Maria Olsson

A novel pathway for amyloid precursor protein processing

Amyloid precursor protein (APP) can be proteolytically processed along two pathways, the amyloidogenic that leads to the formation of the 40-42 amino acid long Alzheimer-associated amyloid β (Aβ) peptide and the non-amyloidogenic in which APP is cut in the middle of the Aβ domain thus precluding Aβ formation. Using immunoprecipitation and mass spectrometry we have shown that Aβ is present in cerebrospinal fluid (CSF) as several shorter isoforms in addition to Aβ1-40 and Aβ1-42. To address the question by which processing pathways these shorter isoforms arise, we have developed a cell model that accurately reflects the Aβ isoform pattern in CSF. Using this model, we determined changes in the Aβ isoform pattern induced by α-, β-, and γ-secretase inhibitor treatment. All isoforms longer than and including Aβ1-17 were γ-secretase dependent whereas shorter isoforms were γ-secretase independent. These shorter isoforms, including Aβ1-14 and Aβ1-15, were reduced by treatment with α- and β-secretase inhibitors, which suggests the existence of a third and previously unknown APP processing pathway involving concerted cleavages of APP by α- and β-secretase.

BACE1 inhibition induces a specific cerebrospinal fluid β-amyloid pattern that identifies drug effects in the central nervous system.

BACE1 is a key enzyme for amyloid-β (Aβ) production, and an attractive therapeutic target in Alzheimers disease (AD). Here we report that BACE1 inhibitors have distinct effects on neuronal Aβ metabolism, inducing a unique pattern of secreted Aβ peptides, analyzed in cell media from amyloid precursor protein (APP) transfected cells and in cerebrospinal fluid (CSF) from dogs by immunoprecipitation-mass spectrometry, using several different BACE1 inhibitors. Besides the expected reductions in Aβ1-40 and Aβ1-42, treatment also changed the relative levels of several other Aβ isoforms. In particular Aβ1-34 decreased, while Aβ5-40 increased, and these changes were more sensitive to BACE1 inhibition than the changes in Aβ1-40 and Aβ1-42. The effects on Aβ5-40 indicate the presence of a BACE1 independent pathway of APP degradation. The described CSF Aβ pattern may be used as a pharmacodynamic fingerprint to detect biochemical effects of BACE1-therapies in clinical trials, which might accelerate development of novel therapies.

Mass spectrometric characterization of amyloid-β species in the 7PA2 cell model of Alzheimer's disease.

The Chinese hamster ovary cell line 7PA2, stably transfected with the 751 amino acid isoform of amyloid-β protein precursor (AβPP) containing the Val → Phe mutation at residue 717, is one of the most used models to study the biochemistry and toxicity of secreted amyloid-β (Aβ) peptides, particularly Aβ oligomers, which are considered to be of relevance to the pathogenesis of Alzheimers disease. Here, we present a detailed immunochemical and mass spectrometric characterization of primary structures of Aβ peptides secreted by 7PA2 cells. Immunoprecipitation and western blot of 7PA2 cell culture media revealed abundant anti-Aβ immunoreactive bands in the molecular weight range of 4-20 kDa. Mass spectrometric analysis showed that these bands contain several AβPP/Aβ peptides, starting at the N-terminal of the Aβ sequence and extending across the BACE1 cleavage site. Treatment of cells with a BACE1 inhibitor decreased the abundance of the Aβ monomer band by western blot and resulted in lower levels of Aβ1-40, Aβ1-42, and sAβPPβ as measured by ELISA. However, western blot bands thought to represent oligomers of Aβ increased in response to BACE1 inhibition. This increase was paralleled by the emergence of N-terminally truncated Aβ species (Aβ5-40 in particular) and Aβ species that spanned the β-secretase site in AβPP according to mass spectrometric analyses. The formation of these AβPP/Aβ peptides may have implications for the use of the 7PA2 cell line as a model for Aβ pathology. The enzyme(s) responsible for this particular BACE1-independent AβPP-processing remains to be identified.

Amyloid-β metabolism in Niemann-Pick C disease models and patients.

Niemann-Pick type C (NPC) is a progressive neurodegenerative lysosomal disease with altered cellular lipid trafficking. The metabolism of amyloid-β (Aβ) - previously mainly studied in Alzheimer’s disease - has been suggested to be altered in NPC. Here we aimed to perform a detailed characterization of metabolic products from the amyloid precursor protein (APP) in NPC models and patients. We used multiple analytical technologies, including immunoassays and immunoprecipitation followed by mass spectrometry (IP-MS) to characterize Aβ peptides and soluble APP fragments (sAPP-α/β) in cell media from pharmacologically (U18666A) and genetically (NPC1−/−) induced NPC cell models, and cerebrospinal fluid (CSF) from NPC cats and human patients. The pattern of Aβ peptides and sAPP-α/β fragments in cell media was differently affected by NPC-phenotype induced by U18666A treatment and by NPC1−/− genotype. U18666A treatment increased the secreted media levels of sAPP-α, AβX-40 and AβX-42 and reduced the levels of sAPP-β, Aβ1-40 and Aβ1-42, while IP-MS showed increased relative levels of Aβ5-38 and Aβ5-40 in response to treatment. NPC1−/− cells had reduced media levels of sAPP-α and Aβ1-16, and increased levels of sAPP-β. NPC cats had altered CSF distribution of Aβ peptides compared with normal cats. Cats treated with the potential disease-modifying compound 2-hydroxypropyl-β-cyclodextrin had increased relative levels of short Aβ peptides including Aβ1-16 compared with untreated cats. NPC patients receiving β-cyclodextrin had reduced levels over time of CSF Aβ1-42, AβX-38, AβX-40, AβX-42 and sAPP-β, as well as reduced levels of the axonal damage markers tau and phosphorylated tau. We conclude that NPC models have altered Aβ metabolism, but with differences across experimental systems, suggesting that NPC1-loss of function, such as in NPC1−/− cells, or NPC1-dysfunction, seen in NPC patients and cats as well as in U18666A-treated cells, may cause subtle but different effects on APP degradation pathways. The preliminary findings from NPC cats suggest that treatment with cyclodextrin may have an impact on APP processing pathways. CSF Aβ, sAPP and tau biomarkers were dynamically altered over time in human NPC patients.

Converging Pathways of Chromogranin and Amyloid Metabolism in the Brain

Much is unknown regarding the regulation of Alzheimer-related amyloid-beta protein precursor (AbetaPP)-processing in the human central nervous system. It has been hypothesized that amyloidogenic AbetaPP-processing preferentially occurs in the regulated secretory pathway of neurons. To test this hypothesis we looked for correlations of AbetaPP-derived molecules in cerebrospinal fluid (CSF) with chromogranin (Cg) derived peptides, representing the regulated secretion. Patients with Alzheimers disease (AD, N=32), multiple sclerosis (MS, N=50), and healthy controls (N= 70) were enrolled. CSF was analyzed for the amyloid peptides Abeta1-42, Abetax-42, Abetax-40, Abetax-38, alpha-cleaved soluble AbetaPP (sAbetaPPalpha), beta-cleaved soluble AbetaPP (sAbetaPPbeta), and peptides derived from CgB and SgII (Secretogranin-II, CgC). We investigated CSF levels of the protease BACE1, which processes AbetaPP into Abeta, in relation to Cg-levels. Finally, we measured Cg levels in cell media from untreated and BACE1-inhibited SH-SY5Y human neuroblastoma cells. CSF Cg levels correlated to sAbetaPP and Abeta peptides in AD, MS, and controls, and to CSF BACE1. Cell medium from BACE1-inhibited cells had decreased CgB levels. These results suggest that a large part of AbetaPP in the human central nervous system is processed in the regulated secretory pathway of neurons.

The performance of Aβ5-X isoforms as novel pharmacodynamic markers of BACE1 inhibition

progressors respectively) (P<0.05). Therewere no differences in composite vascular score, dementia severity (CMMSE and CDR-SOB score), age-related white matter (ARWMC) total score or treatment differences. No differences were noted in APOE status in 6 available data. We found statistically significant increase in GZMB, GZMH, TGFBR3, KLRD1, KIR2DL3 and KIR2DL4 expression (P <0.05) for fast AD-progressors (compared to slow progressors) consistently across 3 time points. Conclusions: Our initial results demonstrate relationship between mRNA levels and fast progression in early AD subjects. This has potential therapeutic implications for the independent role of inflammation and apoptosis in influencing AD progression and we await further confirmation with larger sample analyses.

Amyloid metabolism in Niemann-Pick type C disease

research on the molecular biology of AD. While the conformation adopted by Aß within these aggregates is not known, a ß-hairpin conformation is known to be accessible to monomeric Aß. The ß-hairpin is for instance observed in a complex with Affibody binding protein. Methods: To test the hypothesis that the ß-hairpin is a building block of toxic oligomers, we have engineered a double-cysteine mutant of Aß (called AßCC) in which the ß-hairpin conformation is stabilized by an intramolecular disulfide bond. AßCC oligomers were characterized to determine if they represent a stable model of wild type neurotoxic aggregates. Results: Aß40CC and Aß42CC form stable protofibrils, but are unable to convert into amyloid fibrils. Biochemical and biophysical experiments and assays with conformation-specific antibodies used to detect Aß aggregates in vivo indicate that the wild-type oligomer structure is preserved and stabilized in AßCC oligomers. Stable oligomers are potent inducers of neuronal apoptosis. Conclusions: Stable neurotoxic AßCC oligomers/protofibrils might facilitate studies of their structure and role in the pathogenesis of AD. Stable protofibril preparations might potentially also be used for therapy development based on active or passive immunization.

Convergence of chromogranin and amyloid metabolism in the brain

Background: Much is unknown regarding the regulation of amyloid precursor protein (APP) processing in the human central nervous system. It has been hypothesized that amyloidogenic APP-processing preferentially occurs in the regulated secretory pathway of neurons. To test this hypothesis we looked for correlations of APP-derived molecules in CSF with chromogranin (Cg) derived peptides, representing the regulated secretion. Methods: Patients with Alzheimers disease (AD, N = 32), multiple sclerosis (MS, N = 50) and healthy controls (N = 70) were enrolled. CSF was analyzed for the amyloid peptides Aβ1-42, Aβx-42, Aβx-40, Aβx-38, α-cleaved soluble APP (α-sAPP), β-cleaved soluble APP (β-sAPP), and peptides derived from CgB and SgII (Secretogranin-II, CgC). We investigated CSF levels of the protease BACE1, which processes APP into Aβ, in relation to Cg-levels. Finally, we measured Cg levels in cell media from untreated and BACE1-inhibited SH-SY5Y human neuroblastoma cells. Results: CSF Cg levels correlated to sAPP and Aβ peptides in AD, MS and controls, and to CSF BACE1. Cell medium from BACE1-inhibited cells had decreased CgB levels. Conclusions: These results suggest that a large part of APP in the human central nervous system is processed in the regulated secretory pathway of neurons. (Less)