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J. Biol. Chem., Vol. 277, Issue 18, 15666-15670, May 3, 2002

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Intracellular Amyloid-1-42, but Not Extracellular Soluble Amyloid- Peptides, Induces Neuronal Apoptosis^*

Pascal Kienlen-Campard, Sarah Miolet, Bernadette Tasiaux, and Jean-Noël Octave

From the Université Catholique de Louvain, FARL/UCL 54 10, av Hippocrate 54, B-1200 Brussels, Belgium

Received for publication, January 28, 2002, and in revised form, February 21, 2002

	ABSTRACT

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Alzheimer disease (AD), the most frequent cause of dementia, is characterized by an important neuronal loss. A typical histological hallmark of AD is the extracellular deposition of -amyloid peptide (A), which is produced by the cleavage of the amyloid precursor protein (APP). Most of the gene mutations that segregate with the inherited forms of AD result in increasing the ratio of A42/A40 production. A42 also accumulates in neurons of AD patients. Altogether, these data strongly suggest that the neuronal production of A42 is a critical event in AD, but the intraneuronal A42 toxicity has never been demonstrated. Here, we report that the long term expression of human APP in rat cortical neurons induces apoptosis. Although APP processing leads to production of extracellular A1-40 and soluble APP, these extracellular derivatives do not induce neuronal death. On the contrary, neurons undergo apoptosis as soon as they accumulate intracellular A1-42 following the expression of full-length APP or a C-terminal deleted APP isoform. The inhibition of intraneuronal A1-42 production by a functional -secretase inhibitor increases neuronal survival. Therefore, the accumulation of intraneuronal A1-42 is the key event in the neurodegenerative process that we observed.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

A clear diagnosis of AD¹ can be performed by correlating clinical findings and postmortem examination of brain sections. AD is characterized by a massive neuronal loss in vulnerable brain regions (1, 2). Two typical hallmarks of AD are neurofibrillary tangles and senile plaques (3). The major constituent of the amyloid core of senile plaques is the -amyloid or A peptide. The A peptide is a 39-43-amino acid peptide produced from a larger precursor, the amyloid precursor protein or APP (4). Among the ten identified isoforms of human APP (5), eight contain the A sequence. The isoform that is mainly expressed in the human brain is a 695-amino acid protein known as APP695 (4). APP is processed by the non-amyloidogenic pathway, where secretase activity (6) produces soluble forms of APP, and by the amyloidogenic pathway, where -secretase (7) and -secretase activities allow the release of A. Several identified mutations in the APP and the presenilins genes segregate with inherited forms of AD known as early onset familial Alzheimer disease or FAD (8, 9). Most of these mutations result in an increased production of the A ending at position 42 (10). In vitro studies have shown that A42 rapidly aggregates into fibrils and that extracellular fibrillar A peptides induce apoptosis in cultured neurons (11). On the other hand, recent reports have demonstrated an intraneuronal accumulation of A42 in AD-vulnerable brain regions (12, 13). Intraneuronal A42 accumulation has also been reported in transgenic mice expressing FAD proteins (14) as well as in transgenic mice showing accelerated neurodegeneration without extracellular amyloid deposition (15). Altogether, these data support the idea that A42 accumulation and neuronal loss are closely correlated. Nevertheless, the direct link between A production by neurons and neuronal death has not been clearly established until now.

Here, we report that the long term expression of human APP in rat-cultured neurons induces apoptosis. To understand how APP expression and processing modify neuronal survival, we characterized the extracellular and intracellular A isoforms produced by the processing of different APP constructs. We further demonstrated that APP-induced neuronal apoptosis depends on intraneuronal A1-42 accumulation.

	EXPERIMENTAL PROCEDURES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Cell Cultures and Reagents-- Primary cultures of cortical neurons were prepared from 17-day-old Wistar rat embryos as described previously (16). Cells were plated in 6- or 96-well culture dishes (4 × 10⁵ cells/cm²) or glass coverslips (1.25 × 10⁵ cells/cm²) pretreated with poly(L-lysine) (10 µg/ml in phosphate-buffered saline) and cultured for 6 days in vitro in NEUROBASAL^TM medium supplemented with 2% B-27 and 0.5 mM L-glutamine prior to infection with recombinant adenoviruses. Under these conditions, neuronal cultures (up to 98% of neurons) display high differentiation and survival rates (17). Transfected CHO cell lines expressing human APP695 (18) were cultured in F12 medium containing 10% fetal calf serum for 48 h before the culture medium was collected. DAPT, a functional -secretase inhibitor (19), was kindly provided by Aventis Pharma.

Recombinant Adenoviruses and Neuronal Infection-- The construction of recombinant adenoviruses encoding -galactosidase (AdRSV-gal) has been described previously (20, 21). The pAdRSVAPP695 vector was generated by subcloning the EcoRV-SalI fragment of pHMGAPP695 (22) in a pAdRSV vector (20). The deletion of the APP 695 intracellular domain was generated by PCR amplification of the APP 695 sequence using a 5' primer (5'-AACGAAGTTGAGCCTGTTGATG-3') encoding residues 560-567 of APP 695 and containing a BglII restriction site and a 3' primer (5'-GTCGACCTAGTACTGTTTCTTCTTCAGC-3') complementary to the sequence encoding residues 648-653 followed by a stop codon and a SalI restriction site. pAdRSVAPPC was generated by insertion of the BglII-SalI-digested PCR product in the BglII-SalI sites of pAdRSVAPP695. Production, propagation, and purification of adenoviruses (AdRSVAPP, AdRSVAPPC, and AdRSV-gal) were performed as described previously (20). After 6 days in vitro, neuronal cultures were infected at the multiplicity of infection of 100 for 4 h in a minimal volume of culture medium. Infection medium was then replaced by fresh culture medium for 3-5 days. Under these conditions, at least 75% of neurons express the proteins encoded by recombinant adenoviruses (16).

Survival Assays and Nuclear Staining-- Neuronal survival was measured by the colorimetric MTT assay as described previously (23). Neurons grown in 96-well culture dishes were incubated after infection for 2 h at 37 °C in fresh culture medium containing 0.5 mg/ml MTT. Medium was removed, and dark blue crystals formed were dissolved by adding 100 µl/well of lysis solution (isopropyl alcohol/0.04 N HCl). Outer diameter was measured on a microplate reader (492 nm). For nuclear staining, cells were fixed (0.37% formaldehyde/0.2% glutaraldehyde in phosphate-buffered saline) and incubated for 30 min in the Hoechst 33342 dye (1 µg/ml). Nuclear morphology was analyzed under fluorescence microscopy at excitation/emission wavelengths of 350/450 nm.

Protein Analysis by Western Blot-- Cell culture medium and cell lysates were analyzed by Western blot as described previously (16). Cell lysates (20 µg of protein) and culture medium (15 µl) were subjected to 10% SDS-PAGE and blotted onto nitrocellulose membrane, incubated overnight at 4 °C with human APP-specific primary WO2 antibody at 1 µg/ml (24), washed, and incubated with 1/10,000 anti-mouse Ig horseradish peroxidase-conjugated secondary antibody followed by ECL revelation.

Immunoprecipitation and Quantification of A Production-- A production was monitored by immunoprecipitation of cell culture medium. The quantification of A1-40 and A1-42 isoforms was performed by ELISA. Culture medium was collected, treated with protease inhibitors (1 µg/ml pepstatin, 10 µg/ml leupeptin, 1 mM phenylmethylsulfonyl fluoride), and cleared by centrifugation (16,000 × g, 5 min, 4 °C). One hundred µl of the supernatant was used for A quantification by fluorescent sandwich ELISA according to the manufacturer's instructions (BIOSOURCE, Camarillo, CA). Previous experiments showed that there is no cross-reaction between A1-40 and A1-42 recognition. Fluorescence emission was measured at excitation/emission wavelengths of 485 nm/535 nm. Immunoprecipitation was performed on 1.5 ml of the remaining culture medium with 15 µl/ml anti-A whole rabbit serum (16). The immunoprecipitate was analyzed by Western blot on a 4-12% Nupage^TM gel using the WO2 antibody. A was extracted from cell lysates by a modification of the protocol described previously (25). Neurons (~10⁷ cells) were scraped and pelleted in cold phosphate-buffered saline. Cell pellets were solubilized in 300 µl of formic acid (70%). Formic acid-solubilized cell pellets were cleared (16,000 × g, 5 min, 4 °C) to remove cell debris, and supernatants were centrifuged at 21,000 × g, 4 °C for 20 min. The supernatants were vacuum-dried, and the resulting pellet was resuspended in 1 ml of alkaline carbonate buffer (2% Na₂CO₃, 0.1 N NaOH) and centrifuged (16,000 × g, 3 min, 4 °C). Protein concentration was measured on 50 µl of the resulting supernatant by using the BCA protein assay (Pierce). For immunoprecipitation, 800 µl of the supernatant was neutralized with 800 µl of 1 M Tris-HCl, pH 6.8, and diluted 1:3 in H₂O. Immunoprecipitation was performed as described above. For ELISA, 100 µl of the remaining supernatant was neutralized with 100 µl of 1 M Tris-HCl, pH 6.8, and diluted 1:3 in H₂O containing 10% fetal calf serum, 0.5% Triton X-100, and 0.5% Nonidet P-40 (final concentrations). A1-40 and A1-42 concentrations were measured by ELISA on 100 µl of neutralized extract.

Statistical Analysis-- The number of samples (n) in each experimental condition is indicated in the figure legends. Statistical analysis was performed by one-way analysis of variance (ANOVA) followed by Bonferroni's multiple comparison post-test.

	RESULTS

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Long Term Expression of Human APP in Rat Cortical Neurons Induces Apoptosis-- When rat cortical neurons are infected by AdRSVAPP or AdRSV-gal, a maximal and stable production of APP and -gal is observed at day 3 postinfection (16). APP expression and processing have been analyzed by using the human-specific WO2 antibody (24). Five days after infection by AdRSVAPP, high levels of soluble human APP (sAPP) and A are detected in the culture medium (Fig. 1A). This indicates that human APP is efficiently processed through both non-amyloidogenic and amyloidogenic pathways in rat neurons. The extracellular A was quantified in the culture medium of neurons expressing human APP (Fig. 1B). These neurons secrete about 100 pg/ml of A 1-40, but there is no detectable extracellular A1-42 (not shown). In the same experimental conditions, the MTT survival assay shows that human APP expression induces neuronal death, whereas -gal expression has no effect (Fig. 1C). Taken together, these results demonstrate that a 5-day expression and processing of human APP in rat cortical neurons exert strong neurotoxic effects.

View larger version (40K): [in this window] [in a new window]   Fig. 1.   Expression and processing of human APP triggers apoptosis in rat neurons. A, analysis of neuronal culture medium 5 days after infection by AdRSVAPP (APP) or AdRSVgal (-gal). Western blot (upper panel) showing the accumulation of sAPP in the culture medium of APP-infected neurons (NI = non-infected) and A immunoprecipitation (lower panel) of the same medium. B, quantification of A production by ELISA. Under the sensitivity threshold of the test (15 pg/ml), A is not detectable (). Results are given as mean ±S.E., (n = 6). C, neuronal survival measured by MTT assay. Results (mean ±S.E.) are given as the percentage of survival as compared with non-infected control cultures (**, p < 0.01, as compared with control; n = 8). D, nuclear morphology analysis (Hoechst 33342 staining) of non-infected (upper panel), AdRSVgal-(middle panel), and AdRSVAPP-(lower panel) infected neurons. The nuclear shape of surviving neurons (filled arrow) or apoptotic neurons (open arrow) is indicated. Scale bar = 150 µm. E, quantification of the morphological analysis. For each condition, 10 fields of 3 independent cultures were analyzed. Results are given as the percentage of apoptotic neurons per field (**, p < 0.01, as compared with non-infected (NI) control).

To further study the mechanism of the neuronal death triggered by human APP expression, neurons were stained with the Hoechst 3342 nuclear dye. This morphological analysis of the nuclei allows us to discriminate between the surviving and the apoptotic cells that display high nuclear condensation or fragmentation (26). Nuclear morphology of neurons was analyzed 5 days after infection by AdRSV-gal or AdRSVAPP (Fig. 1D), and the results were quantified to compare the proportion of apoptotic neurons in each condition (Fig. 1E). There is a 2-fold increase of apoptotic nuclei in neurons expressing APP as compared with the non-infected or -gal-expressing neurons. This establishes that a 5-day expression of human APP induces apoptosis in rat-cultured neurons.

APP-induced Neuronal Apoptosis Does Not Involve Extracellular APP and A-- We next analyzed whether the extracellular secretion of APP and A could be responsible for the neurotoxic effects observed. To that end, neurons were incubated in the culture medium of a transfected CHO cell line or neuronal cultures expressing human APP695. The analysis of these two conditioned media indicates that they contain similar amounts of sAPP but different amounts of A (Fig. 2A). The quantification of A shows that the CHO-conditioned medium contains almost 100-fold more A1-40 than the neuronal-conditioned medium. In addition, A1-42 is present in the CHO culture medium, whereas it is undetectable in the neuronal culture medium (Fig. 2B). The treatment of neurons with these conditioned media does not significantly modify neuronal survival (Fig. 2C). Altogether, these results demonstrate that, in our model, the neuronal apoptosis induced by human APP is not triggered by any APP derivative, including A1-40 and A1-42, secreted in the culture medium. This raises the hypothesis that intracellular APP derivatives could be responsible for its neurotoxic effects.

View larger version (18K): [in this window] [in a new window]   Fig. 2.   Extracellular APP and A do not modify neuronal survival. Culture medium from neurons infected by AdRSVAPP (APP neuron) or from a CHO cell line stably expressing APP695 (APP CHO) were used to treat control neurons prior to the survival assay (NI = culture medium of non-infected neurons). A, the presence of sAPP in the conditioned medium was analyzed, before treatment, by Western blot (upper panel), and the presence of A was monitored by immunoprecipitation (lower panel). B, quantification by ELISA of the A present in the culture medium before treatment (mean ±S.E., n = 3). C, neuronal survival measured 2 days after treatment (n = 8).

APP-induced Neuronal Apoptosis Does Not Involve the Intracellular C-terminal Domain of APP-- A possible origin of APP-induced neuronal death could be related to the intracellular C-terminal domain of the protein, which has been demonstrated to induce apoptosis in other cellular models (27, 28). To test this hypothesis, neurons were infected with a recombinant adenovirus (AdRSVAPPC) encoding a human APP isoform deleted in the intracellular C terminus of the protein. Five days after infection by AdRSVAPPC, sAPP and A were detected in the neuronal culture medium (Fig. 3A). Neurons expressing human APPC secrete about 50 pg/ml of A 1-40 (Fig. 3B). This corresponds to half of the concentration of extracellular A produced following the expression of full-length APP (Fig. 1B). In the same experimental conditions, the MTT survival assay shows that human APPC expression induces neuronal death (Fig. 3C). Hoechst staining indicates that APPC, like APP, triggers neuronal apoptosis (not shown). Taken together, these results demonstrate that the intracellular C-terminal domain of human APP is not involved in the neuronal death observed.

View larger version (24K): [in this window] [in a new window]   Fig. 3.   Expression and processing of C-terminal deleted APP display neurotoxic effects. A, analysis of neuronal culture medium 5 days after infection by AdRSVAPPC (APPC) or AdRSVgal (-gal). Western blot (upper panel) showing the accumulation of sAPP in the culture medium of APPC-infected neurons (NI = non-infected) and A immunoprecipitation (lower panel) of the same culture medium. B, quantification of A production by ELISA. Under the sensitivity threshold of the test (15 pg/ml), A is not detectable (). Results are given as mean ±S.E. (n = 6). C, neuronal survival measured by MTT assay. Results (mean ±S.E.) are given as the percentage of survival as compared with non-infected control cultures (**, p < 0.01, as compared with control; n = 8).

Intracellular A1-42 Accumulation Induces Neuronal Apoptosis-- Since the neurotoxic effect of APP does not involve the intracellular domain of the protein, we investigated whether the accumulation of intraneuronal A could trigger apoptosis. An important fraction of intracellular A has been shown to be insoluble (29, 30). Therefore, cells were solubilized in 70% formic acid as described previously (25) to recover all the intraneuronal A peptide. The analysis of formic acid-solubilized cell pellets after 3 days of infection by AdRSVAPP or AdRSVAPPC reveals a similar expression pattern of the intraneuronal human proteins, although APPC is detected in lower amounts as compared with APP (Fig. 4A). In the same experimental conditions, immunoprecipitation of cellular extracts show that intraneuronal A is undetectable (Fig. 4A). The survival assay shows that, after 3 days of infection, neither APP nor APPC expression causes neurotoxic effects (Fig. 4C), indicating that the overexpression of different levels of APP or APPC per se does not induce any neurotoxicity. After 5 days of infection, neurons still express different amounts of APP or APPC, but they accumulate similar amounts of intraneuronal A 1-42. (Fig. 4B) In all these experiments, intraneuronal A1-40 was not detectable (not shown). After 5 days, both APP and APPC induce a massive neuronal death, as compared with non-infected or -gal-expressing neurons (Fig. 4D). Taken together, these results clearly establish that, in our model, APP-induced neuronal death takes place only when intraneuronal A1-42 is detected.

View larger version (27K): [in this window] [in a new window]   Fig. 4.   APP-induced neuronal death is correlated with intraneuronal A1-42 production. Neuronal cultures were infected by AdRSVgal (-gal), AdRSVAPP (APP), or AdRSVAPPC (APPC) for 3 days (3d) or 5 days (5d). A, Western blot of formic acid-solubilized cell pellets showing the presence of full-length intraneuronal APP (upper panel) and A immunoprecipitation of the same cellular extracts (lower panel). B, quantification of intraneuronal A142 production by ELISA (mean ±S.E., n = 4). C, neuronal survival measured by MTT assay 3 days after infection. Results are given as the percentage of neuronal survival as compared with non-infected control cultures (n = 12). D, neuronal survival measured by MTT assay 5 days after infection. Results are given as the percentage of neuronal survival as compared with non-infected control cultures (**, p < 0.001, as compared with control or with -gal; n = 12).

To further demonstrate that intraneuronal A1-42 accumulation leads to neuronal apoptosis, neurons expressing human APP were treated with DAPT, a functional -secretase inhibitor (19). In our experimental conditions, DAPT does not display significant neurotoxicity by itself (not shown). Although DAPT treatment does not modify the secretion of sAPP in the culture medium, it reduces the extracellular A140 concentration to a non-detectable level (Fig. 5A). DAPT also strongly reduces (57%) the production of intraneuronal A1-42 without affecting the levels of APP expression (Fig. 5B). This reduction of intraneuronal A1-42 production is concomitant with a significant recovery (52%) of cell survival (Fig. 5C). Altogether, these results demonstrate that the neuronal apoptosis induced by human APP in our model is triggered by the production and accumulation of intraneuronal A1-42.

View larger version (26K): [in this window] [in a new window]   Fig. 5.   A -secretase inhibitor reduces intracellular A production and restores neuronal survival. Neuronal cultures were treated for 5 days with 250 nM DAPT immediately after infection with AdRSVAPP (APP). A, accumulation of sAPP in the culture medium after 5 days of treatment analyzed by Western blot (top) and quantification of the extracellular A release by ELISA (bottom) under the same conditions (mean ±S.E., n = 3). B, analysis of intracellular APP expression by Western blot (top) and quantification of A accumulation (bottom) in formic acid-solubilized cell pellets (mean ±S.E., n = 6). C, neuronal survival measured by MTT assay 5 days after infection. Results are given as the percentage of neuronal survival as compared with non-infected (NI) control cultures (***, p < 0.001, **, p < 0.01; n = 12).

	DISCUSSION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

It is currently well admitted that APP plays a central role in AD, but less is known about the link existing between APP processing and the massive neuronal death that takes place in the disease. In the present study, we report that long term expression of human APP triggers apoptosis in rat neurons. This neuronal apoptosis is not related to the adenoviral-mediated overexpression of an exogenous protein since the adenoviral-mediated expression of -galactosidase is without effect on neuronal survival. Our results are in line with previous studies showing that the adenoviral expression of human APP695 induces apoptosis in both rat hippocampal neurons (31) and rat brain (32). It is thus very important to understand how human APP could induce neuronal death.

We first investigated the role of secreted A in APP-induced neurotoxicity. We utilized the culture medium of a CHO cell line or neurons expressing different levels of human APP as a source of extracellular A. The concentrations of A1-40 and A1-42 in the CHO culture medium are comparable with those measured in the cerebrospinal fluid of AD patients (24). These concentrations of extracellular A do not induce any neurotoxicity, indicating that extracellular soluble A peptides are not directly responsible for neurodegeneration. It has been demonstrated previously that extracellular A must aggregate into fibrils to acquire neurotoxic properties (11). At micromolar concentrations, fibrillar A provokes oxidative injuries followed by cell death in neuronal and glial cells (33, 34). This extracellular A toxicity could be mediated by the interaction of fibrillar A with APP present at the neuronal membrane (35). In the present study, even when neurons express human APP at their cell surface, the extracellular A produced fails to induce neuronal death.

The fact that APP-induced apoptosis occurs independently of secreted APP derivatives led us to investigate the role of the C-terminal domain of the protein in neuronal death. The APP C terminus is essential for the cell surface APP signaling function (36) and for the APP-dependent axonal anterograde transport (37, 38). Here we show that the neuronal expression of either C-terminal deleted APP (APPC) or full-length APP (APP695) induces apoptosis. In other cellular models, the C-terminal domain of APP has been shown to mediate cytotoxic effects. The cleavage of the intracellular domain of APP by caspases generates a C31 cytotoxic fragment in mouse N2a neuroblastoma cell lines (27). The interaction of the intracellular domain of the V642I APP mutant with G proteins leads to nucleosomal DNA fragmentation in F11 neuronal cell lines (28, 39). Since the neuronal apoptosis observed in this study is not mediated by the C-terminal domain of APP, we conclude that important differences in the metabolism and function of APP may exist between neuronal primary cultures and cell lines.

Another possible origin of APP-induced apoptosis is the intracellular accumulation of A. Neurons have been shown previously to produce intracellular A42 (25, 40). Here we report that neurons expressing human APP or APPC accumulate very similar amounts of intraneuronal A1-42, whereas they produce different amounts of extracellular A1-40. The extracellular A production by neurons expressing APP or APPC is in agreement with previous observations in transfected cells (18, 41). After 3 days of infection, the expression of membrane human APP or APPC at different levels does not induce any neurotoxicity, indicating that the overexpression of APP per se is not toxic. In addition, the amount of intraneuronal A1-42 is probably too low to be detected, and there is no neuronal damage observed. After 5 days of infection, the intraneuronal accumulation of A1-42 in neurons expressing APP or APPC provokes a massive neuronal apoptosis. To further demonstrate that intraneuronal A1-42 accumulation induces neuronal apoptosis, the intraneuronal production of A1-42 was inhibited by a functional -secretase inhibitor, DAPT (19). DAPT was chosen among other functional -secretase inhibitors described (40) because it was the only one that was not neurotoxic by itself in our experimental conditions. Moreover, DAPT specifically inhibits A production without affecting APP expression and processing through the non-amyloidogenic pathway. DAPT was able to reduce extracellular A1-40 production to a non-detectable level and inhibited intraneuronal A1-42 production by 57%. The differential inhibition observed at A40 and A42 sites could be viewed as evidence that different -secretases generate A1-40 and A1-42 or could result from the production of these two peptides in different cellular organelles to which -secretase inhibitors have access with different efficiency (42). The production of intraneuronal APP and A was studied in three different experimental conditions: (i) production of APP without detectable A1-42, (ii) production of APP and A1-42, (iii) production of APP and partial inhibition of the production of A1-42. Our results show that the DAPT-mediated inhibition of intraneuronal A1-42 accumulation (57%) is similar to the DAPT-mediated recovery of neuronal survival (52%). This clearly indicates that the neuronal apoptosis that we observed results from the accumulation of intraneuronal A1-42. The mechanisms by which intraneuronal A accumulation triggers apoptosis are currently unknown. The highly amyloidogenic A1-42 is produced in the endoplasmic reticulum/intermediate compartment of neuronal cells (43). The intracellular accumulation of A1-42 may cause an overload of the endoplasmic reticulum, leading to neuronal cell injury (44).

Intraneuronal A accumulation has been described in transgenic mice expressing FAD mutations. In double transgenic mice expressing human PS1 and APP mutants, intraneuronal A accumulation precedes amyloid deposition (14). Intraneuronal A42 accumulation together with extensive neuronal loss occurs without amyloid deposition in transgenic mice expressing a human PS1 mutation (15). Neuronal loss has also been documented in transgenic mice expressing the Swedish APP mutation that leads to plaque formation (45).

Intraneuronal accumulation of A42 in AD brains has been recently reported (12). Although AD patients show a severe neuronal loss in specific brain regions, the involvement of apoptosis in AD neurodegeneration remains a matter of debate (46). Apoptotic features have been observed in brains of AD patients (47), but it may be very difficult to observe the transient apoptotic state of neurons when looking at the lesions several years after the onset of the disease.

In conclusion, our data, in agreement with other recent reports, strongly support the idea that the intraneuronal production and accumulation of A1-42 are key events in AD. Elucidating how intraneuronal A triggers apoptosis should in turn allow a better understanding of the neurodegeneration occurring in the disease.

	ACKNOWLEDGEMENTS

We thank K. Beyreuther and L. Mercken for the generous gift of the WO2 antibody and DAPT, respectively. Also, we acknowledge A. S. Caumont for the critical reading of the manuscript.

	FOOTNOTES

^* This work was supported by the Belgian Fonds de la Recherche Scientifique Médicale, Pôles d'attraction interuniversitaires/Services fédéraux des Affaires scientifiques, techniques et culturelles, and the Queen Elisabeth Medical Foundation.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed. Tel.: 32-2-7649341; Fax: 32-2-7645460; E-mail: octave@nchm.ucl.ac.be.

Published, JBC Papers in Press, February 22, 2002, DOI 10.1074/jbc.M200887200

	ABBREVIATIONS

The abbreviations used are: AD, Alzheimer disease; FAD, familial AD; APP, amyloid precursor protein; sAPP, soluble human APP; A, -amyloid peptide; CHO, Chinese hamster ovary; ELISA, enzyme-linked immunosorbent assay; -gal, -galactosidase; DAPT, N-[-N-(3,5-difluoro-phenylacetyl)-L-alanyl]-S-phenylglycine t-butyl ester; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide.

	REFERENCES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

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