Age and amyloid effects on human central nervous system amyloid-beta kinetics.

OBJECTIVE: Age is the single greatest risk factor for Alzheimer's disease (AD), with the incidence doubling every 5 years after age 65. However, our understanding of the mechanistic relationship between increasing age and the risk for AD is currently limited. We therefore sought to determine the relationship between age, amyloidosis, and amyloid-beta (Aß) kinetics in the central nervous system (CNS) of humans. METHODS: Aß kinetics were analyzed in 112 participants and compared to the ages of participants and the amount of amyloid deposition. RESULTS: We found a highly significant correlation between increasing age and slowed Aß turnover rates (2.5-fold longer half-life over five decades of age). In addition, we found independent effects on Aß42 kinetics specifically in participants with amyloid deposition. Amyloidosis was associated with a higher (>50%) irreversible loss of soluble Aß42 and a 10-fold higher Aß42 reversible exchange rate. INTERPRETATION: These findings reveal a mechanistic link between human aging and the risk of amyloidosis, which may be owing to a dramatic slowing of Aß turnover, increasing the likelihood of protein misfolding that leads to deposition. Alterations in Aß kinetics associated with aging and amyloidosis suggest opportunities for diagnostic and therapeutic strategies. More generally, this study provides an example of how changes in protein turnover kinetics can be used to detect physiological and pathophysiological changes and may be applicable to other proteinopathies.

Amyloid-ß efflux from the central nervous system into the plasma.

OBJECTIVE: The aim of this study was to measure the flux of amyloid-ß (Aß) across the human cerebral capillary bed to determine whether transport into the blood is a significant mechanism of clearance for Aß produced in the central nervous system (CNS). METHODS: Time-matched blood samples were simultaneously collected from a cerebral vein (including the sigmoid sinus, inferior petrosal sinus, and the internal jugular vein), femoral vein, and radial artery of patients undergoing inferior petrosal sinus sampling. For each plasma sample, Aß concentration was assessed by 3 assays, and the venous to arterial Aß concentration ratios were determined. RESULTS: Aß concentration was increased by ∼7.5% in venous blood leaving the CNS capillary bed compared to arterial blood, indicating efflux from the CNS into the peripheral blood (p < 0.0001). There was no difference in peripheral venous Aß concentration compared to arterial blood concentration. INTERPRETATION: Our results are consistent with clearance of CNS-derived Aß into the venous blood supply with no increase from a peripheral capillary bed. Modeling these results suggests that direct transport of Aß across the blood-brain barrier accounts for ∼25% of Aß clearance, and reabsorption of cerebrospinal fluid Aß accounts for ∼25% of the total CNS Aß clearance in humans. Ann Neurol 2014;76:837-844.

Amyloid-beta isoform metabolism quantitation by stable isotope-labeled kinetics.

Abundant evidence suggests a central role for the amyloid-beta (Aß) peptide in Alzheimer's disease (AD) pathogenesis. Production and clearance of different Aß isoforms have been established as targets of proposed disease-modifying therapeutic treatments of AD. However, previous studies used multiple sequential purification steps to isolate the isoforms individually and quantitate them based on a common mid-domain peptide. We created a method to simultaneously purify Aß isoforms and quantitate them by the specific C-terminal peptides in order to investigate Aß isoform physiology in the central nervous system. By using standards generated from in vitro metabolic labeling, the relative quantitation of four peptides representing total amount of Aß (Aß-Total), Aß38, Aß40, and Aß42 were achieved both in cell culture and in human cerebrospinal fluid (CSF). Standard curves for each isoform demonstrated good sensitivity with very low limits of detection and high accuracy. Because the assay does not require antibody development for each Aß isoform peptide, significant improvements in the throughput and accuracy of isoform quantitation were achieved.

A gamma-secretase inhibitor decreases amyloid-beta production in the central nervous system.

OBJECTIVE: Accumulation of amyloid-beta (Abeta) by overproduction or underclearance in the central nervous system (CNS) is hypothesized to be a necessary event in the pathogenesis of Alzheimer's disease. However, previously, there has not been a method to determine drug effects on Abeta production or clearance in the human CNS. The objective of this study was to determine the effects of a gamma-secretase inhibitor on the production of Abeta in the human CNS. METHODS: We utilized a recently developed method of stable-isotope labeling combined with cerebrospinal fluid sampling to directly measure Abeta production during treatment of a gamma-secretase inhibitor, LY450139. We assessed whether this drug could decrease CNS Abeta production in healthy men (age range, 21-50 years) at single oral doses of 100, 140, or 280mg (n = 5 per group). RESULTS: LY450139 significantly decreased the production of CNS Abeta in a dose-dependent fashion, with inhibition of Abeta generation of 47, 52, and 84% over a 12-hour period with doses of 100, 140, and 280mg, respectively. There was no difference in Abeta clearance. INTERPRETATION: Stable isotope labeling of CNS proteins can be utilized to assess the effects of drugs on the production and clearance rates of proteins targeted as potential disease-modifying treatments for Alzheimer's disease and other CNS disorders. Results from this approach can assist in making decisions about drug dosing and frequency in the design of larger and longer clinical trials for diseases such as Alzheimer's disease, and may accelerate effective drug validation. Ann Neurol 2009.

Human fibroblast and stem cell resource from the Dominantly Inherited Alzheimer Network.

BACKGROUND: Mutations in amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2) cause autosomal dominant forms of Alzheimer disease (ADAD). More than 280 pathogenic mutations have been reported in APP, PSEN1, and PSEN2. However, understanding of the basic biological mechanisms that drive the disease are limited. The Dominantly Inherited Alzheimer Network (DIAN) is an international observational study of APP, PSEN1, and PSEN2 mutation carriers with the goal of determining the sequence of changes in presymptomatic mutation carriers who are destined to develop Alzheimer disease. RESULTS: We generated a library of 98 dermal fibroblast lines from 42 ADAD families enrolled in DIAN. We have reprogrammed a subset of the DIAN fibroblast lines into patient-specific induced pluripotent stem cell (iPSC) lines. These cells were thoroughly characterized for pluripotency markers. CONCLUSIONS: This library represents a comprehensive resource that can be used for disease modeling and the development of novel therapeutics.

A single dose of the γ-secretase inhibitor semagacestat alters the cerebrospinal fluid peptidome in humans.

BACKGROUND: In Alzheimer's disease, beta-amyloid peptides in the brain aggregate into toxic oligomers and plaques, a process which is associated with neuronal degeneration, memory loss, and cognitive decline. One therapeutic strategy is to decrease the production of potentially toxic beta-amyloid species by the use of inhibitors or modulators of the enzymes that produce beta-amyloid from amyloid precursor protein (APP). The failures of several such drug candidates by lack of effect or undesired side-effects underscore the importance to monitor the drug effects in the brain on a molecular level. Here we evaluate if peptidomic analysis in cerebrospinal fluid (CSF) can be used for this purpose. METHODS: Fifteen human healthy volunteers, divided into three groups, received a single dose of placebo or either 140 mg or 280 mg of the γ-secretase inhibitor semagacestat (LY450139). Endogenous peptides in CSF, sampled prior to administration of the drug and at six subsequent time points, were analyzed by liquid chromatography coupled to mass spectrometry, using isobaric labeling based on the tandem mass tag approach for relative quantification. RESULTS: Out of 302 reproducibly detected peptides, 11 were affected by the treatment. Among these, one was derived from APP and one from amyloid precursor-like protein 1. Nine peptides were derived from proteins that may not be γ-secretase substrates per se, but that are regulated in a γ-secretase-dependent manner. CONCLUSIONS: These results indicate that a CSF peptidomic approach may be a valuable tool both to verify target engagement and to identify other pharmacodynamic effects of the drug. Data are available via ProteomeXchange with identifier PXD003075. TRIAL REGISTRATION: NCT00765115 , registered 30/09/2008.

Diurnal patterns of soluble amyloid precursor protein metabolites in the human central nervous system.

The amyloid-ß (Aß) protein is diurnally regulated in both the cerebrospinal fluid and blood in healthy adults; circadian amplitudes decrease with aging and the presence of cerebral Aß deposits. The cause of the Aß diurnal pattern is poorly understood. One hypothesis is that the Amyloid Precursor Protein (APP) is diurnally regulated, leading to APP product diurnal patterns. APP in the central nervous system is processed either via the ß-pathway (amyloidogenic), generating soluble APP-ß (sAPPß) and Aß, or the α-pathway (non-amyloidogenic), releasing soluble APP-α (sAPPα). To elucidate the potential contributions of APP to the Aß diurnal pattern and the balance of the α- and ß- pathways in APP processing, we measured APP proteolytic products over 36 hours in human cerebrospinal fluid from cognitively normal and Alzheimer's disease participants. We found diurnal patterns in sAPPα, sAPPß, Aß40, and Aß42, which diminish with increased age, that support the hypothesis that APP is diurnally regulated in the human central nervous system and thus results in Aß diurnal patterns. We also found that the four APP metabolites were positively correlated in all participants without cerebral Aß deposits. This positive correlation suggests that the α- and ß- APP pathways are non-competitive under normal physiologic conditions where APP availability may be the limiting factor that determines sAPPα and sAPPß production. However, in participants with cerebral Aß deposits, there was no correlation of Aß to sAPP metabolites, suggesting that normal physiologic regulation of cerebrospinal fluid Aß is impaired in the presence of amyloidosis. Lastly, we found that the ratio of sAPPß to sAPPα was significantly higher in participants with cerebral Aß deposits versus those without deposits. Therefore, the sAPPß to sAPPα ratio may be a useful biomarker for cerebral amyloidosis.

Increased in vivo amyloid-ß42 production, exchange, and loss in presenilin mutation carriers.

Alzheimer's disease (AD) is hypothesized to be caused by an overproduction or reduced clearance of amyloid-ß (Aß) peptide. Autosomal dominant AD (ADAD) caused by mutations in the presenilin (PSEN) gene have been postulated to result from increased production of Aß42 compared to Aß40 in the central nervous system (CNS). This has been demonstrated in rodent models of ADAD but not in human mutation carriers. We used compartmental modeling of stable isotope labeling kinetic (SILK) studies in human carriers of PSEN mutations and related noncarriers to evaluate the pathophysiological effects of PSEN1 and PSEN2 mutations on the production and turnover of Aß isoforms. We compared these findings by mutation status and amount of fibrillar amyloid deposition as measured by positron emission tomography (PET) using the amyloid tracer Pittsburgh compound B (PIB). CNS Aß42 to Aß40 production rates were 24% higher in mutation carriers compared to noncarriers, and this was independent of fibrillar amyloid deposits quantified by PET PIB imaging. The fractional turnover rate of soluble Aß42 relative to Aß40 was 65% faster in mutation carriers and correlated with amyloid deposition, consistent with increased deposition of Aß42 into plaques, leading to reduced recovery of Aß42 in cerebrospinal fluid (CSF). Reversible exchange of Aß42 peptides with preexisting unlabeled peptide was observed in the presence of plaques. These findings support the hypothesis that Aß42 is overproduced in the CNS of humans with PSEN mutations that cause AD, and demonstrate that soluble Aß42 turnover and exchange processes are altered in the presence of amyloid plaques, causing a reduction in Aß42 concentrations in the CSF.

Effects of age and amyloid deposition on Aß dynamics in the human central nervous system.

BACKGROUND: The amyloid hypothesis predicts that increased production or decreased clearance of ß-amyloid (Aß) leads to amyloidosis, which ultimately culminates in Alzheimer disease (AD). OBJECTIVE: To investigate whether dynamic changes in Aß levels in the human central nervous system may be altered by aging or by the pathology of AD and thus contribute to the risk of AD. DESIGN: Repeated-measures case-control study. SETTING: Washington University School of Medicine in St Louis, Missouri. PARTICIPANTS: Participants with amyloid deposition, participants without amyloid deposition, and younger normal control participants. MAIN OUTCOME MEASURES: In this study, hourly cerebrospinal fluid (CSF) Aß concentrations were compared with age, status of amyloid deposition, electroencephalography, and video recording data. RESULTS: Linear increases were observed over time in the Aß levels in CSF samples obtained from the younger normal control participants and the older participants without amyloid deposition, but not from the older participants with amyloid deposition. Significant circadian patterns were observed in the Aß levels in CSF samples obtained from the younger control participants; however, circadian amplitudes decreased in both older participants without amyloid deposition and older participants with amyloid deposition. Aß diurnal concentrations were correlated with the amount of sleep but not with the various activities that the participants participated in while awake. CONCLUSIONS: A reduction in the linear increase in the Aß levels in CSF samples that is associated with amyloid deposition and a decreased CSF Aß diurnal pattern associated with increasing age disrupt the normal physiology of Aß dynamics and may contribute to AD.

ß-amyloid dynamics in human plasma.

OBJECTIVES: To investigate dynamic changes in human plasma ß-amyloid (Aß) concentrations, evaluate the effects of aging and amyloidosis on these dynamics, and determine their correlation with cerebrospinal fluid (CSF) Aß concentrations. DESIGN: A repeated plasma and CSF sampling study. SETTING: The Washington University School of Medicine in St Louis, Missouri. PARTICIPANTS: Older adults with amyloid deposition (Amyloid+), age-matched controls without amyloid deposition (Amyloid-), and younger normal controls (YNCs) were enrolled for the study. MAIN OUTCOME MEASURES: Hourly measurements of plasma Aß were compared between groups by age and amyloidosis. Plasma Aß and CSF Aß concentrations were compared for correlation, linear increase, and circadian patterns. RESULTS: Circadian patterns were observed in plasma Aß, with diminished amplitudes with aging. Linear increase of Aß was only observed for CSF Aß in the YNC and Amyloid- groups, but not in the Amyloid+ group. No linear increase was observed for plasma Aß. No significant correlations were found between plasma and CSF Aß concentrations. CONCLUSIONS: Plasma Aß, like CSF, demonstrates a circadian pattern that is reduced in amplitude with increasing age but is unaffected by amyloid deposition. However, we found no evidence that plasma and CSF Aß concentrations were related on an hourly or individual basis.

Amyloid-ß(1-15/16) as a marker for γ-secretase inhibition in Alzheimer's disease.

Amyloid-ß (Aß) producing enzymes are key targets for disease-modifying Alzheimer's disease (AD) therapies since Aß trafficking is at the core of AD pathogenesis. Development of such drugs might benefit from the identification of markers indicating in vivo drug effects in the central nervous system. We have previously shown that Aß(1-15) is produced by concerted ß-and α-secretase cleavage of amyloid-ß protein precursor (AßPP). Here, we test the hypothesis that this pathway is more engaged upon γ-secretase inhibition in humans, and cerebrospinal fluid (CSF) levels of Aß(1-15/16) represent a biomarker for this effect. Twenty healthy men were treated with placebo (n = 5) or the γ-secretase inhibitor semagacestat (100 mg [n = 5], 140 mg [n = 5], or 280 mg [n = 5]). CSF samples were collected hourly over 36 hours and 10 time points were analyzed by immunoassay for Aß(1-15/16), Aß(x-38), Aß(x-40), Aß(x-42), sAßPPα, and sAßPPß. The CSF concentration of Aß(1-15/16) showed a dose-dependent response over 36 hours. In the 280 mg treatment group, a transient increase was seen with a maximum of 180% relative to baseline at 9 hours post administration of semagacestat. The concentrations of Aß(x-38), Aß(x-40), and Aß(x-42) decreased the first 9 hours followed by increased concentrations after 36 hours relative to baseline. No significant changes were detected for CSF sAßPPα and sAßPPß. Our data shows that CSF levels of Aß(1-15/16) increase during treatment with semagacestat supporting its feasibility as a pharmacodynamic biomarker for drug candidates aimed at inhibiting γ-secretase-mediated AßPP-processing.

In vivo human apolipoprotein E isoform fractional turnover rates in the CNS.

Apolipoprotein E (ApoE) is the strongest genetic risk factor for Alzheimer's disease and has been implicated in the risk for other neurological disorders. The three common ApoE isoforms (ApoE2, E3, and E4) each differ by a single amino acid, with ApoE4 increasing and ApoE2 decreasing the risk of Alzheimer's disease (AD). Both the isoform and amount of ApoE in the brain modulate AD pathology by altering the extent of amyloid beta (Aß) peptide deposition. Therefore, quantifying ApoE isoform production and clearance rates may advance our understanding of the role of ApoE in health and disease. To measure the kinetics of ApoE in the central nervous system (CNS), we applied in vivo stable isotope labeling to quantify the fractional turnover rates of ApoE isoforms in 18 cognitively-normal adults and in ApoE3 and ApoE4 targeted-replacement mice. No isoform-specific differences in CNS ApoE3 and ApoE4 turnover rates were observed when measured in human CSF or mouse brain. However, CNS and peripheral ApoE isoform turnover rates differed substantially, which is consistent with previous reports and suggests that the pathways responsible for ApoE metabolism are different in the CNS and the periphery. We also demonstrate a slower turnover rate for CSF ApoE than that for amyloid beta, another molecule critically important in AD pathogenesis.

Decreased clearance of CNS beta-amyloid in Alzheimer's disease.

Alzheimer's disease is hypothesized to be caused by an imbalance between ß-amyloid (Aß) production and clearance that leads to Aß accumulation in the central nervous system (CNS). Aß production and clearance are key targets in the development of disease-modifying therapeutic agents for Alzheimer's disease. However, there has not been direct evidence of altered Aß production or clearance in Alzheimer's disease. By using metabolic labeling, we measured Aß42 and Aß40 production and clearance rates in the CNS of participants with Alzheimer's disease and cognitively normal controls. Clearance rates for both Aß42 and Aß40 were impaired in Alzheimer's disease compared with controls. On average, there were no differences in Aß40 or Aß42 production rates. Thus, the common late-onset form of Alzheimer's disease is characterized by an overall impairment in Aß clearance.