sAPP Alpha Research Articles

Comparison of measurements of medial temporal lobe atrophy in the prediction of Alzheimer's Disease in subjects with MCI

molecules. The interaction of both labelled donor and acceptor molecules induced a fluorescence at 665nm proportional to the sAPP alpha level. A recombinant protein was used as standard. We investigated the CSF from 30 patients with AD, and 30 controls. CSFs were considered as AD profile according to tau protein, phosphorylated tau and As 1-42 results, as previously described (1). Excluded patients on the basis of CSF markers levels were considered as controls. Results: The test did not recognize recombinant sAPPs protein and As1-42 peptide. Sample dilutions (1/5, 1/10, 1/20) allowed to verify the response linearity. The test detected 3 ng/ml of sAPP alpha. 5 ml of sample was sufficient for the CSF quantification. Using this test, we could observed a significant increase of sAPP alpha in the CSF of 30 AD patients comparatively to 30 controls (AD:530 6 36 ng/ml; controls: 3936 28 ng/ml; p< 0.01). Interestingly, sAPP alpha was significantly correlated with As levels in control CSFs only (r2 :0.31; p< 0.001). No significant correlations were observed between tau or phosphorylated tau and sAPP alpha levels in CSF. Conclusions:We developed a very sensitive test for the quantification of sAPP alpha levels in human CSF. Using this test, we observed an increase of sAPP alpha level in the CSF of AD patients in absence of correlation with other markers contrary to that observed in controls. It will be interesting to investigate a larger population of AD and Mild Cognitive Impairment patients to study the putative correlations of sAPP alpha levels with the biomarkers and parameters of the disease. 1: DumurgierJ et al, (2010) NeurobiolDis. 40: 456-459

EHT0202 in Alzheimers Disease: A 3-Month, Randomized, Placebo- Controlled, Double-Blind Study

EHT0202 (etazolate hydrochloride) is a new compound exhibiting both potential disease-modifying and symptomatic treatment properties in Alzheimer's Disease increasing alpha-secretase activity and sAPP alpha secretion, as well as acting as a GABA-A receptor modulator and as a PDE-4 inhibitor. This pilot, randomized, double-blind, placebo-controlled, parallel group, multicentre, Phase IIA study was conducted in 159 randomized patients suffering from mild to moderate Alzheimer's Disease. EHT0202 (40 or 80 mg bid) or placebo was administered as adjunctive therapy to one acetylcholinesterase inhibitor over a 3-month period. This study was designed to assess the clinical safety and tolerability of EHT0202 as a primary objective, with secondary endpoints (cognitive function, daily living activities, behaviour, caregiver burden and global functioning) included to explore clinical efficacy of EHT0202 versus placebo. EHT0202 was shown to be safe and generally well tolerated. Dose-dependent numbers of early withdrawal and central nervous system related adverse events were observed. As expected, since the study was not powered and not designed to show drug efficacy, and except for ratings on the ADCS-ADL scale, no significant differences were seen between treatment groups. These first encouraging safety results do support further development of EHT0202 in order to assess its clinical efficacy and to confirm its tolerability in a larger cohort of Alzheimer patients and for a longer period.

Soluble amyloid precursor proteins in the cerebrospinal fluid as novel potential biomarkers of Alzheimer's disease: a multicenter study

In this report, we present the results of a multicenter study to test analytic and diagnostic performance of soluble forms of amyloid precursor proteins alpha and beta (sAPP alpha and sAPP beta) in the cerebrospinal fluid (CSF) of patients with different forms of dementing conditions. CSF samples were collected from 188 patients with early dementia (mini-mental state examination >or=20 in majority of cases) and mild cognitive impairment (MCI) in 12 gerontopsychiatric centers, and the clinical diagnoses were supported by neurochemical dementia diagnostic (NDD) tools: CSF amyloid beta peptides, Tau and phospho-Tau. sAPP alpha and sAPP beta were measured with multiplexing method based on electrochemiluminescence. sAPP alpha and sAPP beta CSF concentrations correlated with each other with very high correlation ratio (R=0.96, P<0.001). We observed highly significantly increased sAPP alpha and sAPP beta CSF concentrations in patients with NDD characteristic for Alzheimer's disease (AD) compared to those with NDD negative results. sAPP alpha and sAPP beta highly significantly separated patients with AD, whose diagnosis was supported by NDD findings (sAPP alpha: cutoff, 117.4 ng ml(-1), sensitivity, 68%, specificity, 85%, P<0.001; sAPP beta: cutoff, 181.8 ng ml(-1), sensitivity, 75%, specificity, 85%, P<0.001), from the patients clinically assessed as having other dementias and supported by NDD untypical for AD. We conclude sAPP alpha and sAPP beta might be regarded as novel promising biomarkers supporting the clinical diagnosis of AD.

Rationale for the Development of Cholinesterase Inhibitors as Anti- Alzheimer Agents

Alzheimer's disease (AD) is characterized by progressive dementia caused by the loss of the presynaptic markers of the cholinergic system in the brain areas related to memory and learning and brain deposits of amyloid beta peptide (A beta) and neurofibrillary tangles (NFT). A small fraction of early onset familial AD (FAD) is caused by mutations in genes, such as the beta-amyloid precursor protein (APP) and presenilins that increase the load of A beta in the brain. These studies together with findings that A beta is neurotoxic in vitro, provide evidence that some aggregates of this peptide are the key to the pathogenesis of AD. The yield of A beta and the processing and turnover of APP are regulated by a number of pathways including apolipoprotein E, cholesterol and cholinergic agonists. Early studies showed that muscarinic agonists increased APP processing within the A beta sequence (sAPP alpha). More recently, we have presented evidence showing that some, but not all, anticholinesterases reduce secretion of sAPP alpha as well as A beta into the media suggesting that cholinergic agonists modulate A beta levels by multiple mechanisms. Herein we review the recent advances in understanding the function of cholinesterase (ChE) in the brain and the use of ChE-inhibitors in AD. We propose and support the position that the influence of cholinergic stimulation on amyloid formation is critical in light of the early targeting of the cholinergic basal forebrain in AD and the possibility that maintenance of this cholinergic tone might slow amyloid deposition. In this context, the dual action of certain cholinesterase inhibitors on their ability to increase acetylcholine levels and decrease amyloid burden assumes significance as it may identify a single drug to both arrest the progression of the disease as well as treat its symptoms. A new generation of acetyl- and butyryl cholinesterase inhibitors is being studied and tested in human clinical trials for AD. We critically discuss recent trends in AD research, from molecular and genetic to clinical areas, as it relates to the effects of cholinergic agents and their secondary effects on A beta. Finally, we examine different neurobiological mechanisms that provide the basis of new targets for AD drug development.

Therapeutic effects of PKC activators in Alzheimer's disease transgenic mice.

Alzheimer's disease (AD) characteristically presents with early memory loss. Regulation of K(+) channels, calcium homeostasis, and protein kinase C (PKC) activation are molecular events that have been implicated during associative memory which are also altered or defective in AD. PKC is also involved in the processing of the amyloid precursor protein (APP), a central element in AD pathophysiology. In previous studies, we demonstrated that benzolactam (BL), a novel PKC activator, reversed K(+) channels defects and enhanced secretion of APP alpha in AD cells. In this study we present data showing that another PKC activator, bryostatin 1, at subnanomolar concentrations dramatically enhances the secretion of the alpha-secretase product sAPP alpha in fibroblasts from AD patients. We also show that BL significantly increased the amount of sAPP alpha and reduced A beta 40 in the brains of APP[V717I] transgenic mice. In a more recently developed AD double-transgenic mouse, bryostatin was effective in reducing both brain A beta 40 and A beta 42. In addition, bryostatin ameliorated the rate of premature death and improved behavioral outcomes. Collectively, these data corroborate PKC and its activation as a potentially important means of ameliorating AD pathophysiology and perhaps cognitive impairment, thus offering a promising target for drug development. Because bryostatin 1 is devoid of tumor-promoting activity and is undergoing numerous clinical studies for cancer treatment in humans, it might be readily tested in patients as a potential therapeutic agent for Alzheimer's disease.

Folding and Stability of the Extracellular Domain of the Human Amyloid Precursor Protein

The beta-amyloid peptide (A beta), the major component of the senile plaques found in the brains of Alzheimer's disease patients, is derived from proteolytic processing of a transmembrane glycoprotein known as the amyloid precursor protein (APP). Human APP exists in various isoforms, of which the major ones contain 695, 751, and 770 amino acids. Proteolytic cleavage of APP by alpha- or beta-secretases releases the extracellular soluble fragments sAPP alpha or sAPP beta, respectively. Despite the fact that sAPP alpha plays important roles in both physiological and pathological processes in the brain, very little is known about its structure and stability. We have recently presented a structural model of sAPP alpha 695 obtained from small-angle x-ray scattering measurements (Gralle, M., Botelho, M. M., Oliveira, C. L. P., Torriani, I., and Ferreira, S. T. (2002) Biophys. J. 83, 3513-3524). We now report studies on the folding and stabilities of sAPP alpha 695 and sAPP alpha 770. The combined use of intrinsic fluorescence, 4-4'-Dianilino-1,1'binaphthyl-5,5'-disulfonic acid (bis-ANS) fluorescence, circular dichroism, differential ultraviolet absorption, and small-angle x-ray scattering measurements of the equilibrium unfolding of sAPP alpha 695 and sAPP alpha 770 by GdnHCl and urea revealed multistep folding pathways for both sAPP alpha isoforms. Such stepwise folding processes may be related to the identification of distinct structural domains in the three-dimensional model of sAPP alpha. Furthermore, the relatively low stability of the native state of sAPP alpha suggests that conformational plasticity may play a role in allowing APP to interact with a number of distinct physiological ligands.

Non-steroidal Anti-inflammatory Drugs Stimulate Secretion of Non-amyloidogenic Precursor Protein

Chronic inflammatory processes are associated with the pathophysiology of Alzheimer's disease (AD), and it has been proposed that treatment with non-steroidal anti-inflammatory drugs (NSAIDs) reduces the risk for AD. Here we report that various NSAIDs, such as the cyclooxygenase inhibitors, nimesulide, ibuprofen and indomethacin, as well as thalidomide (Thal) and its non-teratogenic analogue, supidimide, significantly stimulated the secretion of the non-amyloidogenic alpha-secretase form of the soluble amyloid precursor protein (sAPP alpha) into the conditioned media of SH-SY5Y neuroblastoma and PC12 cells. These NSAIDs markedly reduced the levels of the cellular APP holoprotein, further accelerating non-amyloidogenic processes. sAPP alpha release, induced by nimesulide and Thal, was modulated by inhibitors of protein kinase C and Erk mitogen-activated protein (MAP) kinase. Furthermore, in results complementary to the inhibitor studies, we show for the first time that NSAIDs can activate the Erk MAP kinase signaling cascade, thus identifying a novel pharmacology mechanism of NSAIDs. Our findings suggest that NSAIDs and Thal might prove useful to favor non-amyloidogenic APP processing by enhancing alpha-secretase activity, thereby reducing the formation of amyloidogenic derivatives, and therefore are of potential therapeutic value in AD.

Constitutive shedding of the amyloid precursor protein ectodomain is up-regulated by tumour necrosis factor-alpha converting enzyme.

The amyloid precursor protein (APP) of Alzheimer's disease is a transmembrane protein that is cleaved within its extracellular domain, liberating a soluble N-terminal fragment (sAPP alpha). Putative mediators of this process include three members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10 and ADAM17/TACE (tumour necrosis factor-alpha converting enzyme). Tumour necrosis factor-alpha protease inhibitor (TAPI-1), an inhibitor of ADAMs, reduced constitutive and muscarinic receptor-stimulated sAPP alpha release in HEK-293 cells stably expressing M3 muscarinic receptors. However, the former was less sensitive to TAPI-1 (IC(50)=8.09 microM) than the latter (IC(50)=3.61 microM), suggesting that these processes may be mediated by different metalloproteases. Constitutive sAPP alpha release was increased several-fold in cells transiently transfected with TACE, and this increase was proportional to TACE expression. In contrast, muscarinic-receptor-activated sAPP alpha release was not altered in TACE transfectants. TACE-dependent constitutive release of co-transfected APP(695) was inhibited by TAPI-1 with an IC(50) of 0.92 microm, a value significantly lower than the IC(50)s for inhibition of either constitutive or receptor-regulated sAPP alpha shedding mediated by endogenous secretases. The results indicate that TACE is capable of catalysing constitutive alpha-secretory cleavage of APP, but it is likely that additional members of the ADAM family mediate endogenous constitutive and receptor-coupled release of sAPP alpha in HEK-293 cells.

Astrocytes down-regulate neuronal beta-amyloid precursor protein expression and modify its processing in an apolipoprotein E isoform-specific manner.

Alzheimer's disease is the most frequent neurodegenerative disorder in the aged population and is characterized by the deposition of the 40/42-residue amyloid beta protein (A beta), a proteolytic fragment of the beta-amyloid precursor protein (APP). A common apolipoprotein E (apoE) polymorphism is associated with an increased risk of developing the disease. In order to assess the putative relationship between apoE and amyloidogenesis in the CNS, we prepared primary cortical neurons overexpressing humanized APP695 bearing the Swedish mutation (hAPP(695sw)) and we analysed APP expression and processing after: (i) coculture with primary astrocytes from wild-type, apoE-deficient (E0) mice, or mice overexpressing human apoE2, E3, or E4; (ii) treatment with conditioned media from apoE0, E2, E3 or E4 astrocytes; and (iii) treatment with human recombinant ApoE or human apoE purified from conditioned media of stably transfected RAW264 cells (E2, E3 and E4). Interestingly, a strong decrease in APP expression was observed only when neurons were cocultured with astrocytes (and independently of the apoE genotype considered), suggesting that cell-cell contact is required. Moreover, apoE4-secreting astrocytes, but not recombinant or purified apoE4, significantly increased A beta production and decrease sAPP alpha secretion only when cultured in direct contact with neurons, whereas apoE2 astrocytes had a protective effect. We conclude that astrocytes: (i) strongly regulate neuronal APP expression in primary neurons, and (ii) promote the amyloidogenic pathway in an apoE4-dependent manner. Thus, apoE and astrocytic factor(s) may modulate the pathogenesis of Alzheimer's disease.

Effect of HIV-1 gp41 peptides on secretion of beta-amyloid precursor protein in human astroglial cell line, T98G.

To understand the mechanism underlying cognitive deficits in AIDS patients, we examined the influence of gp41 peptides on the expression and the secretion of Alzheimer's amyloid precursor protein (APP) in human astroglial cell line T98G. Western blotting analyses demonstrated that treatment of glial cells with a putative immunosuppressive domain (aa 583-599) of gp41 remarkably downregulated the interleukin 1beta- (IL-1beta) induced elevation of the secreted form of APP (sAPP alpha) containing Kunitz-type protease inhibitor (KPI) domain without significant changes of the expression pattern of APP mRNAs as revealed by reverse transcriptase polymerase chain reaction (RT-PCR) analysis. Recombinant gp41 protein encoding for ectodomain, including aa 583-599 residues, also elicited a similar dose-dependent inhibitory effect, whereas the control peptides resulted in little change. The molecular mechanism underlying this gp41-mediated reduction of sAPP alpha secretion appears not to be owing to the difference in the function of extracellular proteases based on the finding of similar proteolytic activities responsible for APP metabolism in vitro present in the conditioned media from the cultures treated with or without gp41 peptide. However, the known PKC inhibitors such as H-7 or staurosporine, partially inhibited the elevation of sAPP alpha secretion in response to protein kinase C (PKC) agonist phorbol 12,13-dibutyrate (PdBu) as well as to IL-1beta, mimicking the immunosuppressive gp41 peptide. These observations implicate that part of the neurodegenerative cascade in AIDS brains may involve the inhibitory effect of gp41 on secretion of sAPP alpha, a potent glial neurotrophic factor, through impaired PKC response.

The deletion of the C-terminal tail and addition of an endoplasmic reticulum targeting signal to Alzheimer's amyloid precursor protein change its localization, secretion, and intracellular proteolysis.

The metabolic pathway of Alzheimer's amyloid precursor protein (APP) involves restricted intracellular proteolysis by secretases, which leads to the secretion of the N-terminal soluble APP (sAPP) and the generation of a cell-associated C-terminal fragment. The precise cellular sites at which these processes occur remain unknown. In this report, we describe the route of APP sorting and the processing site using novel systems with and without sorting signals on the APP molecule. One system involves the replacement of the C-terminal ten amino acids of APP with Adenoviral E19 protein containing an endoplasmic reticulum (ER) retrieval signal (APPE19); the other involves deleting the last ten amino acids corresponding to the replaced site (APPdeltaC10). APPE19 localized mainly within the cis/medial Golgi compartment and exclusively suppresses the secretion of APP. In contrast, deletion of the C-terminal tail promotes sAPP secretion by a constitutive secretion pathway. Metabolic labeling followed by immunoprecipitation with anti-APP antibody revealed that APPE19 is rapidly degraded within 30 min and that the subsequent intracellular turnover rate is decreased with 40% of the protein retained within the cells even after a chase period a 3 h. In contrast, APPdeltaC10 is rapidly eliminated from the intracellular compartments and secreted into the culture medium. The surface internalization and recycling processes of this protein are relatively impaired compared with wild-type APP. The ratios of the levels of production to secretion of sAPP alpha, the N-terminal, soluble APP fragment released by alpha-secretase, are proportional to the secretion efficiencies among APP species, suggesting the localization of alpha-secretase within a compartment late in the constitutive secretion pathway. These secretion mutants which utilize ER targeting signals are useful tools for analyzing the location of secretases and the intracellular degradation system within a constitutive secretion pathway such as ER quality control.

Cellular actions of beta-amyloid precursor protein and its soluble and fibrillogenic derivatives.

beta-Amyloid precursor protein (beta-APP), the source of the fibrillogenic amyloid beta-peptide (A beta) that accumulates in the brain of victims of Alzheimer's disease, is a multifunctional protein that is widely expressed in the nervous system. beta-Amyloid precursor protein is axonally transported and accumulates in presynaptic terminals and growth cones. A secreted form of beta-APP (sAPP alpha) is released from neurons in response to electrical activity and may function in modulation of neuronal excitability, synaptic plasticity, neurite outgrowth, synaptogenesis, and cell survival. A signaling pathway involving guanosine 3',5'-cyclic monophosphate is activated by sAPP alpha and modulates the activities of potassium channels, N-methyl-D-aspartate receptors, and the transcription factor NF kappa B. Additional functions of beta-APP may include modulation of cell adhesion and regulation of proliferation of nonneuronal cells. Alternative enzymatic processing of beta-APP liberates A beta, which has a propensity to form amyloid fibrils; A beta can damage and kill neurons and increase their vulnerability to excitotoxicity. The mechanism involves generation of oxyradicals and impairment of membrane transport systems (e.g., ion-motive ATPases and glutamate and glucose transporters). Genetic mutations or age-related metabolic changes may promote neuronal degeneration in Alzheimer's disease by increasing production of A beta and/or decreasing levels of neuroprotective sAPP alpha.

Secreted form of beta-amyloid precursor protein shifts the frequency dependency for induction of LTD, and enhances LTP in hippocampal slices.

The secreted form of beta-amyloid precursor protein (sAPP alpha) is released from neurons in an activity-dependent manner, and has been reported to modulate neuronal excitability in dissociated hippocampal neurons. We now report that sAPP alpha shifts the frequency dependence for induction of long-term depression of synaptic transmission (LTD) in hippocampal slices from adult rats. Whereas low frequency stimulation (1 Hz) of Schaffer collateral axons induced LTD of the post-synaptic response of CA1 neurons in control slices, it did not induce LTD in slices pretreated with sAPP alpha. On the other hand, whereas a 10 Hz stimulation normally induced neither LTD or LTP, it did induce LTD in slices pretreated with sAPP alpha. sAPP alpha potentiated LTP induced by high frequency stimulation. sAPP alpha induced cGMP production in hippocampal slices, and pretreatment of slices with 8-bromo-cyclic GMP mimicked the effect of sAPP alpha on LTD suggesting a role for cyclic GMP in modulation of LTD. The data suggest an important role for sAPP alpha in modulation of synaptic plasticity in the hippocampus.

Defective neurite extension is caused by a mutation in amyloid ?/A4 (A?) protein precursor found in Familial Alzheimer's Disease

Clonal central nervous system neuronal cells, B103, do not synthesize detectable endogenous APP or APLP. B103 cells transfected with both wild-type (B103/APP) and mutant APP construct (B103/APP delta NL) secreted comparable amounts of soluble forms of APP (sAPP). B103/APP cells produced sAPP and cleaved at amyloid beta/A4 (A beta) 16, the alpha-secretase site, and B103/APP delta NL cells produced sAPP beta cleaved at A beta 1, the beta-secretase site. B103/APP delta NL cells developed fewer neurites than B103/APP cells in a serum-free defined medium. Neurite numbers of parent B103 cells were increased by the 50% conditioned medium (CM) from B103/APP cells but reduced by the CM from B103/APP delta NL cells. Chemically synthesized A beta at concentration levels higher than 1 nM reduced numbers of neurites from B103 or B103/APP delta NL cells. However, A beta at 1-100 nM could not reduce the neurite number of B103/APP cells. The protective activity against A beta's deleterious effect to reduce neurite numbers was attributed to sAPP alpha in the CM. Although sAPP alpha could block the effect of A beta, sAPP beta could not do so under the identical condition, suggesting the importance of the C-terminal 15-amino acid sequence in sAPP alpha. Nevertheless, sAPP alpha's protective activity required the N-terminal sequence around RERMS, previously identified to be the active domain of sAPP beta. The overall effect of APP mutation which overproduced A beta and sAPP beta and underproduced sAPP alpha was a marked decline in the neurotrophic effect of APP. We suggest that the disruption of balance between the detrimental effect of A beta and the trophic effect of sAPP may be important in the pathogenesis of AD caused by this pathogenic APP mutation.

Increased activity-regulating and neuroprotective efficacy of alpha-secretase-derived secreted amyloid precursor protein conferred by a C-terminal heparin-binding domain.

Proteolytic cleavage of beta-amyloid precursor protein (beta APP) by alpha-secretase results in release of one secreted form (sAPP) of APP (sAPP alpha), whereas cleavage by beta-secretase releases a C-terminally truncated sAPP (sAPP beta) plus amyloid beta-peptide (A beta). beta APP mutations linked to some inherited forms of Alzheimer's disease may alter its processing such that levels of sAPP alpha are reduced and levels of sAPP beta increased. sAPP alpha s may play important roles in neuronal plasticity and survival, whereas A beta can be neurotoxic. sAPP alpha was approximately 100-fold more potent than sAPP beta in protecting hippocampal neurons against excitotoxicity, A beta toxicity, and glucose deprivation. Whole-cell patch clamp and calcium imaging analyses showed that sAPP beta was less effective than sAPP alpha in suppressing synaptic activity, activating K+ channels, and attenuating calcium responses to glutamate. Using various truncated sAPP alpha and sAPP beta APP695 products generated by eukaryotic and prokaryotic expression systems, and synthetic sAPP peptides, the activity of sAPP alpha was localized to amino acids 591-612 at the C-terminus. Heparinases greatly reduced the actions of sAPP alpha s, indicating a role for a heparin-binding domain at the C-terminus of sAPP alpha in receptor activation. These findings indicate that alternative processing of beta APP has profound effects on the bioactivity of the resultant sAPP products and suggest that reduced levels of sAPP alpha could contribute to neuronal degeneration in Alzheimer's disease.

Differential Effects of a Rab6 Mutant on Secretory Versus Amyloidogenic Processing of Alzheimer's β-Amyloid Precursor Protein

The Ras-related GTP-binding protein, Rab6, is localized in late Golgi compartments where it mediates intra-Golgi vesicular trafficking. Herein we report that coexpression of Alzheimer's beta-amyloid precursor protein (beta APP751) with a dominant-negative Rab6 mutant (Rab6N126I) in human embryonal kidney 293 cells causes an increase in secretion of the soluble amino-terminal exodomain (s-APP alpha) derived from non-amyloidogenic processing of beta-APP751 by alpha-secretase. The effect was specific to Rab6N126I, since the corresponding mutation in Rab8 (i.e. Rab8N121I), which has been implicated in protein transport to the plasma membrane, caused a modest reduction in s-APP alpha secretion. While Rab6N126I stimulated secretion of APP alpha, the accumulation of amyloid beta peptide (A beta) in the medium was either moderately reduced or unaffected. Similar differential effects of Rab6N126I on secretion of s-APP alpha versus A beta were observed in cell cultures that were overproducing A beta after transfection with a plasmid encoding Swedish variant of beta APP751. Moreover, assays of medium from the latter cultures revealed a marked increase in secretion of s-APP alpha relative to s-APP beta (the immediate product derived from cleavage of beta APP by beta-secretase). The results indicate that vesicular transport events controlled by Rab6 occur at or near a critical juncture in the trans-Golgi network where beta APP is sorted into either the constitutive alpha-secretase pathway or the amyloidogenic beta-secretase pathway.