Dimerization leads to changes in APP (amyloid precursor protein) trafficking mediated by LRP1 and SorLA.

Proteolytic cleavage of the amyloid precursor protein (APP) by α-, ß- and γ-secretases is a determining factor in Alzheimer's disease (AD). Imbalances in the activity of all three enzymes can result in alterations towards pathogenic Aß production. Proteolysis of APP is strongly linked to its subcellular localization as the secretases involved are distributed in different cellular compartments. APP has been shown to dimerize in cis-orientation, affecting Aß production. This might be explained by different substrate properties defined by the APP oligomerization state or alternatively by altered APP monomer/dimer localization. We investigated the latter hypothesis using two different APP dimerization systems in HeLa cells. Dimerization caused a decreased localization of APP to the Golgi and at the plasma membrane, whereas the levels in the ER and in endosomes were increased. Furthermore, we observed via live cell imaging and biochemical analyses that APP dimerization affects its interaction with LRP1 and SorLA, suggesting that APP dimerization modulates its interplay with sorting molecules and in turn its localization and processing. Thus, pharmacological approaches targeting APP oligomerization properties might open novel strategies for treatment of AD.

Molecular mechanisms and therapeutic application of NSAIDs and derived compounds in Alzheimer's disease.

Alzheimer's disease (AD) is the most common form of neurodegenerative dementias worldwide. Amyloid-ß deposition, neurofibrillary tangle formation and Neuroinflammation are the major pathogenetic mechanisms that in concert lead to memory dysfunction and decline of cognition. To date, there is no curative treatment for AD. Epidemiological analysis support the notion that sustained intake of non-steroidal anti-inflammatory drugs (NSAIDs) reduce the risk and delay the onset of AD. In contrast, therapeutic studies testing NSAID efficacy in AD patients have not yielded positive results. This suggests that either the investigated drugs have not addressed the mechanism of action required for mediating beneficial effects or that NSAIDs are effective at stages way before clinical onset of symptoms. The NSAIDs concerned are pleiotrophic in nature and interact with more than one pathomechanism. Therefore evidence for more than one neuroprotective action of NSAIDs has been put forward and it seems likely that some of the drugs act at multiple levels through more than one molecular mechanism. Some, even may not only be beneficial, but negative actions may be overruled by protective effects. Within these mechanisms, modulation of γ-secretase activity, the activation of the peroxisome proliferator-activated receptor-γ, binding to prostaglandin receptors or interactions at the blood-brain barrier may account for the observed protection from AD. This article reviews the current knowledge and views on the above mechanisms and critically discusses current obstacles and the potential as future AD therapeutics.

Blood-brain-barrier models for the investigation of transporter- and receptor-mediated amyloid-ß clearance in Alzheimer's disease.

Alzheimer's disease (AD) is the most common form of dementia in the elderly with more than 26 million people worldwide living with the disease. Besides the main neuropathological hallmarks of AD, provoked by the accumulation of amyloid-ß (Aß) and tau hyperphosphorylation, other cells and cellular systems such as microglia and the neurovascular unit establishing the blood-brain-barrier (BBB) have been implicated to play a role in AD etiopathology. Insulating the brain from the blood stream, the BBB facilitates supply and disposal of nutrients and metabolites by the expression of transporters and transcytotic receptors at the polarized endothelial cell (EC) surface. Recently, several proteins involved in Aß transport across the BBB have been identified in in vitro and in vivo studies. In this review, we summarize recent evidence of receptor- and transporter-mediated Aß clearance across the BBB. Furthermore, we discuss the models used to identify and characterize Aß transport across the BBB in regard to barrier properties and suitability of the models for the experimental investigation of transport mechanisms involved in Aß clearance across an EC barrier.

A subset of NSAIDs lower amyloidogenic Abeta42 independently of cyclooxygenase activity.

Epidemiological studies have documented a reduced prevalence of Alzheimer's disease among users of nonsteroidal anti-inflammatory drugs (NSAIDs). It has been proposed that NSAIDs exert their beneficial effects in part by reducing neurotoxic inflammatory responses in the brain, although this mechanism has not been proved. Here we report that the NSAIDs ibuprofen, indomethacin and sulindac sulphide preferentially decrease the highly amyloidogenic Abeta42 peptide (the 42-residue isoform of the amyloid-beta peptide) produced from a variety of cultured cells by as much as 80%. This effect was not seen in all NSAIDs and seems not to be mediated by inhibition of cyclooxygenase (COX) activity, the principal pharmacological target of NSAIDs. Furthermore, short-term administration of ibuprofen to mice that produce mutant beta-amyloid precursor protein (APP) lowered their brain levels of Abeta42. In cultured cells, the decrease in Abeta42 secretion was accompanied by an increase in the Abeta(1-38) isoform, indicating that NSAIDs subtly alter gamma-secretase activity without significantly perturbing other APP processing pathways or Notch cleavage. Our findings suggest that NSAIDs directly affect amyloid pathology in the brain by reducing Abeta42 peptide levels independently of COX activity and that this Abeta42-lowering activity could be optimized to selectively target the pathogenic Abeta42 species.

The amyloidogenic pathway of amyloid precursor protein (APP) is independent of its cleavage by caspases.

Amyloid beta-protein (A beta) is the main constituent of senile plaques in Alzheimer's disease and is derived by proteolysis from the amyloid precursor protein (APP). Generation and secretion of both A beta 40 and A beta 42 isoforms depend largely on internalization of APP and occurs mainly in the endocytic pathway. Evidence has also been presented (Gervais, F. G., Xu, D., Robertson, G. S., Vaillancourt, J. P., Zhu, Y., Huang, J., LeBlanc, A., Smith, D., Rigby, M., Shearman, M. S., Clarke, E. E., Zheng, H., Van der Ploeg, L. H. T., Ruffolo, S. C., Thornberry, N. A., Xanthoudakis, S., Zamboni, R. J., Roy, S., and Nicholson, D. W. (1999) Cell, 97, 395--406) that caspase cleavage of APP at its cytosolic tail affects its processing such that it is redirected to a more amyloidogenic pathway, resulting in enhanced A beta generation. However, caspase cleavage of APP also results in loss of its endocytosis signal (YENP), an event that would predict a decline in internalization and a concomitant decrease, not an increase, in A beta generation. In the present work, we examined whether caspase cleavage of APP is relevant to amyloidogenesis. We found that 1) caspase cleavage of APP results in reduced internalization and, accordingly, a decline in A beta secretion; 2) masking of the caspase site in APP did not affect A beta levels and, 3) caspase activation in cells by serum withdrawal did not increase A beta secretion. Thus, caspase cleavage of APP is unlikely to play a direct role in amyloidogenesis.

Modulation of amyloid beta-protein clearance and Alzheimer's disease susceptibility by the LDL receptor-related protein pathway.

Susceptibility to Alzheimer's disease (AD) is governed by multiple genetic factors. Remarkably, the LDL receptor-related protein (LRP) and its ligands, apoE and alpha2M, are all genetically associated with AD. In this study, we provide evidence for the involvement of the LRP pathway in amyloid deposition through sequestration and removal of soluble amyloid beta-protein (Abeta). We demonstrate in vitro that LRP mediates the clearance of both Abeta40 and Abeta42 through a bona fide receptor-mediated uptake mechanism. In vivo, reduced LRP expression is associated with LRP genotypes and is correlated with enhanced soluble Abeta levels and amyloid deposition. Although LRP has been proposed to be a clearance pathway for Abeta, this work provides the first in vivo evidence that the LRP pathway may modulate Abeta deposition and AD susceptibility by regulating the removal of soluble Abeta.

Binding and selective detection of the secretory N-terminal domain of the alzheimer amyloid precursor protein on cell surfaces.

The secretory N-terminal domain of the Alzheimer amyloid precursor protein (sAPP) evokes specific responses in cells on binding to their surfaces. Because APP is expressed in a large variety of cell types, the localization of sAPP binding requires detection techniques that selectively recognize sAPP as a ligand. For this purpose, we prepared antibodies against recombinant sAPP695 (sAPPrec) previously expressed in E. coli. Such antibodies were found to distinguish between sAPPrec and cellular APP or sAPP, as shown by immunocytochemistry and by immunoblot. In addition, they allowed the selective localization of bound sAPPrec on cell surfaces without any signal from cellular APP or sAPP. Saturation of sAPPrec binding to cell surfaces, as determined radiometrically, was reached at 10 nM [125I]-sAPPrec. Binding was specific because it was almost completely inhibited by a 100-fold excess of unlabeled sAPPrec. This specificity of binding was confirmed by surface plasmon resonance spectroscopy. Binding of sAPPrec to cell surfaces occurred in patches and was dependent on the state of cell differentiation. The sAPPrec used in this study contains heparin binding sites, but enzymatic removal of cell surface associated heparin did not affect sAPPrec binding. Aldehyde fixation of cells strongly inhibited their ability to bind sAPPrec. The data point to a fixation-sensitive sAPPrec binding protein which is detectable in the form of patches and therefore is part of assembled cell surface microdomains.

From differentiation to proliferation: the secretory amyloid precursor protein as a local mediator of growth in thyroid epithelial cells.

In various species, thyrotropin (TSH) is known to stimulate both differentiation and proliferation of thyroid follicle cells. This cell type has also been shown to express members of the Alzheimer amyloid precursor (APP) protein family and to release the secretory N-terminal domain of APP (sAPP) in a TSH-dependent fashion. In this study on binding to the cell surfaces, exogenously added recombinant sAPP stimulated phosphorylation mediated by mitogen-activated protein kinase and effectively evoked proliferation in the rat thyroid epithelial cell line FRTL-5. To see whether this proliverative effect of sAPP is of physiological relevance, we used antisense techniques to selectively inhibit the expression of APP and the proteolytic release of sAPP by cells grown in the presence of TSH. The antisense-induced inhibition was detected by immunoblot, immunoprecipitation, and immunocytochemical analyses. After the reduced APP expression and sAPP secretion, we observed a strong suppression of the TSH-induced cell proliferation down to 35%. Recombinant sAPP but not TSH was able to overcome this antisense effect and to completely restore cell proliferation, indicating that sAPP acts downstream of TSH, in that it is released from thyroid epithelial cells during TSH-induced differentiation. We propose that sAPP operates as an autocrine growth factor mediating the proliferative effect of TSH on neighboring thyroid epithelial cells.

Growth regulation of rat thyrocytes (FRTL-5 cells) by the secreted ectodomain of beta-amyloid precursor-like proteins.

The TSH-dependent expression of amyloid precursor-like proteins and the secretion of their ectodomain (sAPP) in rat thyroids coincide with increased rates of thyrocyte proliferation. To analyze whether the secretion of sAPP and the proliferation of thyrocytes are regulatorily linked, we employed [3H]thymidine or 5-bromo-2'-deoxyuridine assays and found that conditioned culture medium stimulated the proliferation of FRTL-5 cells depending on the content of sAPP. These observations prompted experiments with sAPP-derived peptides known to stimulate the growth of APP-deficient fibroblasts. Using autoradiography and radiochemical assays, we observed that an iodinated 19-mer sAPP peptide was bound specifically to the surface of FRTL-5 cells. Binding of this peptide was followed by a 2- to 8-fold increase in cell proliferation, which reached a plateau at 1 nM. This effect was significant only when cells were cultured in nonconfluent monolayers, and contact inhibition did not interfere. Our observations indicate that sAPP and sAPP-derived peptides increased the proportion of proliferation-competent FRTL-5 cells and suggest that sAPP may be a new member in the family of peptides involved in the growth regulation of thyrocytes.