from afflicted individuals. Following cell fractionation these unique structures could be detected by gradient sedimentation coupled with immunoblot analysis. They would not be observed in age matched control normal human cells. We now report that this procedure has been successfully used to determine a unique subcellular alteration of beta-amyloid precursor protein (beta-APP) structure in Alzheimer's disease (AD) cells. The latter was not observed in normal cells. Similar structural alterations were observed for glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a protein known to bind to beta-APP in vitro. The utility of this model system to interrelate aberrant protein interactions of neurodegenerative disease proteins and their subcellular localization is considered.

Amyloid beta peptide load is correlated with increased beta-secretase activity in sporadic Alzheimer's disease patients

Whether elevated beta-secretase (BACE) activity is related to plaque formation or amyloid beta peptide (Abeta) production in Alzheimer's disease (AD) brains remains inconclusive. Here, we report that we used sandwich enzyme-linked immunoabsorbent assay to quantitate various Abeta species in the frontal cortex of AD brains homogenized in 70% formic acid. We found that most of the Abeta species detected in rapidly autopsied brains (<3 h) with sporadic AD were Abeta(1-x) and Abeta(1-42), as well as Abeta(x-42). To establish a linkage between Abeta levels and BACE, we examined BACE protein, mRNA expression and enzymatic activity in the same brain region of AD brains. We found that both BACE mRNA and protein expression is elevated in vivo in the frontal cortex. The elevation of BACE enzymatic activity in AD is correlated with brain Abeta(1-x) and Abeta(1-42) production. To examine whether BACE elevation was due to mutations in the BACE-coding region, we sequenced the entire ORF region of the BACE gene in these same AD and nondemented patients and performed allelic association analysis. We found no mutations in the ORF of the BACE gene. Moreover, we found few changes of BACE protein and mRNA levels in Swedish mutated amyloid precursor protein-transfected cells. These findings demonstrate correlation between Abeta loads and BACE elevation and also suggest that as a consequence, BACE elevation may lead to increased Abeta production and enhanced deposition of amyloid plaques in sporadic AD patients.

X11α impairs γ- but not β-cleavage of amyloid precursor protein

The phosphotyrosine binding domain of the neuronal protein X11alpha/mint-1 binds to the C-terminus of amyloid precursor protein (APP) and inhibits catabolism to beta-amyloid (Abeta), but the mechanism of this effect is unclear. Coexpression of X11alpha or its PTB domain with APPswe inhibited secretion of Abeta40 but not APPsbetaswe, suggesting inhibition of gamma- but not beta-secretase. To further probe cleavage(s) inhibited by X11alpha, we coexpressed beta-secretase (BACE-1) or a component of the gamma-secretase complex (PS-1Delta9) with APP, APPswe, or C99, with and without X11alpha, in HEK293 cells. X11alpha suppressed the PS-1Delta9-induced increase in Abeta42 secretion generated from APPswe or C99. However, X11alpha did not impair BACE-1-mediated proteolysis of APP or APPswe to C99. In contrast to impaired gamma-cleavage of APPswe, X11alpha or its PTB domain did not inhibit gamma-cleavage of NotchDeltaE to NICD (the Notch intracellular domain). The X11alpha PDZ-PS.1Delta9 interaction did not affect gamma-cleavage activity. In a cell-free system, X11alpha did not inhibit the catabolism of APP C-terminal fragments. These data suggest that X11alpha may inhibit Abeta secretion from APP by impairing its trafficking to sites of active gamma-secretase complexes. By specifically targeting substrate instead of enzyme X11alpha may function as a relatively specific gamma-secretase inhibitor.

Metabolic cholesterol depletion hinders cell-surface trafficking of the nicotinic acetylcholine receptor

The effects of metabolic inhibition of cholesterol biosynthesis on the trafficking of the nicotinic acetylcholine receptor (AChR) to the cell membrane were studied in living CHO-K1/A5, a Chinese hamster ovary clonal line that heterologously expresses adult alpha2betadeltaepsilon mouse AChR. To this end, we submitted CHO-K1/A5 cells to long-term cholesterol deprivation, elicited by Mevinolin, a potent inhibitor of 3-hydroxy-3-methyl-glutaryl-CoA reductase and applied a combination of biochemical, pharmacological and fluorescence microscopy techniques to follow the fate of the AChR. When CHO-K1/A5 cells were grown for 48 h in lipid-deficient medium supplemented with 0.5 microM Mevinolin, total cholesterol was significantly reduced (40%). Concomitantly, the maximum number of binding sites (Bmax) of the cell-surface AChR for the competitive antagonist alpha-bungarotoxin was reduced from 647+/-30 to 352+/-34 fmol/mg protein, i.e. by 46%. The apparent dissociation constant (Kdapp) for alpha-bungarotoxin of the AChRs remaining at the cell surface was not modified by cholesterol depletion. Similarly, the half-concentration inhibiting the specific binding of the radioligand (IC50) for another competitive antagonist, d-tubocurarine, did not differ from that in control cells. The decrease in cell-surface AChR was paralleled by an increase in intracellular AChR levels, which rose from 44+/-2.1% in control cells to 74+/-3.3% in Mevinolin-treated cells. When analyzed by wide-field fluorescence microscopy, the fluorescence signal arising from alpha-bungarotoxin labeled cell-surface AChRs was reduced by approximately 70% in Mevinolin-treated cells. The distribution of intracellular AChR also changed: Alexa594-alpha-bungarotoxin-labeled AChR exhibited a highly compartmentalized pattern, concentrating at the perinuclear and Golgi-like regions. Temperature-arrest of protein trafficking magnified this effect, emphasizing the Golgi localization of the AChR. Colocalization studies using the transiently expressed fluorescent trans-Golgi/trans-Golgi network marker pEYFP/human beta1,4-galactosyltransferase and the trans-Golgi network marker syntaxin 6 provided additional support for the Golgi localization of intracellular AChRs. The low AChR cell-surface expression and the increase in intracellular AChR pools in cholesterol-depleted cells raise the possibility that cholesterol participates in the trafficking of the receptor protein to the plasmalemma and its stability at this surface location.

Age-related changes of Alzheimer's disease-associated proteins in cynomolgus monkey brains

We characterized senile plaques (SPs) immunohistochemically in cynomolgus monkey brains and also examined age-related biochemical changes of Alzheimer's disease (AD)-associated proteins in these brains from monkeys of various ages. In the neocortex of aged monkeys (>20 years old), we found SPs but no neurofibrillary tangles (NFTs). Antibodies against beta-amyloid precursor protein (APP) or apolipoprotein E (ApoE) stained SPs; however, the pattern of immunostaining was different for the two antigens. APP was present only in swollen neurites, but ApoE was present throughout all parts of SPs. Western blot analysis revealed that the pattern of APP expression changed with age. Although full-length APP695 protein was mainly expressed in brains from young monkeys (4-years-old), the expression of full-length APP751 protein was increased in brains from older monkeys (>20 years old). Biochemical analyses also showed that levels of various AD-associated proteins increased significantly with age in nerve ending fractions. Both SP-associated (APP) and NFT-associated proteins (tau, activated glycogen synthase kinase 3beta, cyclin dependent kinase 5, p35, and p25) accumulated in the nerve ending fraction with increasing age; however, we found no NFTs or paired helical filaments of tau in aged cynomolgus monkey brains. This age-related accumulation of these proteins in the nerve ending fraction was similar to that observed in our laboratory previously for presenilin-1 (PS-1). The accumulation of these SP-associated proteins in this fraction may be a causal event in the spontaneous formation of SPs; thus, SPs may be formed initially in nerve endings. Taken together, these results suggest that intensive investigation of age-related changes in the nerve ending and in axonal transport will contribute to a better understanding of the pathogenesis of neurodegenerative disorders such as AD.