Metabolism of Alzheimer beta-amyloid precursor protein: regulation by protein kinase A in intact cells and in a cell-free system

APP-Mediated Signaling Prevents Memory Decline in Alzheimer's Disease Mouse Model

Amyloid precursor protein (APP) and its metabolites play key roles in Alzheimer's disease (AD) pathophysiology. Whereas short amyloid-β (Aβ) peptides derived from APP are pathogenic, the APP holoprotein serves multiple purposes in the nervous system through its cell adhesion and receptor-like properties. Our studies focused on the signaling mediated by the APP cytoplasmic tail. We investigated whether sustained APP signaling during brain development might favor neuronal plasticity and memory process through a direct interaction with the heterotrimeric G-protein subunit Gα_S (stimulatory G-protein alpha subunit). Our results reveal that APP possesses autonomous regulatory capacity within its intracellular domain that promotes APP cell surface residence, precludes Aβ production, facilitates axodendritic development, and preserves cellular substrates of memory. Altogether, these events contribute to strengthening cognitive functions and are sufficient to modify the course of AD pathology.

Sexual dimorphism in predisposition to Alzheimer's disease

Clinical studies indicate that Alzheimer's disease (AD) disproportionately affects women in both disease prevalence and rate of symptom progression, but the mechanisms underlying this sexual divergence are unknown. Although some have suggested this difference in risk is a reflection of the known differences in longevity between men and women, mounting clinical and preclinical evidence supports women also having intrinsic susceptibilities toward the disease. Although a number of potential risk factors have been hypothesized to mediate these differences, none have been definitively verified. In this review, we first summarize the epidemiologic studies of prevalence and incidence of AD among the sexes. Next, we discuss the most likely risk factors to date that interact with biological sex, including (1) genetic factors, (2) sex hormones (3) deviations in brain structure, (4) inflammation and microglia, and (5) and psychosocial stress responses. Overall, though differences in life span are likely to account for part of the divide between the sexes in AD prevalence, the abundance of preclinical and clinical evidence presented here suggests an increase in intrinsic AD risk for women. Therefore, future studies focusing on the underlying biological mechanisms for this phenomenon are needed to better understand AD pathogenesis in both sexes, with the eventual goal of sex-specific prevention and treatment strategies.

Sex differences in chronic stress responses and Alzheimer's disease

Clinical studies indicate that Alzheimer's disease (AD) disproportionately affects women in both disease prevalence and severity, but the mechanisms underlying this sex divergence are unknown. Though some have suggested this difference in risk is a reflection of known differences in longevity between men and women, mounting clinical and preclinical evidence supports women also having intrinsic susceptibilities towards the disease. While a number of potential risk factors have been hypothesized to affect these differences in risks, none have been definitively verified. In this review, we discuss a novel hypothesis whereby women's susceptibility to chronic stress also mediates increased risk for AD. As stress is a risk factor for AD, and women are twice as likely to develop mood disorders where stress is a major etiology, it is possible that sex dimorphisms in stress responses contribute to the increase in women with AD. In line with this, sex divergence in biochemical responses to stress have been noted along the hypothalamic-pituitary-adrenal (HPA) axis and among known molecular effectors of AD, with crosstalk between these processes also being likely. In addition, activation of the cortical corticotrophin-releasing factor receptor 1 (CRF1) signaling pathway leads to distinct female-biased increases in molecules associated with AD pathogenesis. Therefore, the different biochemical responses to stress between women and men may represent an intrinsic, sex-dependent risk factor for AD.

Effects of corticotrophin-releasing factor receptor 1 antagonists on amyloid-β and behavior in Tg2576 mice

RATIONALE

Previous studies indicate that psychosocial stressors could accelerate amyloid-β (Aβ) levels and accelerate plaque deposition in mouse models of Alzheimer disease (AD). Stressors enhanced the release of corticotrophin-releasing factor (CRF), and exogenous CRF administration mimicked the effects of stress on Aβ levels in mouse models of AD. However, whether CRF receptor 1 (CRF1) antagonists could influence the stress-induced acceleration of an AD-like process in mouse models has not been well studied.

OBJECTIVE

We sought to examine whether CRF1 antagonists inhibit the effects of isolation stress on tissue Aβ levels, Aβ plaque deposition, and behaviors related to anxiety and memory in Tg2576 mice, and to investigate the molecular mechanism underlying such effects.

METHODS

Cohorts of Tg2576 mouse pups were isolated or group-housed at 21 days of age, and then the subgroups of these cohorts received daily intraperitoneal injections of the CRF1 antagonists, antalarmin or R121919 (5, 10, and 20 mg/kg), or vehicle for 1 week. Other cohorts of Tg2576 mouse pups were isolated or group-housed at 21 days of age, and then at 4 months of age, subgroups of these mice were administered antalarmin (20 mg/kg) or vehicle in their drinking water for 6 months. Finally, cultured primary hippocampal neurons from regular Tg2576 pups (P0) were incubated with CRF (0.1, 1, and 10 nM), antalarmin (100 nM) or H-89 (1 μM) for 48 h. Brain tissues or cultured neurons were collected for histological and biochemical analyses, and behavioral measures were collected in the cohorts of mice that were chronically stressed.

RESULTS

Administration of antalarmin at 20 mg/kg dose for 1 week significantly reduced Aβ1-42 levels in isolation stressed mice. Administration of antalarmin for 6 months significantly decreased plasma corticosterone levels, tissue Aβ1-42 levels, and Aβ plaque deposition in the brain and blocked the effects of isolation stress on behaviors related to anxiety and memory. Finally, incubation of neurons with 100 nM antalarmin inhibited the ability of 10 nM CRF to increase Aβ1-42 levels and protein kinase A IIβ expression. The effect of CRF1 on Aβ1-42 levels was also diminished by treatment with H-89, a c-AMP/PKA inhibitor.

CONCLUSIONS

These results suggest that CRF1 antagonists can slow an AD-like process in Tg2576 mice and that the c-AMP/PKA signaling pathway may be involved in this effect.

Phosphodiesterase III inhibitor promotes drainage of cerebrovascular β-amyloid

OBJECTIVE

Brain amyloidosis is a key feature of Alzheimer's disease (AD). It also incorporates cerebrovascular amyloid β (Aβ) in the form of cerebral amyloid angiopathy (CAA) involving neurovascular dysfunction. We have recently shown by retrospective analysis that patients with mild cognitive impairment receiving a vasoactive drug cilostazol, a selective inhibitor of phosphodiesterase (PDE) III, exhibit significantly reduced cognitive decline. Here, we tested whether cilostazol protects against the disruption of the neurovascular unit and facilitates the arterial pulsation-driven perivascular drainage of Aβ in AD/CAA.

METHODS

We explored the expression of PDE III in postmortem human brain tissue followed by a series of experiments examining the effects of cilostazol on Aβ metabolism in transgenic mice (Tg-SwDI mice) as a model of cerebrovascular β-amyloidosis, as well as cultured neurons.

RESULTS

We established that PDE III is abnormally upregulated in cerebral blood vessels of AD and CAA subjects and closely correlates with vascular amyloid burden. Furthermore, we demonstrated that cilostazol treatment maintained cerebral hyperemic and vasodilative responses to hypercapnia and acetylcholine, suppressed degeneration of pericytes and vascular smooth muscle cells, promoted perivascular drainage of soluble fluorescent Aβ1-40, and rescued cognitive deficits in Tg-SwDI mice. Although cilostazol decreased endogenous Aβ production in cultured neurons, C-terminal fragment of amyloid precursor protein expression was not altered in cilostazol-treated Tg-SwDI mice.

INTERPRETATION

The predominant action of cilostazol on Aβ metabolism is likely to facilitate Aβ clearance due to the sustained cerebrovascular function in vivo. Our findings mechanistically demonstrate that cilostazol is a promising therapeutic approach for AD and CAA.

Cyclic adenosine monophosphate as an endogenous modulator of the amyloid-β precursor protein metabolism

Besides playing a pathogenic role in Alzheimer disease, amyloid-beta peptides are normally produced in low amounts in the brain, and several lines of evidence suggest that they can modulate synaptic plasticity and memory. As cyclic adenosine monophosphate (cAMP) is known to be involved in the same processes and the blockade of its degradation by phosphodiesterase 4 inhibitors has consistently shown beneficial effects on cognition, we investigated the possible correlation between this second messenger and Aβ peptides in neuronal N2a cells overexpressing the amyloid-β precursor protein (APP). We herein report that the elevation of endogenous cAMP by rolipram increased APP protein expression and both its amyloidogenic and nonamyloidogenic processing. The effects of rolipram were reproduced by both the cAMP membrane-permeant analog 8Br-cAMP and the forskolin-induced activation of adenylyl cyclase but were not affected by the PKA inhibitor H-89. Our results demonstrate that, in neuronal cells, APP metabolism is physiologically modulated by cAMP and suggest that this might represent an additional mechanism through which the second messenger could influence memory functions.

Differences in the cellular uptake and intracellular itineraries of amyloid beta proteins 40 and 42: ramifications for the Alzheimer's drug discovery

Mounting evidence suggests that the pathological hallmarks of Alzheimer's disease (AD), neurofibrillary tangles and parenchymal amyloid plaques, are downstream reflections of neurodegeneration caused by the intraneuronal accumulation of amyloid-β proteins (Aβ), particularly Aβ42 and Aβ40. While the neurotoxicity of more amyloidogenic but less abundant Aβ42 is well documented, the effect of Aβ40 on neurons has been understudied. The Aβ40 expression in the presymptomatic AD brain is ten times greater than that of Aβ42. However, the Aβ40:42 ratio decreases with AD progression and coincides with increased amyloid plaque deposition in the brain. Hence, it is thought that Aβ40 protects neurons from the deleterious effects of Aβ42. The pathophysiological pathways involved in the neuronal uptake of Aβ40 or Aβ42 have not been clearly elucidated. Lack of such critical information obscures therapeutic targets and thwarts rational drug development strategies aimed at preventing neurodegeneration in AD. The current study has shown that fluorescein labeled Aβ42 (F-Aβ42) is internalized by neurons via dynamin dependent endocytosis and is sensitive to membrane cholesterol, whereas the neuronal uptake of F-Aβ40 is energy independent and nonendocytotic. Following their uptake, both F-Aβ40 and F-Aβ42 did not accumulate in early/recycling endosomes; F-Aβ42 but not F-Aβ40 accumulated in late endosomes and in the vesicles harboring caveolin-1. Furthermore, F-Aβ42 demonstrated robust accumulation in the lysosomes and damaged their integrity, whereas F-Aβ40 showed only a sparse lysosomal accumulation. Such regulated trafficking along distinct pathways suggests that Aβ40 and Aβ42 exercise differential effects on neurons. These differences must be carefully considered in the design of a pharmacological agent intended to block the neurodegeneration triggered by Aβ proteins.

APP processing in Alzheimer's disease

An important pathological feature of Alzheimer's disease (AD) is the presence of extracellular senile plaques in the brain. Senile plaques are composed of aggregations of small peptides called β-amyloid (Aβ). Multiple lines of evidence demonstrate that overproduction/aggregation of Aβ in the brain is a primary cause of AD and inhibition of Aβ generation has become a hot topic in AD research. Aβ is generated from β-amyloid precursor protein (APP) through sequential cleavages first by β-secretase and then by γ-secretase complex. Alternatively, APP can be cleaved by α-secretase within the Aβ domain to release soluble APPα and preclude Aβ generation. Cleavage of APP by caspases may also contribute to AD pathologies. Therefore, understanding the metabolism/processing of APP is crucial for AD therapeutics. Here we review current knowledge of APP processing regulation as well as the patho/physiological functions of APP and its metabolites.

HH domain of Alzheimer's disease Abeta provides structural basis for neuronal binding in PC12 and mouse cortical/hippocampal neurons

A key question in understanding AD is whether extracellular Aβ deposition of parenchymal amyloid plaques or intraneuronal Aβ accumulation initiates the AD process. Amyloid precursor protein (APP) is endocytosed from the cell surface into endosomes where it is cleaved to produce soluble Aβ which is then released into the brain interstitial fluid. Intraneuronal Aβ accumulation is hypothesized to predominate from the neuronal uptake of this soluble extracellular Aβ rather than from ER/Golgi processing of APP. We demonstrate that substitution of the two adjacent histidine residues of Aβ40 results in a significant decrease in its binding with PC12 cells and mouse cortical/hippocampal neurons. These substitutions also result in a dramatic enhancement of both thioflavin-T positive fibril formation and binding to preformed Aβ fibrils while maintaining its plaque-binding ability in AD transgenic mice. Hence, alteration of the histidine domain of Aβ prevented neuronal binding and drove Aβ to enhanced fibril formation and subsequent amyloid plaque deposition - a potential mechanism for removing toxic species of Aβ. Substitution or even masking of these Aβ histidine residues might provide a new therapeutic direction for minimizing neuronal uptake and subsequent neuronal degeneration and maximizing targeting to amyloid plaques.

A functional mouse retroposed gene Rps23r1 reduces Alzheimer's beta-amyloid levels and tau phosphorylation

Senile plaques consisting of beta-amyloid (Abeta) and neurofibrillary tangles composed of hyperphosphorylated tau are major pathological hallmarks of Alzheimer's disease (AD). Elucidation of factors that modulate Abeta generation and tau hyperphosphorylation is crucial for AD intervention. Here, we identify a mouse gene Rps23r1 that originated through retroposition of ribosomal protein S23. We demonstrate that RPS23R1 protein reduces the levels of Abeta and tau phosphorylation by interacting with adenylate cyclases to activate cAMP/PKA and thus inhibit GSK-3 activity. The function of Rps23r1 is demonstrated in cells of various species including human, and in transgenic mice overexpressing RPS23R1. Furthermore, the AD-like pathologies of triple transgenic AD mice were improved and levels of synaptic maker proteins increased after crossing them with Rps23r1 transgenic mice. Our studies reveal a new target/pathway for regulating AD pathologies and uncover a retrogene and its role in regulating protein kinase pathways.

Mechanism of neuronal versus endothelial cell uptake of Alzheimer's disease amyloid beta protein

Alzheimer's disease (AD) is characterized by significant neurodegeneration in the cortex and hippocampus; intraneuronal tangles of hyperphosphorylated tau protein; and accumulation of β-amyloid (Aβ) proteins 40 and 42 in the brain parenchyma as well as in the cerebral vasculature. The current understanding that AD is initiated by the neuronal accumulation of Aβ proteins due to their inefficient clearance at the blood-brain-barrier (BBB), places the neurovascular unit at the epicenter of AD pathophysiology. The objective of this study is to investigate cellular mechanisms mediating the internalization of Aβ proteins in the principle constituents of the neurovascular unit, neurons and BBB endothelial cells. Laser confocal micrographs of wild type (WT) mouse brain slices treated with fluorescein labeled Aβ40 (F-Aβ40) demonstrated selective accumulation of the protein in a subpopulation of cortical and hippocampal neurons via nonsaturable, energy independent, and nonendocytotic pathways. This groundbreaking finding, which challenges the conventional belief that Aβ proteins are internalized by neurons via receptor mediated endocytosis, was verified in differentiated PC12 cells and rat primary hippocampal (RPH) neurons through laser confocal microscopy and flow cytometry studies. Microscopy studies have demonstrated that a significant proportion of F-Aβ40 or F-Aβ42 internalized by differentiated PC12 cells or RPH neurons is located outside of the endosomal or lysosomal compartments, which may accumulate without degradation. In contrast, BBME cells exhibit energy dependent uptake of F-Aβ40, and accumulate the protein in acidic cell organelle, indicative of endocytotic uptake. Such a phenomenal difference in the internalization of Aβ40 between neurons and BBB endothelial cells may provide essential clues to understanding how various cells can differentially regulate Aβ proteins and help explain the vulnerability of cortical and hippocampal neurons to Aβ toxicity.

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

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

Evidence against roles for phorbol binding protein Munc13-1, ADAM adaptor Eve-1, or vesicle trafficking phosphoproteins Munc18 or NSF as phospho-state-sensitive modulators of phorbol/PKC-activated Alzheimer APP ectodomain shedding

BACKGROUND

Shedding of the Alzheimer amyloid precursor protein (APP) ectodomain can be accelerated by phorbol esters, compounds that act via protein kinase C (PKC) or through unconventional phorbol-binding proteins such as Munc13-1. We have previously demonstrated that application of phorbol esters or purified PKC potentiates budding of APP-bearing secretory vesicles at the trans-Golgi network (TGN) and toward the plasma membrane where APP becomes a substrate for enzymes responsible for shedding, known collectively as alpha-secretase(s). However, molecular identification of the presumptive "phospho-state-sensitive modulators of ectodomain shedding" (PMES) responsible for regulated shedding has been challenging. Here, we examined the effects on APP ectodomain shedding of four phorbol-sensitive proteins involved in regulation of vesicular membrane trafficking of APP: Munc13-1, Munc18, NSF, and Eve-1.

RESULTS

Overexpression of either phorbol-sensitive wildtype Munc13-1 or phorbol-insensitive Munc13-1 H567K resulted in increased basal APP ectodomain shedding. However, in contrast to the report of Rossner et al (2004), phorbol ester-dependent APP ectodomain shedding from cells overexpressing APP and Munc13-1 wildtype was indistinguishable from that observed following application of phorbol to cells overexpressing APP and Munc13-1 H567K mutant. This pattern of similar effects on basal and stimulated APP shedding was also observed for Munc18 and NSF. Eve-1, an ADAM adaptor protein reported to be essential for PKC-regulated shedding of pro-EGF, was found to play no obvious role in regulated shedding of sAPPalpha.

CONCLUSION

Our results indicate that, in the HEK293 system, Munc13-1, Munc18, NSF, and EVE-1 fail to meet essential criteria for identity as PMES for APP.

Diabetes and Alzheimer's disease - is there a connection?

It has been known for some time that diabetes may be associated with impaired cognitive function. During the last decade, epidemiological data have emerged suggesting a linkage between diabetes, particularly type 2 diabetes, and Alzheimer's disease (AD). There is evidence to suggest that impaired activities of neurotrophic factors such as insulin, IGF-1 and NGF, which occur in both diabetes and AD, may provide a mechanistic link between the two disorders. An additional probable factor that has been less evaluated to date is hypercholesterolemia, a common accompaniment to type 2 diabetes. Increased cholesterol availability is believed to play a crucial role in the abnormal metabolism of amyloid precursor protein leading to accumulation of amyloid-beta. Impaired insulin signaling in particular appears to be involved in hyperphosphorylation of the tau protein, which constitutes neurofibrillary tangles in AD. The linkage between abnormal amyloid metabolism and phosphor-tau is likely to be provided by the activation of caspases both by increased amyloid-beta and by impaired insulin signaling. Although the details of many of these components still await evaluation, it appears clear that commonalities exist in the underlying pathogenesis of diabetes and Alzheimer's disease. In this review we provide a brief update on linkages between these two diverse but common disorders.

Ectodomain Shedding of the Amyloid Precursor Protein: Cellular Control Mechanisms and Novel Modifiers

Proteolytic cleavage in the ectodomain of the amyloid precursor protein (APP) is a key regulatory step in the generation of the Alzheimer's disease amyloid-beta (Abeta) peptide and occurs through two different protease activities termed alpha- and beta-secretase. Both proteases compete for APP cleavage, but have opposite effects on Abeta generation. At present, little is known about the cellular pathways that control APP alpha- or beta-secretase cleavage and thus Abeta generation. To explore the contributory pathways in more detail we have recently employed an expression cloning screen and identified several activators of APP cleavage by alpha- or beta-secretase. Among them were known activators of APP cleavage, for example protein kinase A, and novel activators, such as endophilin and the APP homolog amyloid precursor-like protein 1 (APLP1). Mechanistic analysis revealed that both endophilin and APLP1 reduce the rate of APP endocytosis and strongly increase APP cleavage by alpha-secretase. This review summarizes the results of the expression cloning screen in the context of recent developments in our understanding of the cellular regulation of APP alpha-secretase cleavage. Moreover, it highlights the particular importance of endocytic APP trafficking as a prime modulator of APP shedding.

Immunology and immunotherapy of Alzheimer's disease

Although Alzheimer's disease is considered to be a degenerative brain disease, it is clear that the immune system has an important role in the disease process. As discussed in this Review, immune-based therapies that are designed to remove amyloid-beta peptide from the brain have produced positive results in animal models of the disease and are being tested in humans with Alzheimer's disease. Although immunotherapy holds great promise for the treatment of Alzheimer's disease, clinical trials of active amyloid-beta vaccination of patients with Alzheimer's disease were discontinued after some patients developed meningoencephalitis. New immunotherapies using humoral and cell-based approaches are currently being investigated for the treatment and prevention of Alzheimer's disease.

Transcriptional regulation of APH-1A and increased gamma-secretase cleavage of APP and Notch by HIF-1 and hypoxia

The proteolytic cleavage of Alzheimer beta-amyloid precursor protein (APP) and signaling receptor Notch is mediated by the PS/gamma-secretase complex, which consists of presenilins, nicastrin, APH-1, and PEN-2. Although the four components are known to coordinately regulate each other at the protein level, information regarding their transcription regulation is scarce. Here we characterized the 5'-flanking region of the human APH-1A gene and identified a 271-bp fragment containing the transcription initiation site for the promoter activity. Sequence analysis, mutagenesis, and gel shift studies revealed a binding of AP4 and HIF-1 to the promoter, which affects the promoter activity. Activation of HIF-1 by short-term NiCl2 treatments (a condition of chemical hypoxia) dramatically increased APH-1A mRNA and protein expression, accompanied by increased secretion of Abeta and decreased APP CTFs formation, indicative of an increase in gamma-secretase activity. NiCl2 treatments had little effect on APP and the other three components of the gamma-secretase complex. The cellular concentration of Notch intracellular domain (NICD) was also increased by the hypoxic treatment. Our results demonstrate that APH-1A expression and the gamma-secretase mediated Abeta and Notch NICD generation are regulated by HIF-1, and the specific control of APH-1A expression may imply physiological functions uniquely assigned to APH-1A.

Transcriptional regulation of PEN-2, a key component of the gamma-secretase complex, by CREB

Gamma-secretase, which is responsible for the intramembranous cleavage of Alzheimer's beta-amyloid precursor protein (APP), the signaling receptor Notch, and many other substrates, is a multiprotein complex consisting of at least four components: presenilin (PS), nicastrin, APH-1, and PEN-2. Despite the fact that PEN-2 is known to mediate endoproteolytic cleavage of full-length PS and APH-1 and nicastrin are required for maintaining the stability of the complex, the detailed physiological function of each component remain elusive. Unlike that of PS, the transcriptional regulation of PEN-2, APH-1, and nicastrin has not been investigated. Here, we characterized the upstream regions of the human PEN-2 gene and identified a 238-bp fragment located 353 bp upstream of the translational start codon as the key region necessary for the promoter activity. Further analysis revealed a CREB binding site located in the 238-bp region that is essential for the transcriptional activity of the PEN-2 promoter. Mutation of the CREB site abolished the transcriptional activity of the PEN-2 promoter. Electrophoretic mobility shift assays and chromatin immunoprecipitation analysis showed the binding of CREB to the PEN-2 promoter region both in vitro and in vivo. Activation of the CREB transcriptional factor by forskolin dramatically promoted the expression of PEN-2 mRNA and protein, whereas the other components of the gamma-secretase complex remained unaffected. Forskolin treatment slightly increases the secretion of soluble APPalpha and Abeta without affecting Notch cleavage. These results demonstrate that expression of PEN-2 is regulated by CREB and suggest that the specific control of PEN-2 expression may imply additional physiological functions uniquely assigned to PEN-2.

Expression cloning screen for modifiers of amyloid precursor protein shedding

Ectodomain shedding of the amyloid precursor protein (APP) is a key regulatory step in the generation of the amyloid beta peptide (Abeta), which is thought to provoke the pathogenesis of Alzheimer's disease. To better understand the cellular processes that regulate ectodomain shedding of APP we used human embryonic kidney 293 cells and applied a sib-selection expression cloning approach. In addition to a known activator of APP shedding -- protein kinase A -- the following cDNAs were identified: the endocytic proteins endophilin A1 and A3, the metabotropic glutamate receptor 3 (mGluR3), palmitoyl-protein thioesterase 1 (PPT1), Numb-like and the kinase MEKK2. Endophilins A1 and A3, as well as mGluR3 activated APP shedding relatively specifically. They had little or no effect on the shedding of the unrelated membrane proteins TNF receptor 2 and P-selectin glycoprotein ligand-1. In contrast, MEKK2 and PKA also increased shedding of TNF receptor 2, suggesting that these kinases contribute to a general program regulating ectodomain shedding. The strongest activator of APP shedding, endophilin A3, reduced the rate of APP endocytosis and specifically increased APP shedding by the protease alpha-secretase, as measured in an antibody uptake assay and by immunoblot analysis. This suggests that endophilin A3 is a novel modulator of APP trafficking affecting access of APP to alpha-secretase. In summary, this study shows that expression cloning is a suitable way to identify proteins controlling ectodomain shedding of membrane proteins.

The neuropeptide PACAP promotes ?‐secretase pathway for processing Alzheimer amyloid precursor protein

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has neurotrophic as well as anti-apoptotic properties and is involved in learning and memory processes. Its specific G protein-coupled receptor PAC1 is expressed in several central nervous system (CNS) regions, including the hippocampal formation. Here we examined the effect of PAC1 receptor activation on alpha-secretase cleavage of the amyloid precursor protein (APP) and the production of secreted APP (APPsalpha). Stimulation of endogenously expressed PAC1 receptors with PACAP in human neuroblastoma cells increased APPsalpha secretion, which was completely inhibited by the PAC1 receptor specific antagonist PACAP-(6-38). In HEK cells stably overexpressing functional PAC1 receptors, PACAP-27 and PACAP-38 strongly stimulated alpha-secretase cleavage of APP. The PACAP-induced APPsalpha production was dose dependent and saturable. This increase of alpha-secretase activity was completely abolished by hydroxamate-based metalloproteinase inhibitors, including a preferential ADAM 10 inhibitor. By using several specific protein kinase inhibitors, we show that the MAP-kinase pathway [including extracellular-regulated kinase (ERK) 1 and ERK2] and phosphatidylinositol 3-kinase mediate the PACAP-induced alpha-secretase activation. Our findings provide evidence for a role of the neuropeptide PACAP in stimulation of the nonamyloidogenic pathway, which might be related to its neuroprotective properties.

Short-term interleukin-1β increases the release of secreted APPα via MEK1/2-dependent and JNK-dependent α-secretase cleavage in neuroglioma U251 cells

Several lines of neuroimmunological evidence correlate the development of the inflammatory responses of the brain with the formation of amyloid plaques associated with the pathogenesis of neurodegenerative disorders such as Alzheimer's disease. Within this context, we tested the ability of interleukin-1beta (IL-1beta) to regulate the processing of beta-amyloid precursor protein (beta-APP) in neuroglioma U251 cells. Our findings have shown that short-term treatment with IL-1beta (2 hr) resulted in a concentration-dependent decrease in the amount of the cell-associated form of beta-APP in U251 cells as compared to untreated cells, whereas a 2-hr treatment with IL-1beta led to increased release of secreted APP(alpha) fragment (sAPP(alpha)) into the conditioned media of the cells. The fact that sAPP(alpha) is an alpha-secretase cleavage metabolite of the cell-associated form of beta-APP, and the observation that IL-1beta-induced sAPP(alpha) release could be blocked by tissue inhibitors of metalloproteinases-1 (alpha-secretase inhibitors), suggested that alpha-secretase might be involved in IL-1beta-induced-sAPP(alpha) release. Moreover, to determine whether an intracellular signaling pathway mediates the IL-1beta-induced increase in sAPP(alpha) secretion, we used various specific signaling inhibitors and found that sAPP(alpha) release is significantly blocked by the mitogen-activated protein kinase (MEK1/2) inhibitor PD98059 and the c-Jun N-terminal kinase inhibitor SP600125. These findings suggested that the mechanism of IL-1beta-induced-sAPP(alpha) release is dependent on MEK1/2- and JNK-activated alpha-secretase cleavage in neuroglioma U251 cells.

Presenilins and gamma-secretase inhibitors affect intracellular trafficking and cell surface localization of the gamma-secretase complex components

The intramembranous cleavage of Alzheimer beta-amyloid precursor protein and the signaling receptor Notch is mediated by the presenilin (PS, PS1/PS2)-gamma-secretase complex, the components of which also include nicastrin, APH-1, and PEN-2. In addition to its essential role in gamma-secretase activity, we and others have reported that PS1 plays a role in intracellular trafficking of select membrane proteins including nicastrin. Here we examined the fate of PEN-2 in the absence of PS expression or gamma-secretase activity. We found that PEN-2 is retained in the endoplasmic reticulum and has a much shorter half-life in PS-deficient cells than in wild type cells, suggesting that PSs are required for maintaining the stability and proper subcellular trafficking of PEN-2. However, the function of PS in PEN-2 trafficking is distinct from its contribution to gamma-secretase activity because inhibition of gamma-secretase activity by gamma-secretase inhibitors did not affect the PEN-2 level or its egress from the endoplasmic reticulum. Instead, membrane-permeable gamma-secretase inhibitors, but not a membrane-impermeable derivative, markedly increased the cell surface levels of PS1 and PEN-2 without affecting that of nicastrin. In support of its role in PEN-2 trafficking, PS1 was also required for the gamma-secretase inhibitor-induced plasma membrane accumulation of PEN-2. We further showed that gamma-secretase inhibitors specifically accelerated the Golgi to the cell surface transport of PS1 and PEN-2. Taken together, we demonstrate an essential role for PSs in intracellular trafficking of the gamma-secretase components, and that selective gamma-secretase inhibitors differentially affect the trafficking of the gamma-secretase components, which may contribute to an inactivation of gamma-secretase.

Modulation of β-amyloid metabolism by non-steroidal anti-inflammatory drugs in neuronal cell cultures

Alzheimer disease (AD) is characterized by cerebral deposits of beta-amyloid (Abeta) peptides, which are surrounded by neuroinflammatory cells. Epidemiological studies have shown that prolonged use of non-steroidal anti-inflammatory drugs (NSAIDs) reduces the risk of developing AD. In addition, biological data indicate that certain NSAIDs specifically lower Abeta42 levels in cultures of peripheral cells independently of cyclooxygenase (COX) activity and reduce cerebral Abeta levels in AD transgenic mice. Whether other NSAIDs, including COX-selective compounds, modulate Abeta levels in neuronal cells remains unexploited. Here, we investigated the effects of compounds from every chemical class of NSAIDs on Abeta40 and Abeta42 secretion using both Neuro-2a cells and rat primary cortical neurons. Among non-selective NSAIDs, flurbiprofen and sulindac sulfide concentration-dependently reduced the secretion not only of Abeta42 but also of Abeta40. Surprisingly, both COX-2 (celecoxib; sc-125) or COX-1 (sc-560) selective compounds significantly increased Abeta42 secretion, and either did not alter (sc-560; sc-125) or reduced (celecoxib) Abeta40 levels. The levels of betaAPP C-terminal fragments and Notch cleavage were not altered by any of the NSAIDs, indicating that gamma-secretase activity was not overall changed by these drugs. The present findings show that only a few non-selective NSAIDs possess Abeta-lowering properties and therefore have a profile potentially relevant to their clinical use in AD.

Protein phosphorylation and APP metabolism

Numerous lines of evidence place signal transduction cascades at the core of many processes having a direct role in neurodegeneration and associated disorders. Key players include neurotransmitters, growth factors, cytokines, hormones, and even binding and targeting proteins. Indeed, abnormal phosphorylation of key control proteins has been detected in many cases and is thought to underlie the associated cellular dysfunctions. Several signaling cascades have been implicated, affecting processes as varied as protein processing, protein expression, and subcellular protein localization, among others. The Alzheimer's amyloid precursor protein (APP) is a phosphoprotein, with well-defined phosphorylation sites but whose function is not clearly understood. The factors and pathways regulating the processing of APP have been particularly elusive, both in normal ageing and the Alzheimer's disease (AD) condition. Not surprisingly, the physiological function(s) of the protein remain(s) to be elucidated, although many hypotheses have been advanced. Nonetheless, considerable data has accumulated over the last decade, placing APP in key positions to be modulated both directly and indirectly by phosphorylation and phosphorylation-dependent events. The pathological end product of APP processing is the main proteinaceous component of the hallmark senile plaques found in the brains of AD patients, that is, a toxic peptide termed Abeta. In this minireview we address the importance of phosphorylation and signal transduction cascades in relation to APP processing and Abeta production. The possible use of the identified molecular alterations as therapeutic targets is also addressed.

Glucagon-like peptide-1 decreases endogenous amyloid-beta peptide (Abeta) levels and protects hippocampal neurons from death induced by Abeta and iron

Glucagon-like peptide-1(7-36)-amide (GLP-1) is an endogenous insulinotropic peptide that is secreted from the gastrointestinal tract in response to food. It enhances pancreatic islet beta-cell proliferation and glucose-dependent insulin secretion and lowers blood glucose and food intake in patients with type 2 diabetes mellitus. GLP-1 receptors, which are coupled to the cyclic AMP second messenger pathway, are expressed throughout the brains of rodents and humans. It was recently reported that GLP-1 and exendin-4, a naturally occurring, more stable analogue of GLP-1 that binds at the GLP-1 receptor, possess neurotrophic properties and can protect neurons against glutamate-induced apoptosis. We report here that GLP-1 can reduce the levels of amyloid-beta peptide (Abeta) in the brain in vivo and can reduce levels of amyloid precursor protein (APP) in cultured neuronal cells. Moreover, GLP-1 and exendin-4 protect cultured hippocampal neurons against death induced by Abeta and iron, an oxidative insult. Collectively, these data suggest that GLP-1 can modify APP processing and protect against oxidative injury, two actions that suggest a novel therapeutic target for intervention in Alzheimer's disease.

Amyloid precursor protein beta-secretase (BACE) mRNA expression in human neural cell lines following induction of neuronal differentiation and exposure to cytokines and growth factors

Recently, a novel amyloid precursor protein beta-secretase (designated BACE) was identified. Because activated microglia and astrocytes play a role in amyloidogenesis in Alzheimer's disease, the constitutive and glial cytokine/growth factor-regulated expression of BACE was studied in human neural cell lines. By reverse transcription-polymerase chain reaction (RT-PCR) analysis, BACE mRNA expression was identified in various human neural and non-neural cell lines. By northern blot analysis, the expression of BACE mRNA composed of five distinct transcripts (>8.0, 7.0, 6.0, 4.4 and 2.6 kb) was elevated markedly in NTera2 teratocarcinoma cells following retinoic acid-induced neuronal differentiation. But the levels of three major BACE mRNA species (7.0, 6.0 and 4.4 kb) were not significantly altered in NTera2-derived neurons, SK-N-SH neuroblastoma or U-373MG astrocytoma following exposure to tumor necrosis factor-alpha, interleukin (IL)-1beta, IL-6, interferon-gamma, transforming growth factor-beta1, epidermal growth factor, basic fibroblast growth factor, brain-derived neurotrophic factor, dibutyryl cyclic adenosine monophosphate or phorbol 12-myristate 13-acetate. These results indicate that BACE mRNA is expressed constitutively in human neural cells and its expression is upregulated during neuronal differentiation, but it is unlikely to be regulated by activated glia-derived cytokines and growth factors.

Oxidative stress induces increase in intracellular amyloid beta-protein production and selective activation of betaI and betaII PKCs in NT2 cells

Amyloid beta-protein (Abeta) aggregation produces an oxidative stress in neuronal cells that, in turn, may induce an amyloidogenic shift of neuronal metabolism. To investigate this hypothesis, we analyzed intra- and extracellular Abeta content in NT2 differentiated cells incubated with 4-hydroxy-2,3-nonenal (HNE), a major product of lipid peroxidation. In parallel, we evaluated protein kinase C (PKC) isoenzymes activity, a signaling system suspected to modulate amyloid precursor protein (APP) processing. Low HNE concentrations (0.1-1 microM) induced a 2-6 fold increase of intracellular Abeta production that was concomitant with selective activation of betaI and betaII PKC isoforms, without affecting either cell viability or APP full-length expression. Selective activation of the same PKC isoforms was observed following NT2 differentiation. Our findings suggest that PKC beta isoenzymes are part of cellular mechanisms that regulate production of the intracellular Abeta pool. Moreover, they indicate that lipid peroxidation fosters intracellular Abeta accumulation, creating a vicious neurodegenerative loop.

The phosphatidylinositol 3-kinase inhibitor wortmannin alters the metabolism of the Alzheimer's amyloid precursor protein

One of the hallmarks of Alzheimer's disease is the accumulation of senile plaques in brain, extracellular lesions comprised mostly of aggregates of the amyloid beta-peptide (Abeta). Abeta is proteolytically derived from the Alzheimer's amyloid precursor protein (APP). The generation of Abeta and nonamyloidogenic derivatives of APP involves utilization of alternative processing pathways and multiple subcellular compartments. To improve our understanding of the regulation of APP processing, we investigated the effects of wortmannin, a phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, on APP processing. PI3-kinases form a multifaceted family of enzymes that represent converging points for multiple signal transduction pathways and also act as key regulators of vesicular trafficking. In N2a neuroblastoma cells expressing either wild-type APP or the "Swedish" familial Alzheimer's disease-associated mutant variant of APP, wortmannin treatment resulted in decreased release of both Abeta and soluble APPalpha. In parallel, full-length APP and both processed derivatives accumulated inside the cells. These effects were not present at nanomolar concentrations of wortmannin, but only at micromolar concentrations, implying the possible involvement of a recently described trans-Golgi network (TGN)-associated PI3-kinase that is resistant to nanomolar concentrations of the inhibitor, but sensitive to micromolar concentrations. All effects were reversible when the drug was removed from the cell culture medium. Given the suspected site of action of this novel PI3-kinase activity at the TGN, it is tempting to speculate that the unexpected increase in the levels of both intracellular soluble APPalpha and intracellular Abeta might be due to wortmannin-induced covesiculation of APP together with its respective secretase enzymes within the TGN, leading to the execution of alpha-, beta-, and gamma-secretase reactions.

Rationalizing a pharmacological intervention on the amyloid precursor protein metabolism

The treatment of Alzheimer's disease remains a major challenge because of our incomplete understanding of the triggering events that lead to the selective neurodegeneration characteristic of Alzheimer's brains. The rational design of a pharmacological intervention is therefore a great theoretical challenge. One approach involves the study of the pharmacological modulation of the amyloid precursor protein metabolism, in which the goal is to reduce the formation of beta-amyloid in the hope of reducing the formation of a potentially neurotoxic peptide. Such an approach has led to the identification of a complex intracellular mechanism that can be regulated by neurotransmitters and other ligands.

Abnormalities of protein kinases in neurodegenerative diseases

In neurodegenerative diseases such as ALS and AD there is evidence for abnormal regulation of protein kinases. In these diseases, altered activities and protein levels of several specific kinases suggest that abnormal phosphorylation is present and this aberrant phosphorylation may be involved in the pathogenesis of these diseases. The observation that regulation of the NMDA receptor ion channel is altered in tissue from ALS patients may arise from the abnormal phosphorylation state of the protein kinase regulating NMDA receptor function. Whether the abnormalities of these protein kinases is a primary event leading to altered receptor regulation or vice versa is still poorly understood. The seemingly multiple pathogenic mechanisms of ALS and AD create complexity in assessing a primary cause that may lead to cell death. The mechanisms causing cell death (apoptosis or necrosis) may be overlapping with integrated events among the components interacting and contributing to a final pathway for neuron death. Thus, evidence of impairment in protein kinase signalling in these diseases may be a primary cause, a secondary event, or a compensatory mechanism. To further study this issue, different model systems could be beneficial to obtain a better understanding of these diseases.

Regulation of amyloid precursor protein cleavage

Multiple lines of evidence suggest that increased production and/or deposition of the beta-amyloid peptide, derived from the amyloid precursor protein, contributes to Alzheimer's disease. A growing list of neurotransmitters, growth factors, cytokines, and hormones have been shown to regulate amyloid precursor protein processing. Although traditionally thought to be mediated by activation of protein kinase C, recent data have implicated other signaling mechanisms in the regulation of this process. Moreover, novel mechanisms of regulation involving cholesterol-, apolipoprotein E-, and stress-activated pathways have been identified. As the phenotypic changes associated with Alzheimer's disease encompass many of these signaling systems, it is relevant to determine how altered cell signaling may be contributing to increasing brain amyloid burden. We review the myriad ways in which first messengers regulate amyloid precursor protein catabolism as well as the signal transduction cascades that give rise to these effects.

Post-transcriptional contribution of a cAMP-dependent pathway to the formation of alpha- and beta/gamma-secretases-derived products of beta APP maturation in human cells expressing wild-type and Swedish mutated beta APP

BACKGROUND

The physiopathological maturation of the beta-amyloid precursor protein can be modulated by effectors targeting a protein kinase C-dependent pathway. These agents increase the recovery of APP alpha, the physiological alpha-secretase-derived product of beta APP processing, and concomittantly lower the production of the pathogenic beta/gamma-secretase-derived A beta fragment.

METHODS

We set up stably transfected HEK293 cells expressing wild-type or Swedish mutated beta APP. By combined metabolic labeling and/or immunoprecipitation procedures, we assessed the effect of various cAMP effectors on the production of the beta APP maturation products A beta 40, A beta 42, APP alpha, and its C-terminal counterpart.

RESULTS

We show here that the cAMP-dependent protein kinase (PKA) effectors, dibutyryl-cAMP (dBut-cAMP) and forskolin, but not the inactive analog dideoxyforskolin, enhance the secretion of APP alpha and the intracellular production of its C-terminal counterpart (p10) in stably transfected HEK293 cells. The above agonists also drastically increase both A beta 40 and A beta 42 secretions and intracellular A beta recovery. The same influence was observed with HEK293 cells overexpressing the Swedish mutated beta APP. We attempted to delineate the relative contribution of transcriptional and post-transcriptional events in the cAMP-mediated response. We show here that the dBut-cAMP and forskolin-induced increase of APP alpha and A beta s secretions is not prevented by the transcription inhibitor actinomycin D.

CONCLUSION

Our data suggest a major contribution of post-transcriptional events in the cAMP-dependent effect on beta APP maturation. It appears likely that cAMP triggers the PKA-dependent phosphorylation of a protein involved in beta APP maturation and occurring upstream to alpha- and beta/gamma-secretase cleavages.

Evidence that tumor necrosis factor alpha converting enzyme is involved in regulated alpha-secretase cleavage of the Alzheimer amyloid protein precursor

The amyloid protein, Abeta, which accumulates in the brains of Alzheimer patients, is derived by proteolysis of the amyloid protein precursor (APP). APP can undergo endoproteolytic processing at three sites, one at the amino terminus of the Abeta domain (beta-cleavage), one within the Abeta domain (alpha-cleavage), and one at the carboxyl terminus of the Abeta domain (gamma-cleavage). The enzymes responsible for these activities have not been unambiguously identified. By the use of gene disruption (knockout), we now demonstrate that TACE (tumor necrosis factor alpha converting enzyme), a member of the ADAM family (a disintegrin and metalloprotease-family) of proteases, plays a central role in regulated alpha-cleavage of APP. Our data suggest that TACE may be the alpha-secretase responsible for the majority of regulated alpha-cleavage in cultured cells. Furthermore, we show that inhibiting this enzyme affects both APP secretion and Abeta formation in cultured cells.

The use of permeabilized cells to investigate secretory granule biogenesis

To investigate the mechanism of secretory granule biogenesis in endocrine cells, our laboratory used rat anterior pituitary GH3 cells which secrete growth hormone and prolactin. Here we describe a simple and rapid procedure for generating permeabilized cells to dissect molecular mechanisms involved in nascent secretory vesicle budding from the trans-Golgi network (TGN). Using this system, we demonstrate that vesicle budding is temperature, energy, and cytosol dependent; in addition, cytosol from a variety of cells, including yeast (Saccharomyces cerevisiae), can support vesicle release. The budding of nascent secretory vesicles from the TGN is stimulated by a phospholipase D activity that is associated with Golgi membranes. Our results suggest that phospholipid metabolism plays an important role in the release of nascent secretory vesicles from the TGN.

Analysis of constitutive and constitutive-like secretion in semi-intact pituitary cells

To study biosynthetic transport through the constitutive and regulated secretory pathways, we have designed a semi-intact mammalian cell system that restores the transport of secretory proteins from the trans-Golgi/trans-Golgi network (TGN) to the cell surface. The mouse pituitary AtT-20 cell line is a suitable model to biochemically analyze molecular sorting in the secretory pathway. The prohormone proopiomelanocortin is sulfated on N-linked carbohydrate chains in the trans-Golgi prior to proteolytic processing in the secretory granule. Radiolabeling with [35S]sulfate therefore provides a convenient tool to selectively follow molecular events in the regulated secretory pathway without interference from earlier steps. Likewise, transport through the constitutive secretory pathway may be monitored using sulfate-labeled glycosaminoglycan chains. We show that export from the TGN is efficiently reconstituted in cells made semi-intact with streptolysin O, and is dependent on temperature, ATP and GTP hydrolysis, and cytosol. Packaging of proopiomelanocortin into immature secretory granules also activates the proteolytic processing machinery which eventually converts the prohormone to its bioactive mature product, adrenocorticotropic hormone. In addition, a large fraction of incompletely processed proopiomelanocortin is secreted as the processing intermediates from immature secretory granules. This process of constitutive-like secretion can be clearly distinguished from direct constitutive secretion from the trans-Golgi network by kinetic and compositional criteria. Furthermore, we have found that specific inhibitors of different protein phosphatases and kinases are potent blockers of constitutive and constitutive-like secretion. This experimental model should provide a valuable system to elucidate the molecular mechanism regulating post-Golgi traffic during secretory granule biogenesis.

Biogenesis of secretory granules in the trans-Golgi network of neuroendocrine and endocrine cells

Secretory granule formation requires selection of soluble and membrane proteins into nascent secretory granules, and exclusion of proteins not required for the function of secretory granules. Both selection and exclusion presumably can occur in the compartment where assembly of the secretory granule begins, the trans most cisternae of the Golgi complex. Current research focused on the initial stages of secretory granule formation includes a search for the 'signals' which may mediate active sorting of components into secretory granules, and the role of aggregation of regulated secretory proteins in sorting. In addition, the temporal sequence of the sorting events in the Golgi, and post-Golgi compartments has gained much attention, as summarized by the alternative but not mutually exclusive 'sorting for entry' vs. 'sorting by retention' models. 'Sorting for entry' which encompasses the most popular models requires selection of cargo and membrane and exclusion of non-secretory granule proteins in the TGN prior to secretory granule formation. 'Sorting by retention' stipulates that protein selection or exclusion may occur after secretory granule formation: secretory granule specific components are retained during maturation of the granule while non-secretory granule molecules are removed in vesicles which bud from maturing secretory granules. Finally, some progress has been made in the identification of cytosolic components involved in the budding of nascent secretory granules from the TGN. This review will focus on the recent data concerning the events in secretory granule formation which occur, in the trans-Golgi network.

Casein secretion in mammary tissue: tonic regulation of basal secretion by protein kinase A

Despite its quantitative importance in the secretion of lactoproteins, little is known about the triggering and control mechanisms that initiate, regulate and terminate the operation of the basal pathway of lactoprotein secretion throughout the lactation cycle. This study investigated the possible modulation by cAMP-mediated mechanisms, of cellular transit of newly-synthesised caseins and their basal secretion in explants of mammary tissue from lactating rats and rabbits. Enhancement of the rate of secretion of newly-synthesised caseins occurs when mammary explants are challenged in vitro with agents that activate protein kinase A (PKA). Inhibition of PKA slows casein secretion. The PKA-sensitive step(s) in casein secretion is early in the exocytosis pathway but inhibition of PKA does not impair casein maturation. Ultrastructural, immunochemical and biochemical methods locate PKA on membranes of vesicles situated in the Golgi region. Exposure of tissue to a cell-permeant PKA inhibitor results in morphological modification of these vesicular structures. We conclude that PKA mediates tonic positive regulation of the basal secretory pathway for lactoproteins in the mammary epithelial cell.

Regulation of amyloid precursor protein secretion by glutamate receptors in human Ntera 2 neurons

The amyloid precursor protein (APP) can be cleaved by a beta-secretase to generate a beta-amyloid peptide, which has been implicated in the pathogenesis of Alzheimer's disease. However, APP can also be cleaved by an alpha-secretase to form a non-amyloidogenic secreted form of APP (APP-S). APP-S secretion can be physiologically regulated. This study examined the glutamatergic regulation of APP in the human neuronal Ntera 2 (NT2N) cell line. Metabotropic glutamate receptor subtypes 1alpha/beta and 5alpha were identified in the NT2N neurons by reverse transcription-polymerase chain reaction. Stimulation of these phosphatidylinositol-linked receptors with glutamate or specific receptor agonists resulted in a dose- and time-dependent increase in the secretion of the amyloid precursor protein (APP-S), measured by the immunoprecipitation of APP-S from the medium of [35S]methionine-labeled NT2N neurons. The glutamate-induced APP-S secretion was maximal at 30 min and at a concentration of 1 mM glutamate. Glutamate-induced APP-S secretion required activation of phospholipase C, which resulted in inositol 1, 4,5-trisphosphate production, as shown by the rapid glutamate-induced accumulation of inositol 1,4,5-trisphosphate. Glutamate also caused an increase in intracellular Ca2+. The protein kinase C activator phorbol 12-myristate 13-acetate, a phorbol ester, as well as 1-oleoyl-2-acetoyl-3-glycerol, a cell-permeable diacylglycerol analog, also stimulated APP-S secretion. These findings suggest that APP-S secretion from NT2N neurons can be regulated by the activation of phosphatidylinositol-linked metabotropic glutamate receptor signaling pathway.

Estrogen reduces neuronal generation of Alzheimer beta-amyloid peptides

Alzheimer's disease (AD) is characterized by the accumulation of cerebral plaques composed of 40- and 42-amino acid beta-amyloid (Abeta) peptides, and autosomal dominant forms of AD appear to cause disease by promoting brain Abeta accumulation. Recent studies indicate that postmenopausal estrogen replacement therapy may prevent or delay the onset of AD. Here we present evidence that physiological levels of 17beta-estradiol reduce the generation of Abeta by neuroblastoma cells and by primary cultures of rat, mouse and human embryonic cerebrocortical neurons. These results suggest a mechanism by which estrogen replacement therapy can delay or prevent AD.

Regulation of amyloid precursor protein catabolism involves the mitogen-activated protein kinase signal transduction pathway

Catabolic processing of the amyloid precursor protein (APP) is subject to regulatory control by protein kinases. We hypothesized that this regulation involves sequential activation of the enzymes mitogen-activated protein kinase kinase (MEK) and extracellular signal-regulated protein kinase (ERK). In the present investigation, we provide evidence that MEK is critically involved in regulating APP processing by both nerve growth factor and phorbol esters. Western blot analysis of the soluble N-terminal APP derivative APPs demonstrated that the synthetic MEK inhibitor PD 98059 antagonized nerve growth factor stimulation of both APPs production and ERK activation in PC12 cells. Moreover, PD 98059 inhibited phorbol ester stimulation of APPs production and activation of ERK in both human embryonic kidney cells and cortical neurons. Furthermore, overexpression of a kinase-inactive MEK mutant inhibited phorbol ester stimulation of APP secretion and activation of ERK in human embryonic kidney cell lines. Most important, PD 98059 antagonized phorbol ester-mediated inhibition of Abeta secretion from cells overexpressing human APP695 carrying the "Swedish mutation." Taken together, these data indicate that MEK and ERK may be critically involved in protein kinase C and nerve growth factor regulation of APP processing. The mitogen-activated protein kinase cascade may provide a novel target for altering catabolic processing of APP.

Regulation of amyloid precursor protein catabolism involves the mitogen-activated protein kinase signal transduction pathway

Catabolic processing of the amyloid precursor protein (APP) is subject to regulatory control by protein kinases. We hypothesized that this regulation involves sequential activation of the enzymes mitogen-activated protein kinase kinase (MEK) and extracellular signal-regulated protein kinase (ERK). In the present investigation, we provide evidence that MEK is critically involved in regulating APP processing by both nerve growth factor and phorbol esters. Western blot analysis of the soluble N-terminal APP derivative APPs demonstrated that the synthetic MEK inhibitor PD 98059 antagonized nerve growth factor stimulation of both APPs production and ERK activation in PC12 cells. Moreover, PD 98059 inhibited phorbol ester stimulation of APPs production and activation of ERK in both human embryonic kidney cells and cortical neurons. Furthermore, overexpression of a kinase-inactive MEK mutant inhibited phorbol ester stimulation of APP secretion and activation of ERK in human embryonic kidney cell lines. Most important, PD 98059 antagonized phorbol ester-mediated inhibition of Abeta secretion from cells overexpressing human APP695 carrying the "Swedish mutation." Taken together, these data indicate that MEK and ERK may be critically involved in protein kinase C and nerve growth factor regulation of APP processing. The mitogen-activated protein kinase cascade may provide a novel target for altering catabolic processing of APP.

Alzheimer's alpha-secretase may be a calcium-dependent protease

Proteolytic processing of beta-amyloid precursor protein (APP) is believed to be fundamental to the understanding of Alzheimer's disease. The identities and the regulatory elements of the proteases involved in the process, known as alpha/beta/gamma secretases, are unclear. In this study, by examining reported data, we found some indications suggesting that the putative alpha-secretase may be a calcium-dependent protease, and that this enzyme may play a primary role in the regulation of APP processing. Based on this, we proposed a model for the membrane orientations of the secretases for further discussions.