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Vol. 18, Issue 9, 3591-3600, September 2007
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–degrading Enzyme Neprilysin
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*Department of Cellular Neurology, Hertie-Institute for Clinical Brain Research, University of Tübingen, D-72076 Tübingen, Germany; and Axxima Pharmaceuticals AG, D-81377 Munich, Germany
Submitted January 16, 2007;
Revised May 21, 2007;
Accepted June 28, 2007
Monitoring Editor: Jonathan Weissman
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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(A) deposition is a major pathological hallmark of Alzheimer's disease. Gleevec, a known tyrosine kinase inhibitor, has been shown to lower A secretion, and it is considered a potential basis for novel therapies for Alzheimer's disease. Here, we show that Gleevec decreases A levels without the inhibition of Notch cleavage by a mechanism distinct from 
-secretase inhibition. Gleevec does not influence -secretase activity in vitro; however, treatment of cell lines leads to a dose-dependent increase in the amyloid precursor protein intracellular domain (AICD), whereas secreted A is decreased. This effect is observed even in presence of a potent -secretase inhibitor, suggesting that Gleevec does not activate AICD generation but instead may slow down AICD turnover. Concomitant with the increase in AICD, Gleevec leads to elevated mRNA and protein levels of the A-degrading enzyme neprilysin, a potential target gene of AICD-regulated transcription. Thus, the Gleevec mediated-increase in neprilysin expression may involve enhanced AICD signaling. The finding that Gleevec elevates neprilysin levels suggests that its A-lowering effect may be caused by increased A-degradation. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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(A) peptides and the formation of intracellular neurofibrillary tangles, accompanied by neuron loss and dementia (Selkoe, 2001
). A is generated by sequential proteolytic cleavages of the amyloid precursor protein (APP) by -secretase (BACE) and -secretase. The -secretase cleavage occurs within the membrane, releasing the APP intracellular domain (AICD) into the cytosol. AICD, together with its binding partners Fe65 and Tip60, is considered to be involved in transcriptional regulation (Cao and Sudhof, 2001). Putative target genes of AICD signaling have been suggested (Baek et al., 2002; Kim et al., 2003; von Rotz et al., 2004; Pardossi-Piquard et al., 2005; Ryan and Pimplikar, 2005; Muller et al., 2007), although results for some of these genes are controversial (Hass and Yankner, 2005; Hebert et al., 2006; Chen and Selkoe, 2007; Pardossi-Piquard et al., 2007). One potential AICD target gene is the A-degrading enzyme neprilysin (Pardossi-Piquard et al., 2005, 2006), a metalloprotease that is one of the main A-degrading enzymes in the brain (Carson and Turner, 2002).
-Secretase is a multiprotein complex, processing several type I integral membrane proteins, including APP and the Notch receptor (Kopan and Ilagan, 2004). Therapeutic strategies aimed at lowering A include the development of selective -secretase inhibitors (Evin et al., 2006). However, long-term treatment with -secretase inhibitors has shown severe side effects in preclinical animal studies due to inhibition of Notch processing and signaling (Searfoss et al., 2003; Wong et al., 2004).
Recently, Gleevec (signal transduction inhibitor 571, STI571, imantinib mesylate), a tyrosine kinase inhibitor, has been described to lower A in a cell-free system, in N2A cells expressing human APP, in rat primary neurons, and in guinea pig brain without inhibiting Notch cleavage (Netzer et al., 2003). Gleevec is an approved drug for the treatment of chronic myeloid leukemia, and it inhibits primarily c-Abl, the platelet-derived growth factor receptors (PDGFRs), and c-Kit (Druker et al., 1996; Buchdunger et al., 2000; Mauro et al., 2002). The A-lowering effect of Gleevec has been shown not to be dependent on Abl kinase (Netzer et al., 2003). It has been proposed that Gleevec may act as an APP-selective -secretase inhibitor (Netzer et al., 2003), whereas others found no direct inhibition of -secretase activity in vitro (Fraering et al., 2005). The exact mechanism by which Gleevec leads to the reduction in A is unknown.
Here, we confirm that Gleevec lowers A levels without inhibiting Notch cleavage. In addition, we propose a mechanism distinct from -secretase inhibition.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-secretase inhibitor L-685,458 dissolved in DMSO, and synthetic C50 peptide, representing the C-terminal 50 amino-acid-long AICD sequence of APP (APP721–770), were purchased from Calbiochem (San Diego, CA). The following antibodies were used: 6E10, and biotinylated 4G8 anti-APP monoclonal antibodies (Signet Laboratories, Dedham, MA), A8717 anti-APP C-terminal polyclonal antibody (Sigma-Aldrich), T9026 monoclonal anti-
-tubulin antibody (Sigma-Aldrich), 9E10 anti-c-myc monoclonal antibody (mAb) (Roche Diagnostics, Mannheim, Germany), 56C6 anti-neprilysin mAb (Novocastra, Newcastle, United Kingdom), and anti-Fe65 antibody E-20 and horseradish peroxidase-conjugated secondary antibodies (Santa Cruz Biotechnology, Santa Cruz, CA).
Cell Lines and Treatment
H4 human neuroglioma cells stably transfected with human APP751 (H4-APPwt), H4 cells stably overexpressing the Swedish FAD mutation (K670N/M671L) in human APP695 (H4-APPswe), and U373 astrocytoma cells stably transfected with human APP751 (U373-APPwt) were kindly provided by Boehringer Ingelheim (Ingelheim, Germany). H4-Fe65i cells express human Fe65-Myc under the control of the tet off system (Gossen and Bujard, 1992). Expression is turned off by cultivation of cells with 100 ng/ml doxycyline and induced by washing out doxycyline from the culture medium and subsequent cell culture for 3 d. All cells were cultured in DMEM supplemented with 10% fetal bovine serum. For experiments, cells were supplemented with fresh medium containing compounds at concentrations and for durations indicated. DMSO concentrations between samples were kept consistent, and DMSO-treated cells served as a control.
A-Enzyme-linked Immunosorbent Assay (ELISA)
Levels of total A in conditioned cell medium of H4-APPwt cells were measured using a sandwich ELISA based on the mAb 6E10 and the biotinylated mAb 4G8. Capturing antibody 6E10, recognizing an epitope within amino acids 1–17 of human A, was used to coat plastic dishes, whereas 4G8, which is reactive to amino acids 17–24 of A, was used as detection antibody. Each data point was measured in triplicate. Percentage of remaining A from Gleevec-treated cells was calculated in relation to conditioned cell medium from DMSO-treated cells as positive control (=100%) and tissue culture medium as negative control (=0%).
3-(4,5-Dimethylthiazol-2-yl)-5-(3-carboxymeth-oxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, Inner Salt (MTS)-Assay
Cell viability was measured using CellTiter 96 Aqueous NonRadioactive Cell Proliferation Assay (Promega, Madison, WI) according to the manufacturer's protocol. The tetrazolium compound MTS is bioreduced by cells into a formazan product, which is directly proportional to the number of living cells in the culture. Percentage of viable cells after Gleevec treatment was calculated in relation to DMSO-treated control cells.
Immunoprecipitations and Western Blotting
A was immunoprecipitated from equal volumes of conditioned cell culture medium of H4-APPwt cells by incubation with 6E10 antibody at 4°C overnight and subsequently with GammaBind Plus Sepharose (GE Healthcare, Little Chalfont, Buckinghamshire, United Kingdom) at 4°C for 1 h. Beads were washed, and proteins were denatured in sample buffer. Equal volumes of conditioned cell medium were directly analyzed by SDS-polyacrylamide gel electrophoresis (PAGE) to detect A from H4-APPswe cells or soluble -secretase cleaved APP (APPs-) from cell media. For detection of proteins from cell lysates, cells were lysed in radioimmunoprecipitation assay (RIPA) buffer supplemented with protease and phosphatase inhibitors (10 mM Tris, pH 8.0, 150 mM NaCl, 1% NP-40, 0.5% deoxycholate, 0.1% SDS, 1x Complete inhibitor mix [Roche Diagnostics], 5 mM EDTA, 2 mM 1,10-phenanthroline [Sigma-Aldrich], 10 mM NaF, 1 mM Na-pyrophosphate, 1 mM -glycerophosphate, and 1 mM Na-orthovanadate). APP, APPs-, neprilysin, and Notch cleavage products were analyzed by 10% or 8% Tris-glycine SDS-PAGE. For detection of AICD and other APP C-terminal fragments, lysates were separated on 16.5%T 6% C Tricine SDS gels containing 6 M urea (Schagger and von Jagow, 1987). A40 and A42 were analyzed by 10% T 5% C Bicine/Tris, 8 M urea, SDS-PAGE (Wiltfang et al., 1997). For Western Blot detection, proteins were transferred to polyvinylidene difluoride or nitrocellulose membranes. After antibody incubation, SuperSignal West Pico reagents (Pierce Chemical, Rockford, IL) were used for detection. Representative blots from at least three independent experiments are shown.
Generation and Analysis of A and AICD In Vitro
A and AICD were generated in vitro from cell membrane preparations according to previously described procedures (Pinnix et al., 2001) with some changes. In brief, H4-APPswe cells were incubated with 100 nM -secretase inhibitor L-685,458 for 24 h to accumulate C-terminal fragments of APP. Cell pellets were resuspended (850 µl/15-cm dish) in hypotonic buffer (15 mM citrate buffer, pH 6.4, 5 mM EDTA, and 1x Complete protease inhibitor mix). Cells were homogenized and a postnuclear supernatant (PNS) was prepared as described previously (Steiner et al., 1998). Membranes were pelleted from PNS by centrifugation at 16,000 x g for 30 min at 4°C, and then they were resuspended (1 ml/15-cm plate) in assay buffer (50 mM citrate, pH 6.4, 5 mM EDTA, 1x Complete inhibitor mix, and 2 mM 1,10-phenanthroline). To allow A and AICD generation, 80 µl/assay was incubated at 37°C for 15 h. Control samples were kept on ice. After incubation, membranes were pelleted at 16,000 x g for 30 min at 4°C. Supernatant (1 µl) was analyzed for AICD by Western blot analysis with antibody A8717. For A detection, membranes were resuspended in sample buffer and analyzed by 10% T 5% C Bicine/Tris, 8 M urea, SDS-PAGE (Wiltfang et al., 1997) and detection with 6E10 antibody.
Western Blot Quantification
Densitometric values of band intensities were analyzed using the public domain software ImageJ, version 1.34 (www.rsb.info.nih.gov/ij/). Statistical analysis was performed by the unpaired Student's t test by using the StatView 5.0 software (SAS Institute, Cary, NC), and p values < 0.05 were considered as statistically significant.
Analysis of Notch Cleavage
Cells were transfected using FuGENE6 transfection reagent according to the manufacturer's protocol (Roche Diagnostics). The plasmid used for Notch
E-expression, pSC2EMV-6MT (Schroeter et al., 1998), was a kind gift of Raphael Kopan (Washington University, St. Louis, MO). After transfection and treatment with the indicated compounds for 24 h, cells were lysed, and NotchE and Notch intracellular domain (NICD) levels were detected by Western blot with 9E10 antibody. Blots were quantified, and the ratio of NICD/NotchE was calculated.
Reverse Transcription and Quantitative Real-Time Polymerase Chain Reaction (PCR)
Cells were grown with DMSO or Gleevec treatment for 15 h. Total RNA was isolated with the RNeasy Mini kit (QIAGEN, Hilden, Germany), and first-strand cDNA from 1 µg of RNA was synthesized with the Omniscript RT kit (QIAGEN) according to the manufacturer's protocol. For real-time PCR reactions, 5 µl of 1:50 diluted cDNA per sample was mixed with 2xQuantiTect SYBR Green PCR Master Mix (QIAGEN), 2.5 µl of QuantiTect Primer Assay (QIAGEN) MME for neprilysin detection, or glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as a housekeeping gene, in a total volume of 25 µl. PCR reactions were performed according to the manufacturer's protocol on a ABI PRISM 7000 machine (Applied Biosystems, Foster City, CA). Each data point was measured in triplicate. Relative mRNA expression was calculated of the mean value with the comparative Ct method, and neprilysin expression of each sample was normalized to GAPDH expression. The -fold induction of neprilysin expression in Gleevec-treated cells compared with controls was calculated. The Pair Wise Fixed Reallocation Randomisation Test (Pfaffl et al., 2002) was used for statistical analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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but Not APPs- secreted into cell media was measured by sandwich-ELISA (Figure 1A). We observed a dose-dependent decrease in total secreted A with increasing Gleevec concentrations. The IC50 for inhibition of A secretion was determined to be 9.5 µM. Cell viability was not impaired by Gleevec concentrations up to 20 µM, ruling out a reduction of A due to cytotoxicity (Figure 1A, dark gray bars). Immunoprecipitation of secreted A40 and A42 from conditioned cell medium followed by Western blot analysis revealed that treatment of cells with 10 µM Gleevec led to a decrease in both A40 and A42 by 
50% (Figure 1B). Thus, Gleevec reduced total secreted A without altering the A40/42 ratio. In comparison the amount of secreted APPs- remained unchanged by Gleevec treatment (Figure 1B), indicating that Gleevec did not affect -secretase cleavage of APP.
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-secretase cleavage product AICD after treatment of cells with Gleevec (Figure 2B). Western Blot quantification revealed an approximate 10-fold increase in AICD with 10 µM Gleevec (Figure 2C), a concentration where A decrease was around twofold (50% remaining; Figure 1, A and B). This result was unexpected, because it was proposed that Gleevec might inhibit -secretase (Netzer et al., 2003). However, inhibition of -secretase activity should result in a decrease in AICD rather than an increase, indicating that Gleevec might work via different mechanisms to decrease A than was initially thought. We also analyzed untransfected H4 cells, which express low levels of endogenous APP, H4-APPswe cells overexpressing APP695 carrying the Swedish FAD mutation as well as U373-APPwt cells, which overexpress APP751. In all three cell lines Gleevec mediated an increase in AICD, C83, C89, and C99, while leaving levels of full-length APP unaffected (Figure 2, D and E). Thus, the observed effects seem independent of APP overexpression and occurred in different cell lines.
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-Secretase Activity In Vitro-secretase complex. We therefore tested the effect of Gleevec on -secretase activity in vitro, by using membrane preparations from H4-APPswe cells containing intact -secretase and a high amount of C99 fragments serving as the -secretase substrate. Incubation of membrane fractions at 37°C resulted in the generation of A40 and a lesser amount of A42, and in the generation of AICD, none of which were produced at 4°C (Figure 3). A and AICD generation were strongly inhibited with the potent -secretase inhibitor L-685,458 (Shearman et al., 2000). In contrast, incubation of membrane fractions with Gleevec did not inhibit the in vitro generation of A nor did it influence the in vitro generation of AICD. We did not observe an effect of Gleevec at 10 µM, which was effective in cells, and even a higher concentration of 30 µM showed no effect on -secretase cleavage in vitro (Figure 3). Our results do not indicate a direct action of Gleevec on the -secretase complex to enhance AICD or inhibit A generation.
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-Secretase InhibitorE construct that is constitutively processed by -secretase. NotchE and the generated NICD were detected from cell lysates by Western blot analysis. Incubation of cells with Gleevec had no effect on NICD generation (Figure 4, A and B), whereas AICD, C83, C89, and C99 increased and A decreased (Figure 4A), as described above. As expected, treatment of cells with the -secretase inhibitor L-685,458 effectively inhibited the generation of all three -secretase cleavage products, NICD, AICD, and A, and it led to a strong accumulation of C83, C89, and C99 fragments (Figure 4A). A striking observation was that in cells treated with both Gleevec and L-685,458, we still found a prominent AICD increase, even in the presence of the -secretase inhibitor. Gleevec did not prevent -secretase inhibition by L-685,458, because NICD and A generation were still reduced, and C99 and C83 still accumulated as seen with L-685,458 treatment alone (Figure 4A, compare last two lanes). These findings indicate that the Gleevec-mediated AICD increase is not caused by enhanced -secretase cleavage. In higher exposures we observed that small amounts of A were still detected even when cells were treated with -secretase inhibitor (Figure 4, bottom-most panel, last two lanes), suggesting that low -secretase activity still produced small amounts of A and AICD. Similarly, a residual production of AICD was seen after -secretase inhibition in the above-mentioned in vitro experiments (Figure 3). These results suggest that Gleevec treatment of cells might slow down the turnover of AICD, such that low amounts of AICD are rendered more stable and accumulate over time. Similarly, the increase in C83 and C99 fragments after Gleevec treatment might be caused by a slowed turnover of these fragments by mechanisms not involving -secretase cleavage.
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-degrading Enzyme Neprilysin after Gleevec Treatment, which seemed independent of -secretase inhibition. Enhanced APP-cleavage by -secretase could also not account for the A-lowering effect of Gleevec, because levels of APPs- remained unchanged (Figure 1B). Thus, we sought an alternative mechanism by which Gleevec might mediate the decrease in A. It is known that not only A production but also A degradation plays an important role in the regulation of A levels (Turner et al., 2004). Several proteases have been found to degrade A, one of which is the metalloprotease neprilysin (Carson and Turner, 2002). Moreover, AICD has been implicated in activation of gene expression (Cao and Sudhof, 2001) and recently the A-degrading enzyme neprilysin has been described as a potential target gene of AICD signaling (Pardossi-Piquard et al., 2005, 2006). Because Gleevec led to greatly enhanced AICD levels in our cells, we next tested whether neprilysin expression was changed by Gleevec. As shown in Figure 5A, neprilysin protein levels in H4-APPswe cells increased with increasing Gleevec, and concurrently increasing AICD concentrations. A similar rise in neprilysin levels was found in H4-APPwt cells (Figure 5B), and in untransfected H4 cells (data not shown). Concomitant with elevated neprilysin protein levels, neprilysin mRNA levels were also significantly elevated after Gleevec treatment, as measured by real-time PCR analyses (Figure 5C). Increases in AICD and neprilysin were both already detectable after 4 h of Gleevec treatment and levels of AICD and neprilysin further accumulated over time (Figure 5D, top and middle). Together, these results show that the AICD and neprilysin increase were correlated in dose and time, suggesting that increased AICD levels might lead to enhanced neprilysin gene expression. Analysis of secreted A from conditioned cell media showed that the Gleevec-mediated decrease in A was already detected after 4 h and followed a similar time course as neprilysin and AICD up-regulation (Figure 5D, bottom).
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Decrease Occur in Different Cell Lines by enhancing A-degradation. To further investigate this correlation, we quantified neprilysin and A levels from H4-APPswe cells after 4 and 24 h of Gleevec treatment, as used in the time course experiments described above, and in H4-APPwt cells after 24 h of Gleevec treatment. The results show a significant reduction in A secreted from H4-APPswe cells 4 and 24 h after treatment, which was in the range of 61 and 70% compared with control cells (Figure 6A). In H4-APPwt cells, a higher reduction in A to 48% of control was observed (Figure 6A), in line with the ELISA results described above (Figure 1). Neprilysin was up-regulated in both cell lines to a similar extend or slightly higher in H4-APPswe cells 24 h after treatment (Figure 6A). Less reduction in A in H4-APPswe cells could be explained by the fact that these cells secrete six to eightfold higher levels of A than H4-APPwt cells (Figure 6B), due to the Swedish mutation (Citron et al., 1992; Cai et al., 1993). A reduction in secreted A from these cells can be expected to be less efficient by comparable amounts of neprilysin, because higher amounts of A have to be degraded. In U373-APPwt cells, a similar neprilysin increase and A-decrease as in H4-APPwt cells was observed (Figure 6C). Together, these results suggest that Gleevec might lower A by increasing levels of neprilysin and thereby enhancing A-degradation.
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Decrease decrease or both effects could be independent of each other. There is evidence in the literature favoring AICD-mediated neprilysin transcription (Pardossi-Piquard et al., 2005, 2006, 2007) and contradicting reports (Hebert et al., 2006; Chen and Selkoe, 2007). The mechanism of AICD signaling has not been fully elucidated. Recently, a study was published demonstrating that alkalizing drugs, that impair endosomal/lysosomal degradation, lead to an increase in AICD and APP C-terminal fragments and a decrease in A (Vingtdeux et al., 2007), similar to the results we obtained with Gleevec. To further investigate the correlation between an AICD increase and neprilysin up-regulation, H4-APPwt cells were treated with the alkalizing agent NH4Cl. Subsequently, AICD, APP–carboxyl-terminal fragment (CTFs), neprilysin, and secreted A levels were analyzed by Western blot. The results confirm the findings of Vingtdeux et al. (2007), and they show a pronounced increase in AICD and APP C-terminal fragments. Concomitant with the AICD increase, neprilysin levels were up-regulated and secreted A was decreased (Figure 7, A and B). Possibly impaired AICD degradation via the endosomal/lysosomal system after NH4Cl treatment may lead to enhanced AICD-mediated transcription of neprilysin and thus to an A-decrease via similar mechanisms as Gleevec.
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) and Fe65 binding may stabilize AICD (Kimberly et al., 2001, 2005); however, this effect was not observed in all reports (Cupers et al., 2001; Nakaya and Suzuki, 2006). To further investigate whether Fe65 could be a player in enhanced AICD stability, neprilysin expression, and concomitant A decrease, we analyzed the effect of Fe65 on AICD stability and neprilysin expression. We used H4-Fe65i cells overexpressing Fe65 under the control of the "tet-off system" (Gossen and Bujard, 1992). Induction of Fe65 overexpression in these cells led to higher AICD levels; however, APP-CTFs were found to be decreased (Figure 7C). This finding could point to a higher turnover of APP-CTFs by -secretase, thus leading to higher AICD generation. In line with that, A levels were increased after Fe65 overexpression (Figure 7C). Although levels of AICD were increased, neprilysin was not significantly up-regulated in these cells (Figure 7, C and D). Thus, the effect of Fe65 overexpression on APP metabolism in H4 cells clearly differs from that of Gleevec treatment. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-lowering drugs in the treatment of Alzheimer's disease. In the present study, we investigated the mechanism by which Gleevec influences A levels. Gleevec treatment led to a dose-dependent decrease in cell-secreted A, and it did not inhibit Notch cleavage, in line with previously published results (Netzer et al., 2003). However, simultaneously, a dose-dependent increase in the -secretase cleavage product AICD was observed. This novel result cannot be explained by either direct or indirect inhibition of -secretase by Gleevec, because this should result in a decrease in both AICD and A. In addition, Gleevec did not directly influence -secretase activity in vitro, an observation that has also been reported by others (Fraering et al., 2005). Differential effects on - and 
-cleavage have been described for some presenilin (PS) mutants linked to familial AD cases (Chen et al., 2002; Moehlmann et al., 2002; Walker et al., 2005; Bentahir et al., 2006). Possibly, Gleevec might indirectly influence -secretase activity and cause a shift in cleavage by activating -cleavage, resulting in more AICD, and inhibiting -cleavage, resulting in lower A. However, Gleevec treatment of cells increased C99 and C83, which would be expected to decrease if Gleevec led to enhanced -cleavage. In addition, modulation of -secretase activity by PS mutations, in the reports cited above, always involved changes in the A40/A42 ratio, which we and others (Netzer et al., 2003) did not observe with Gleevec. Together, these data strongly suggest that Gleevec lowers A levels and increases AICD by a mechanism distinct from -secretase inhibition or modulation.
As an alternative mechanism leading to elevated AICD levels in cells, we propose that in Gleevec-treated cells, the stability of AICD may be highly increased. Supporting this, Gleevec still led to higher AICD levels, even when coincubated with a potent -secretase inhibitor. We interpret this finding in the way that small amounts of AICD still produced under these conditions have a slower rate of turnover and accumulate in Gleevec-treated cells. The mechanism by which AICD may be stabilized by Gleevec is not known. Possibly, it might involve changes in phosphorylation of AICD and the interaction with its binding partners. The C-terminus of APP contains a consensus motif (682YENPTY687) that interacts with the phosphotyrosine binding (PTB) domains of several cytoplasmic adaptor proteins, including Fe65, X11, JIP1-B, JIP2, mDab1, Numb and ShcA (Borg et al., 1996; Howell et al., 1999; Roncarati et al., 2002; Russo et al., 2002; Scheinfeld et al., 2002; Tarr et al., 2002b). Phosphorylation of AICD at T668 influences AICD stability and leads to destabilization of AICD during differentiation of primary neurons in culture (Kimberly et al., 2005). Among the several kinases that have been described to phosphorylate APP at T668 (Suzuki et al., 1994; Aplin et al., 1996; Iijima et al., 2000; Taru and Suzuki, 2004; Kimberly et al., 2005), the c-Jun NH2-terminal kinase (JNK) seems to play an important role in vivo (Kimberly et al., 2005). JNK is a serine/threonine kinase, but it may be activated by pathways involving receptor tyrosine kinases, e.g., the PDGFR (Yu et al., 2003) and c-Kit (Hong et al., 2004), which can be inhibited by Gleevec. Binding of the adaptor protein Fe65 may be regulated by phosphorylation of APP at T668 (Ando et al., 2001; Kimberly et al., 2005), and it has been implicated in AICD stabilization (Kimberly et al., 2001, 2005). In other reports, Fe65 did not show a stabilizing effect on AICD (Cupers et al., 2001; Nakaya and Suzuki, 2006). We found that overexpression of Fe65 in H4 cells increased the level of AICD. However this effect seemed to be caused by enhanced -cleavage of APP and not by AICD stabilization, because APP C-terminal fragments were decreased, and the amount of secreted A increased. This finding is in line with previously published results, that overexpression of Fe65L1 in H4 cells enhances -secretase processing of APP (Chang et al., 2003). Concluding from these results, the stabilizing effect of Gleevec on AICD is probably not mediated by enhanced binding of Fe65. Apart from Fe65 also ShcA binds to APP in a phosphorylation-dependent manner, possibly involving the receptor tyrosine kinase TrkA, and it has been reported to decrease levels of AICD (Tarr et al., 2002a). Binding of other partners to the APP-C terminus occurs independently of APP phosphorylation. Alternatively to changing the binding of AICD binding proteins, Gleevec might interfere with enzymes involved in AICD degradation. Insulin degrading enzyme (IDE) has been shown to mediate degradation of AICD (Edbauer et al., 2002; Farris et al., 2003) and also of A. Inhibition of its activity should lead to an increase in both AICD and A, as is observed in IDE knockout mice (Farris et al., 2003). Recently, it has been described that alkalizing drugs induce the accumulation of AICD, likely mediated by the endosome/lysosome pathway (Vingtdeux et al., 2007). We found that treatment of cells with the alkalizing agent NH4Cl led to a strong increase in AICD and APP C-terminal fragments very similarly to the effect observed with Gleevec treatment. Thus, it seems possible that Gleevec interferes with endosomal/lysosomal degradation of AICD. Also, the degradation of APP C-terminal fragments that accumulate after Gleevec treatment has been attributed to lysosomes (Golde et al., 1992; Haass et al., 1992). A possible target might be the vacuolar H+-ATPase, because Gleevec can bind to and block ATP binding sites (Mauro et al., 2002).
Apart from the inhibition of -secretase activity, A-reduction can also result from activation of the nonamyloidogenic pathway of APP processing, which may occur via protein kinase C and lead to enhanced -cleavage of APP (Buxbaum et al., 1993). Although Gleevec led to accumulation of the -secretase cleavage product C83, we and others (Netzer et al., 2003) did not observe changes in secreted APPs- from Gleevec-treated cells. These results indicate that Gleevec does not lower A via enhancing the nonamyloidogenic pathway of APP processing. As discussed above, the observed increase in C83 is presumed to occur via impaired degradation of APP C-terminal fragments.
The Gleevec-mediated decrease in A seems to involve other mechanisms than inhibition of -secretase or activation of -secretase. Because AICD has been implicated in transcriptional regulation (Cao and Sudhof, 2001), increased AICD levels after Gleevec treatment may lead to changes in AICD-regulated gene expression causing the observed A-lowering effect. Supporting this, we found a Gleevec dose-dependent increase in the A-degrading enzyme neprilysin, a putative target gene of AICD signaling (Pardossi-Piquard et al., 2005, 2006), which correlated in dose and time with higher AICD levels. In line with transcriptional activation, increased neprilysin mRNA levels were also observed after Gleevec treatment. Further support for a correlation of increased AICD levels and neprilysin up-regulation comes from the observation that alkalizing drug treatment, leading to increased AICD levels similar to Gleevec treatment, also up-regulated neprilysin levels and decreased secreted A. In contrast, overexpression of Fe65, which is thought to be involved in transcriptional activation via AICD (Cao and Sudhof, 2001), led to an increase in AICD and A levels, probably via enhanced -secretase processing. Although AICD levels were increased, neprilysin levels remained unchanged. The effect of Fe65 overexpression clearly differs from the effect of Gleevec treatment, so that Gleevec-mediated changes in the binding of Fe65 to APP or AICD seem unlikely to be the cause of AICD stabilization and neprilysin up-regulation. Concluding from these results, mere elevation of AICD levels may not be sufficient to up-regulate neprilysin expression in H4 cells. Other factors, such as changes in phosphorylation or cellular localization of AICD or its binding partners, might be involved that could be caused by Gleevec and NH4Cl treatment but not by Fe65 overexpression. Results presented here point to a role of AICD in neprilysin up-regulation; however, it cannot be completely ruled out that Gleevec treatment may lead to neprilysin up-regulation independent of AICD.
The balance between anabolism and catabolism of A determines actual A levels, such that a reduction in amyloid levels may be achieved by enhanced A-degradation. The proteases neprilysin (Hama et al., 2001; Iwata et al., 2001; Leissring et al., 2003; Marr et al., 2004), IDE (Farris et al., 2003), and endothelin-converting enzyme (Eckman et al., 2003) have been implicated in the degradation of A peptides. Because we found a dose-dependent increase in levels of neprilysin after Gleevec treatment, our results suggest that the concomitant dose-dependent decrease in A may be caused by enhanced A-degradation by neprilysin. In line with that, neprilysin-up-regulation also correlated with A-decrease in time course experiments. Gleevec-mediated reduction in secreted A from H4-APPswe cells was less efficient than in H4-APPwt cells, probably because H4-APPswe cells secrete six- to eightfold higher A levels, such that higher amounts of A have to be degraded by comparable neprilysin levels.
Together, we propose the following working model (Figure 8). Gleevec treatment increases levels of AICD by slowing down its rate of turnover. Neprilysin expression is increased, due to enhanced AICD signaling or an alternative mechanism, leading to increased A degradation.
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without influencing -secretase cleavage of APP or Notch signaling and thus meets important safety criteria required from a potential therapeutic drug. Because Gleevec itself does not cross the blood-brain barrier, it cannot be used as a drug to reduce A in the brain of patients; however, it represents a very useful tool to investigate new mechanisms involved in the regulation of A levels. An attractive therapeutic strategy for the treatment of AD may be the up-regulation of neprilysin expression in the brain (Hama et al., 2001; Leissring et al., 2003; Marr et al., 2003). The presented results provide the basis for future analyses of the underlying signaling mechanisms leading to AICD stabilization and neprilysin upregulation, and it may lead to new strategies of increasing neprilysin expression in the brain and the discovery of new targets for A-lowering drugs.
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ACKNOWLEDGMENTS |
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EMV-6MT construct; and H. Steiner (Munich, Germany) and M. Calhoun and J. Coomaraswamy (Tuebingen, Germany) for critical comments on the manuscript. This work was supported by the University of Tuebingen, Fortuene grant F1314009 (to E.K.). |
Footnotes |
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Present address: GPC Biotech AG, 82152 Martinsried, Germany. 
Address correspondence to: Ellen Kilger (ellen.kilger{at}uni-tuebingen.de).
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