Cerebrospinal Aβ11-x and 17-x levels as indicators of mild cognitive impairment and patients' stratification in Alzheimer's disease

Genetic and chemical disruption of amyloid precursor protein processing impairs zebrafish sleep maintenance

Amyloid precursor protein (APP) is a brain-rich, single pass transmembrane protein that is proteolytically processed into multiple products, including amyloid-beta (Aβ), a major driver of Alzheimer disease (AD). Although both overexpression of APP and exogenously delivered Aβ lead to changes in sleep, whether APP processing plays an endogenous role in regulating sleep is unknown. Here, we demonstrate that APP processing into Aβ40 and Aβ42 is conserved in zebrafish and then describe sleep/wake phenotypes in loss-of-function appa and appb mutants. Larvae with mutations in appa had reduced waking activity, whereas larvae that lacked appb had shortened sleep bout durations at night. Treatment with the γ-secretase inhibitor DAPT also shortened night sleep bouts, whereas the BACE-1 inhibitor lanabecestat lengthened sleep bouts. Intraventricular injection of P3 also shortened night sleep bouts, suggesting that the proper balance of Appb proteolytic processing is required for normal sleep maintenance in zebrafish.

A Cystatin C Cleavage ELISA Assay as a Quality Control Tool for Determining Sub-Optimal Storage Conditions of Cerebrospinal Fluid Samples in Alzheimer's Disease Research

Background:

An N-terminal octapeptide cleavage of the cystatin C protein was discovered by mass spectrometry when cerebrospinal fluid (CSF) was stored at –20°C for 3 months, which did not occur when CSF was stored at –80°C.

Objective:

The aim was to develop an immunoassay as quality assessment tool to detect this –20°C cleavage of cystatin C in CSF and support Alzheimer’s disease research.

Methods:

A specific monoclonal antibody and a double indirect sandwich ELISA were developed: one assay quantifies the octapeptide uncleaved protein specifically and the other quantifies the total cystatin C present in the biological fluid (both cleaved and uncleaved forms). The ratio of these concentrations was calculated to assess the extent of cleavage of cystatin C. The novel ELISA was validated and applied in a short-term (up to 4 weeks) and mid-term (up to one year) stability study of CSF stored at 4°C, –20°C, –80°C, and liquid nitrogen. Impact of freeze-thaw cycles, adsorption, and protease inhibitors were tested.

Results:

The ratio of truncated protein was modified following –20°C storage and seemed to reach a plateau after 6 months. The ratio was impacted neither by freeze-thaw cycles nor adsorption. The –20°C specific cleavage was found to be protease related.

Conclusion:

Using this novel double indirect sandwich ELISA, absolute levels of the total and uncleaved cystatin C and the ratio of truncated cystatin C can be measured. This assay is an easily applicable tool which can be used to confirm that CSF biospecimen are fit-for-purpose for Alzheimer’s disease research.

Expression of N-Terminal Cysteine Aβ42 and Conjugation to Generate Fluorescent and Biotinylated Aβ42

Fluorescent derivatives of the β-amyloid peptides (Aβ) are valuable tools for studying the interactions of Aβ with cells. Facile access to labeled expressed Aβ offers the promise of Aβ with greater sequence and stereochemical integrity, without impurities from amino acid deletion and epimerization. Here, we report methods for the expression of Aβ42 with an N-terminal cysteine residue, Aβ(C1-42), and its conjugation to generate Aβ42 bearing fluorophores or biotin. The methods rely on the hitherto unrecognized observation that expression of the Aβ(MC1-42) gene yields the Aβ(C1-42) peptide, because the N-terminal methionine is endogenously excised by Escherichia coli. Conjugation of Aβ(C1-42) with maleimide-functionalized fluorophores or biotin affords the N-terminally labeled Aβ42. The expression affords ∼14 mg of N-terminal cysteine Aβ from 1 L of bacterial culture. Subsequent conjugation affords ∼3 mg of labeled Aβ from 1 L of bacterial culture with minimal cost for labeling reagents. High-performance liquid chromatography analysis indicates the N-terminal cysteine Aβ to be >97% pure and labeled Aβ peptides to be 94-97% pure. Biophysical studies show that the labeled Aβ peptides behave like unlabeled Aβ and suggest that labeling of the N-terminus does not substantially alter the properties of the Aβ. We further demonstrate applications of the fluorophore-labeled Aβ peptides by using fluorescence microscopy to visualize their interactions with mammalian cells and bacteria. We anticipate that these methods will provide researchers convenient access to useful N-terminally labeled Aβ, as well as Aβ with an N-terminal cysteine that enables further functionalization.

Is the p3 Peptide (Aβ17-40, Aβ17-42) Relevant to the Pathology of Alzheimer's Disease?1

Despite the vast heterogeneity of amyloid plaques isolated from the brains of those with Alzheimer's Disease (AD), the basis of the Amyloid Cascade Hypothesis targets a single peptide, the amyloid-β (Aβ) peptide. The countless therapeutic efforts targeting the production and aggregation of this specific peptide have been met with disappointment, leaving many to question the role of Aβ in AD. An alternative cleavage product of the Amyloid-β protein precursor, called the p3 peptide, which has also been isolated from the brains of AD patients, has been largely absent from most Aβ-related studies. Typically referred to as non-amyloidogenic and even suggested as neuroprotective, the p3 peptide has garnered little attention aside from some conflicting findings on cytotoxicity and potential self-assembly to form higher order aggregates. Herein, we report an extensive analysis of the findings surrounding p3 and offer some evidence as to why it may not be as innocuous as previously suggested.

Are N- and C-terminally truncated Aβ species key pathological triggers in Alzheimer's disease?

The histopathology of Alzheimer's disease (AD) is characterized by neuronal loss, neurofibrillary tangles, and senile plaque formation. The latter results from an exacerbated production (familial AD cases) or altered degradation (sporadic cases) of 40/42-amino acid-long β-amyloid peptides (Aβ peptides) that are produced by sequential cleavages of Aβ precursor protein (βAPP) by β- and γ-secretases. The amyloid cascade hypothesis proposes a key role for the full-length Aβ42 and the Aβ40/42 ratio in AD etiology, in which soluble Aβ oligomers lead to neurotoxicity, tau hyperphosphorylation, aggregation, and, ultimately, cognitive defects. However, following this postulate, during the last decade, several clinical approaches aimed at decreasing full-length Aβ42 production or neutralizing it by immunotherapy have failed to reduce or even stabilize AD-related decline. Thus, the Aβ peptide (Aβ40/42)-centric hypothesis is probably a simplified view of a much more complex situation involving a multiplicity of APP fragments and Aβ catabolites. Indeed, biochemical analyses of AD brain deposits and fluids have unraveled an Aβ peptidome consisting of additional Aβ-related species. Such Aβ catabolites could be due to either primary enzymatic cleavages of βAPP or secondary processing of Aβ itself by exopeptidases. Here, we review the diversity of N- and C-terminally truncated Aβ peptides and their biosynthesis and outline their potential function/toxicity. We also highlight their potential as new pharmaceutical targets and biomarkers.

Role of the N-terminus for the stability of an amyloid-β fibril with three-fold symmetry

A key player in Alzheimer’s disease is the peptide amyloid-beta (Aβ), whose aggregation into small soluble oligomers, protofilaments, and fibrils finally leads to plaque deposits in human brains. The aggregation behavior of Aβ is strongly modulated by the nature and composition of the peptide’s environment and by its primary sequence properties. The N-terminal residues of Aβ play an important role, because they are known to change the peptide’s aggregation propensity. Since these residues are for the first time completely resolved at the molecular level in a three-fold symmetric fibril structure derived from a patient, we chose that system as template for a systematic investigation of the influence of the N-terminus upon structural stability. Using atomistic molecular dynamics simulations, we examined several fibrillar systems comprising three, six, twelve and an infinite number of layers, both with and without the first eight residues. First, we found that three layers are not sufficient to stabilize the respective Aβ topology. Second, we observed a clear stabilizing effect of the N-terminal residues upon the overall fibril fold: truncated Aβ systems were less stable than their full-length counterparts. The N-terminal residues Arg5, Asp7, and Ser8 were found to form important interfilament contacts stabilizing the overall fibril structure of three-fold symmetry. Finally, similar structural rearrangements of the truncated Aβ species in different simulations prompted us to suggest a potential mechanism involved in the formation of amyloid fibrils with three-fold symmetry.

Glutaminyl cyclase activity correlates with levels of Aβ peptides and mediators of angiogenesis in cerebrospinal fluid of Alzheimer's disease patients

Background

Pyroglutamylation of truncated Aβ peptides, which is catalysed by enzyme glutaminyl cyclase (QC), generates pE-Aβ species with enhanced aggregation propensities and resistance to most amino-peptidases and endo-peptidases. pE-Aβ species have been identified as major constituents of Aβ plaques and reduction of pE-Aβ species is associated with improvement of cognitive tasks in animal models of Alzheimer’s disease (AD). Pharmacological inhibition of QC has thus emerged as a promising therapeutic approach for AD. Here, we question whether cerebrospinal fluid (CSF) QC enzymatic activity differs between AD patients and controls and whether inflammatory or angiogenesis mediators, some of which are potential QC substrates, and/or Aβ peptides may serve as pharmacodynamic read-outs for QC inhibition.

Methods

QC activity, Aβ peptides and inflammatory or angiogenesis mediators were measured in CSF of a clinically well-characterized cohort of 20 mild AD patients, 20 moderate AD patients and 20 subjective memory complaints (SMC) controls. Correlation of these parameters with core diagnostic CSF AD biomarkers (Aβ42, tau and p-tau) and clinical features was evaluated.

Results

QC activity shows a tendency to decrease with AD progression (p = 0.129). The addition of QC activity to biomarkers tau and p-tau significantly increases diagnostic power (ROC-AUC_TAU = 0.878, ROC-AUC_TAU&_QC = 0.939 and ROC-AUC_pTAU = 0.820, ROC-AUC_pTAU&_QC = 0.948). In AD and controls, QC activity correlates with Aβ38 (r = 0.83, p < 0.0001) and Aβ40 (r = 0.84, p < 0.0001), angiogenesis mediators (Flt1, Tie2, VEGFD, VCAM-1 and ICAM-1, r > 0.5, p < 0.0001) and core diagnostic biomarkers (r > 0.35, p = <0.0057). QC activity does not correlate with MMSE or ApoE genotype.

Conclusions

Aβ38, Aβ40 and angiogenesis mediators (Flt1, Tie2, VEGFD, VCAM-1 and ICAM-1) are potential pharmacodynamic markers of QC inhibition, because their levels closely correlate with QC activity in AD patients. The addition of QC activity to core diagnostic CSF biomarkers may be of specific interest in clinical cases with discordant imaging and biochemical biomarker results.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-017-0266-6) contains supplementary material, which is available to authorized users.

Differential levels of p75NTR ectodomain in CSF and blood in patients with Alzheimer's disease: a novel diagnostic marker

Alzheimer's disease (AD) is the primary cause of dementia in the elderly. The ectodomain of p75 neurotrophin receptor (p75NTR-ECD) has been suggested to play important roles in regulating beta-amyloid (Aβ) deposition and in protecting neurons from the toxicity of soluble Aβ. However, whether and how the serum and cerebrospinal fluid (CSF) levels of p75NTR-ECD change in patients with AD are not well documented. In the present study, we determined the concentrations of serum p75NTR-ECD in an AD group, a Parkinson disease group and a stroke group, as well as in a group of elderly controls without neurological disorders (EC). We also determined the levels of CSF p75NTR-ECD in a subset of the AD and EC groups. Our data showed that a distinct p75NTR-ECD profile characterized by a decreased CSF level and an increased serum level was present concomitantly with AD patients but not with other diseases. p75NTR-ECD levels in both the serum and CSF were strongly correlated with Mini-Mental State Examination (MMSE) scores and showed sound differential diagnostic value for AD. Moreover, when combining CSF Aβ42, CSF Aβ42/40, CSF ptau181 or CSF ptau181/Aβ42 with CSF p75NTR-ECD, the area under the receiver operating characteristic curve (AUC) and diagnostic accuracies improved. These findings indicate that p75NTR-ECD can serve as a specific biomarker for AD and the determination of serum and CSF p75NTR-ECD levels is likely to be helpful in monitoring AD progression.

Body fluid biomarkers in Alzheimer's disease

A heterogeneous and slowly progressive disease with extracellular amyloid-β (Aβ) deposits and intracellular hyperphosphorylated tau protein aggregates, Alzheimer's disease (AD) is already a hard nut to crack, featured with cognitive decline and memory lapse. Body fluid biomarkers are proved to be useful in exploring further study of AD, might benefit for a full comprehension of the etiopathogenesis, an improved precision of the prognosis and diagnosis, and a positive response of treatments. The cerebrospinal fluid biomarkers Aβ, total tau, and hyperphosphorylated tau reflect the main pathologic changes of AD. We also review data from several novel biomarkers, such as, β-site APP cleaving enzyme 1, soluble amyloid precursor proteins α and β, soluble Aβ oligomers and so on, which are associated with the occurrence and deterioration of this disease and couldn't be ignored. The rationale for the clinical use of those biomarkers, the challenges faced with and the properties of the most appropriate biomarkers are also summarized in the paper. We aim to find several ideal biomarkers to improve the diagnosis and optimize the treatment respectively.

Study on Aβ34 biology and detection in transgenic mice brains

The β-amyloid precursor protein undergoes cleavages by β- and γ-secretasses yielding amyloid-β peptides (Aβ) that accumulate in Alzheimer's disease. Subsequently, Aβ peptides are targets of additional truncations or endoproteolytic cleavages explaining the diversity of Aβ-related fragments recovered in cell media or pathologic human fluids. Here, we focused on Aβ1-34 (Aβ34) that has been detected both in vitro and in vivo and that derives from the hydrolysis of Aβ by β-secretase. We have obtained and fully characterized by immunologic and biochemical approaches, a polyclonal antibody that specifically recognizes the C-terminus of Aβx-34. We present immunohistochemical evidence for the presence of Aβx-34 in the brain of 3xTg mice and Alzheimer's disease-affected human brains. Finally, we demonstrate a neprilysin-mediated degradation process of Aβ34 and the ability of synthetic Aβ34 to protect HEK cells overexpressing either wild type or Swedish-mutated β-amyloid precursor protein from apoptosis.