New highly sensitive rodent and human tests for soluble amyloid precursor protein alpha quantification: preclinical and clinical applications in Alzheimer’s disease
© Rose et al.; licensee BioMed Central Ltd. 2012
Received: 16 May 2012
Accepted: 23 July 2012
Published: 23 July 2012
Amyloid precursor protein (APP), a key molecule in Alzheimer’s disease (AD), is metabolized in two alternative cleavages, generating either the amyloidogenic peptides involved in AD pathology or the soluble form of APP (sAPPα). The level of amyloidogenic peptides in human cerebrospinal fluid (CSF) is considered to be a biomarker of AD, whereas the level of sAPPα in CSF as a biomarker has not been clearly established. sAPPα has neurotrophic and neuroprotective properties. Stimulating its formation and secretion is a promising therapeutic target in AD research. To this end, very sensitive tests for preclinical and clinical research are required.
The tests are based on homogenous time-resolved fluorescence and require no washing steps.
We describe two new rapid and sensitive tests for quantifying mouse and human sAPPα. These 20 μl-volume tests quantify the levels of: i) endogenous mouse sAPPα in the conditioned medium of mouse neuron primary cultures, as well as in the CSF of wild-type mice, ii) human sAPPα in the CSF of AD mouse models, and iii) human sAPPα in the CSF of AD and non-AD patients. These tests require only 5 μl of conditioned medium from 5 × 104 mouse primary neurons, 1 μl of CSF from wild-type and transgenic mice, and 0.5 μl of human CSF.
The high sensitivity of the mouse sAPPα test will allow high-throughput investigations of molecules capable of increasing the secretion of endogenous sAPPα in primary neurons, as well as the in vivo validation of molecules of interest through the quantification of sAPPα in the CSF of treated wild-type mice. Active molecules could then be tested in the AD mouse models by quantifying human sAPPα in the CSF through the progression of the disease. Finally, the human sAPPα test could strengthen the biological diagnosis of AD in large clinical investigations. Taken together, these new tests have a wide field of applications in preclinical and clinical studies.
KeywordsAlzheimer’s disease Soluble amyloid precursor protein alpha Homogeneous time-resolved fluorescence Rodent Human Cerebrospinal fluid Primary neurons Sensitivity
Alzheimer’s disease (AD) is a neurodegenerative disease characterized by cognitive deficits leading to a progressive loss of functional autonomy. The neuropathological hallmarks of the disease are extracellular amyloid deposits and intracellular neurofibrillar tangles containing phosphorylated Tau (pTau) . According to recently proposed AD diagnostic criteria, diagnosis is based on clinical criteria, medical history, neuropsychological evaluation, morphological neuroimaging, cerebrospinal (CSF) biomarkers and/or molecular imaging (PET scan) . AD is biologically characterized by the combination of low Aβ1-42 peptide levels and high levels of total Tau and Tau phosphorylated on threonine 181 (pTau181) in the CSF [3–6]. However, new biomarkers are needed to better discriminate AD from non-AD patients and to improve the diagnosis of AD in patients with atypical CSF profiles.
Amyloid precursor protein (APP) is a key molecule in AD pathology. Proteolytic cleavages of APP by the sequential actions of β- and γ-secretases form the neurotoxic amyloid beta (Aβ) peptide, which typically consists of 40 or 42 amino acid residues (the amyloidogenic pathway). An alternative cleavage by α-secretase, which splits the APP sequence in the middle of the amyloid peptide sequence, prevents formation of the Aβ peptide and releases the resulting APP N-terminal ectodomain (sAPPα)[7–9]. The physiological regulation of β and α cleavages is poorly established. However, recent papers describe an α cleavage following the β cleavage, as the C-terminal fragment (C99) released after β-secretase cleavage can be processed in α-secretase to generate the C83 fragment . In addition, truncated Aβ peptide of 14 to 16 residues generated from the sequential actions of β and α-secretases have been identified in the CSF . Interestingly, an inhibitory domain for γ-secretase activity in the C83 fragment has been identified able to down regulate the production of Aβ peptide . In addition to its regulation of Aβ peptide, α-secretase cleavage appears very important as sAPPα is involved in several functions [13, 14]. sAPPα is known for its neurotrophic and neuroprotective properties [13, 14]. In addition, it stimulates the proliferation of neuroblasts from the sub-ventricular zone and increases long-term potentiation [15, 16].
Recent studies have investigated sAPPα as a putative biomarker of AD [17, 18] and tested the hypothesis that dysregulation of the alternative cleavage of APP may contribute to the pathophysiology of the disease. In the past, human sAPPα has been evaluated by western blotting [19–22], ELISA [23, 24], and multiplex technology [17, 18, 25]. However, some of the findings regarding sAPPα levels in human CSF have been contradictory. Such discordant results signal the need for an ideal test that allows very sensitive and specific detection of sAPPα in small amounts of human CSF. Recently, a new technology (Alpha ScreenTM)  requiring no washing and using low sample volume has been applied to sAPPα (AlphaLISA) but no results have been published yet.
From a research perspective, sAPPα quantification in preclinical studies in rodents is also necessary to better understand AD pathophysiology and to develop new therapeutic targets. Different cell lines, which include neuroblastoma cells, are currently being used to study APP metabolism in vitro. Although rodent primary neurons produce low amounts of Aβ peptide , this model remains interesting to study the response of sAPPα to AD therapeutic drug candidates before their in vivo validation in mice or rats. To effectively evaluate these models, a sensitive, reproducible and rapid test is needed to accurately assess sAPPα levels in the conditioned medium of rodent neuron primary cultures and to create high-throughput drug screening assays. Tests are also needed for the in vivo evaluation of candidate drugs identified in in vitro screens, both in wild-type mice and in transgenic AD mouse models.
We describe two new rapid and sensitive sAPPα tests that are based on homogenous time-resolved fluorescence (HTRF) and can be used to quantify the levels of: i) endogenous rodent sAPPα in the conditioned medium of mouse primary neurons, as well as in the CSF of wild type mice; ii) human sAPPα in the CSF of AD mouse models; and iii) human sAPPα in the CSF of AD and non-AD patients. These tests require only 5 μl of conditioned medium from 5 × 104 mouse primary neurons, 1 μl of CSF from wild-type and transgenic AD mouse models or 0.5 μl of human CSF.
Results and discussion
Analytical performance of the HTRF sAPPα assays
Intra- and inter-assay imprecision in the h-sAPPα
Human CSF sample
705 ± 13
432 ± 18
434 ± 16
556 ± 21
270 ± 18
Human CSF sample
703 ± 24
424 ± 21
503 ± 17
493 ± 41
338 ± 17
Intra- and inter-assay imprecision in the r-sAPPα assay
Neuron conditioned medium
23.4 ± 1.4
15.9 ± 0.7
17.4 ± 0.6
Neuron conditioned medium
24.7 ± 1.5
14.8 ± 1.7
18.6 ± 1.2
Stability of h-sAPPα in CSF stored at different conditions
Effect of storing CSF in polypropylene and polystyrene tubes
669 ± 20
659 ± 41
98 ± 6
399 ± 20
391 ± 24
98 ± 6
456 ± 2.5
412 ± 16
90 ± 3
535 ± 16
482 ± 18
90 ± 3
236 ± 2
212 ± 7
89 ± 3
Stability of h-sAPPα in CSF stored at different conditions
Effect of storage temperature
100 ± 6
101 ± 5
100 ± 11
93 ± 1
103 ± 2
100 ± 7
93 ± 9
95 ± 15
85 ± 14
93 ± 6
100 ± 1
77 ± 1
82 ± 4
80 ± 4
76 ± 4
100 ± 5
113 ± 2
80 ± 4
109 ± 3
99 ± 5
100 ± 3
86 ± 5
81 ± 4
70 ± 5
83 ± 1
sAPPα assay development: in vitro applications
We used our sAPPα assays to measure the sAPPα levels in the conditioned media of human neuroblastoma cells and mouse neuron primary cultures.
Mouse primary neurons were plated in 96 well-plates at the density of 5 × 104 cells per well. We checked that r-sAPPα secreted was proportional to the number of neurons and to the time of incubation (Figure 5B,C). Treatment of primary neurons with 30 μM EGCG or with 1 μM PACAP-27 produced a strong increase in the level of endogenous r-sAPPα in the incubation medium (Figure 5D). These results from our rodent test validate previously reported western blotting data from treated neuroblastoma cells [34, 35].
The rodent test detected sAPPα in only 5 μl of conditioned medium from 5 × 104 primary neurons cultured in 96-well plates, without concentration of the proteins. Five CSF samples were divided into polypropylene or polystyrene tubes and stored for 24 hours at −80°C. Mean ± SEM of h-sAPPα in ng/ml and (%) compared to the aliquot stored in polypropylene tubes. In previously published reports, evaluating sAPPα in conditioned media required higher numbers of cells combined with trichloracetic acid precipitation of proteins , media lyophilization  or protein concentration with centrifugal filters .
sAPPα assay development: in vivo applications in mice
Measurement of sAPPα in mouse CSF
137 ± 17
3xTg AD (7)
105 ± 8#
34 ± 6
sAPPα assay development: in vivo applications in human
Age ± SEM
M + F
MMSE ± SEM
Non AD (28)
68.9 ± 2.0
14 + 14
23.5 ± 0.7
72.4 ± 1.6
12 + 19
19.3 ± 1.0 **
Furthermore, our test detected h-sAPPα in as little as 0.5 μl of CSF, which is a vast improvement over ELISA tests and multiplex technology, which require sample volumes of 100 μl and 25 μl, respectively. The low amounts of CSF required by our test will allow large clinical and longitudinal investigations in patients to validate sAPPα as an AD biomarker.
This is the first description of two highly sensitive tests that can be used to detect sAPPα in a variety of samples. Samples include the conditioned media of human neuroblastoma cells and mouse primary neurons, as well as very small samples of biological fluids, such as mouse and human CSF.
These new tools will have very wide applications from pre-clinical to clinical studies. Together, their rapidity, sensitivity and small incubation volumes will facilitate high-throughput investigations of drugs capable of increasing the secretion of endogenous sAPPα from primary neurons. The tests may be subsequently validated for in vivo analyses by using them to evaluate at different time points sAPPα levels in the CSF of wild-type mice or AD mouse models treated with molecules that exhibit activity in in vitro screening assays. Finally, the human sAPPα test will allow sAPPα levels to be measured in the CSF of patients participating in large clinical investigations.
The quantification of human and rodent sAPPα was achieved by adapting the HTRF technology developed by Cisbio international company (Bagnol ssur Cèze, France). It is a single step immunoassay that employs two primary antibodies. The first antibody for both tests is directed to an epitope in the N-terminal domain of the APP protein, which is common to humans and rodents (22C11; MAB 348 from Chemicon, Billerica, MA, USA), and coupled to Europium cryptate, a donor fluorophore. The second antibody in each assay is specific for the C-terminal domain of either h-sAPPα (6E10) or r-sAPPα (SIG-39153; both antibodies are from Signet laboratories, and purchased from Covance, Dedham, MA, USA). Both of the secondaries antibodies were coupled to an acceptor fluorophore, such as D2 (Figure 1). When the donor and acceptor fluorophores are brought together by a biomolecular interaction, i.e., when sAPPα is present in the sample, a portion of the energy captured by the donor fluorophore during excitation is released through a fluorescence emission at 620 nm, while the remaining energy is transferred to the acceptor. This energy is then released by the acceptor as a specific fluorescence at 665 nm and is proportional to the concentration of sAPPα in the sample.
Recombinant h-sAPPα, r-sAPPα and h-sAPPβ were produced in bacteria using a p-Gex vector and were purified from bacterial lysates using the glutathione-S-transferase system (GE Healthcare, Uppsala, Sweden), as previously described . Their purity was checked by 10% SDS-PAGE (Bio-Rad, Hercules, CA, USA). The Aβ1-42 peptide was purchased from Bachem (Weil am Rhein, Germany). Fluorophore labeling of antibodies was done by Cisbio.
The assay was performed in a final volume of 20 μl in a 384-well white plate (Proxiplate; Perkin Elmer, Waltham, MA, USA). 5 μl of conditioned medium from SH-SY5Y neuroblastoma cell cultures or from mouse neurons in primary culture, 5 μl of rodent CSF dilutions (1/2.5, 1/5, 1/10), 5 μl of human CSF dilutions (1/2.5, 1/5, 1/10, 1/20), or a calibration curve (human or rodent recombinants APPα, at concentrations of 3–200 ng/ml) in a 50 mM phosphate buffer pH 7.0 containing 0.2% BSA and 10 mM DTT, were added to 5 ng of the cryptate conjugate antibody and 20 ng of the D2 conjugate antibody. Both antibodies were diluted in 50 mM phosphate buffer pH 7.0 containing 0.4 M potassium fluorure. All measurements were performed in duplicate. For the negative control, phosphate buffer replaced the calibration curve. The plate covered with a plate sealer was centrifuged for 1 min at 1000 x g and incubated overnight at room temperature. Then, the plate sealer was removed, and the plate was read on a compatible HTRF reader (Envision; Perkin Elmer, Waltham, MA, USA) at 620 and 665 nm. Results were calculated from the 665 nm/620 nm ratio and expressed in delta F: Delta F = ([test sample or calibration sample 665 nm/620 nm ratio − negative control 665 nm/620 nm ratio]/[negative control 665 nm/620 nm ratio])×100. Delta F is proportional to the sAPPα concentration.
SH-SY5Y human neuroblastoma cells were obtained from the ATCC and maintained at 37°C under 5% carbon dioxide/95% humidified air incubator in a DMEM medium, supplemented with Glutamax, 10% fetal calf serum (FCS), 0.1% penicillin/streptomycin. Cells were plated (3 × 104 cells per well) in 96-well plates coated with 10 μg/ml collagen. The next day, when cells were 60–80% confluent, the conditioned medium was discarded and replaced by the same medium containing 1% N2 (Gibco, Paisley, UK) in place of FCS. Then, 10 μM GF-109203 (Sigma, St. Louis, MO, USA) and 30 μM TAPI 0 (Calbiochem, San Diego, CA, USA) were added 1 hour before adding 0.3 μM PDBu (Sigma, St. Louis, MO, USA) and the cells were incubated for 24 hours. After this period, conditioned media were collected, centrifuged at 800 ×g for 5 min at room temperature to remove any dead cells and the supernatant was immediately supplemented with 1X complete protease inhibitor cocktail (Roche, Bâle, Switzerland) and 5 μl duplicate aliquots were tested in the h-sAPPα assay or were stored at −20°C until the h-sAPPα assay.
Cortical neuron primary cultures were obtained from E16 Swiss mouse embryos. Neurons were grown in a defined medium free of serum and supplemented with hormones, proteins, and salts as previously described . Briefly, dissociated cells were plated (5 × 104 neurons per well) in 96-well plates coated with 1.5 μg/ml polyornithin. After six days in vitro, 60% of the conditioned medium was discarded and replaced by fresh medium. Then, 30 μM EGCG or 1 μM PACAP-27 (Sigma, St. Louis, MO, USA) were added for a 24 hours incubation. At the end of this period, conditioned media were collected, centrifuged at 800 × g for 5 min at room temperature and the supernatant was immediately supplemented with 1X complete protease inhibitor cocktail and 5 μl duplicate aliquots were tested in the r-sAPPα assay or were stored at −20°C until the r-sAPPα assay.
60 patients were recruited from the Clinical and Research Memory Center (CMRR) in Paris Lariboisière Hospital, which is specialized in the care management of patients with cognitive disorders. We included in this study AD and non-AD patients for whom a lumbar puncture (LP) had been performed between November 1st, 2008 and November 1st, 2011 to investigate potential cognitive disorders. All patients had an extensive examination including clinical and neuropsychological evaluations, biological measurements and brain imaging. Based on all available information, patients were classified into two groups (32 AD and 28 non-AD). Complex or unclear cases were discussed, and diagnoses were then set by a multidisciplinary team of neurologists, geriatricians and neuropsychologists. AD patients fit the criteria for probable AD, as defined by the NINCDS-ADRDA , and had biochemical CSF results associated with AD (low Aβ1-42 levels, and high total Tau and pTau181 levels) [3–6].
Non-AD patients were diagnosed with other cognitive disorders, according to validated criteria, including: vascular dementia (n = 7), frontotemporal degeneration (n = 3), depression (n = 2), anxiety (n = 4), Lewy body disease (n = 2), intoxication (n = 1), supranuclear palsy (n = 1), chronic hydrocephalus (n = 2), alcoholism (n = 5), and sleep apnea syndrome (n = 1). Patients with unknown clinical diagnoses or who were not able to undergo testing or a magnetic resonance imaging were excluded. There was no Mild Cognitive Impairment patients in our study. LPs were performed on fasting patients in the month following their clinical diagnosis. CSF was collected in 12 ml polypropylene tubes (TC10PCS; CML, Nemours, France) in standardized conditions and transferred within 4 hours to the laboratory. It was then rapidly centrifuged at 1800 × g for 10 min at 4°C. A small amount of CSF was used for routine analysis, including total cell count, bacteriologic exam, total protein and glucose concentration. CSF was aliquoted in polypropylene tubes of 1 ml and stored at −80°C until use for further analysis.
Measurements of Aβ1-42, Tau and pTau181 levels in CSF were performed by sandwich ELISA according to the manufacturer’s protocols (Innogenetics, Ghent, Belgium). A neurobiological AD profile was established for all the patients in this study.
SDS-PAGE and western blotting analysis
Human and rodent sAPPα recombinant proteins (0.5 μg), were separated in 10% SDS-PAGE (Criterion; Bio-Rad, Hercules, CA, USA), and stained with SimplyBlueSafeStain (Invitrogen Life Technologies, Paisley, UK).
For western blotting analysis, 0.6 ml of conditioned medium at six days in vitro from 4 × 105 cortical neurons primary cultures were centrifuged at 800 x g for 5 min at room temperature. The supernatant was precipitated overnight at −20°C with five volumes of a cold mixture 10% trichloracetic acid/90% acetone. After centrifugation at 15,000 × g for 20 min at 4°C, the pellet was washed with cold acetone and dried. Then, the pellet was resuspended in Laemmli buffer and an equal volume was loaded in two wells in 10% SDS-PAGE. After migration, the proteins were transferred onto polyvinylidene fluoride (PVDF) membranes (Immobilon; Millipore, Billerica, MA, USA) and blocked in 5% low fat milk in Tris buffer saline, 0.02% Tween 20 (TBST). Membranes were incubated overnight with the appropriate primary antibody, 22C11 (1 μg/ml) or the rodent specific SIG-39153 (2 μg/ml) in TBST. Then after 5 washings in TBST, species-specific peroxidase-conjugated secondary antibodies were incubated 45 min at room temperature. After 5 washings the peroxidase signal was visualized using ECL (Western Blotting Detection Reagents; GE Healthcare, Uppsala, Sweden).
Human cerebrospinal fluid (4 μg protein) was diluted in Laemmli buffer and loaded in 10% SDS-PAGE. After migration, and transfer onto PVDF membranes, the membranes were processed as above with the appropriate primary antibody, 22C11 (1 μg/ml) or human specific 6E10 (2 μg/ml).
All animal procedures followed the French and European Union regulations. The protocols of animal anesthesia and biological fluid punctures were performed according to French government ethical laws decree 86/109 and approved by the local Ethics Committee (Direction départementale des services vétérinaires de Paris, service de la protection et santé animales et de la protection de l’environnement), permit number: 75–354.
For the clinical study, the research project was approved by the Ethics Committee of Paris University Hospitals (CEERB Bichat University Hospital, Paris, France). All patients or caregivers gave their written informed consent for CSF assessment.
Mouse CSF sampling
We used seven triple transgenic mice (3xTg AD, 12-months-old) harboring three mutant genes: β-amyloid precursor protein (APPswe), presenilin-1 (M146V) knock-in, and Tau (P301L)  and five non-transgenic controls (14-months-old). CSF from anesthetized mice was taken from the cisterna magna using a glass capillary, according to the method of Fisher et al. . The procedure was performed to keep blood contamination to a minimum. Samples of 2 to 5 μl were transferred directly to a microvial, centrifuged, and the supernatant immediately frozen on dry ice, and then stored at −20°C until rodent and human sAPPα assays were performed.
Statistical differences between groups were analyzed using the unpaired Student’s t-test. Differences were considered significant if p < 0.05 (GraphPad Prism software, La Jolla, CA, USA).
β-amyloid peptide 1–42
bovine serum albumin
Clinical and Research Memory Center
Coefficient of variation
Enzyme-linked immunosorbent assay
Mini-mental state examination
Pituitary adenylatecyclase-activating polypeptide (27aa)
Phorbol 12,13 dibutyrate
Protein kinase C
- pTau 181:
Tau phosphorylated on threonine 181
Tris buffer saline
Soluble amyloid precursor protein alpha
Soluble amyloid precursor protein beta
Standard error of the mean.
This work was supported by Association Internationale pour la Recherche sur la Maladie d’Alzheimer and INSERM.
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