Randall J. Bateman

Clinical and Biomarker Changes in Dominantly Inherited Alzheimer's Disease

BACKGROUND The order and magnitude of pathologic processes in Alzheimers disease are not well understood, partly because the disease develops over many years. Autosomal dominant Alzheimers disease has a predictable age at onset and provides an opportunity to determine the sequence and magnitude of pathologic changes that culminate in symptomatic disease. METHODS In this prospective, longitudinal study, we analyzed data from 128 participants who underwent baseline clinical and cognitive assessments, brain imaging, and cerebrospinal fluid (CSF) and blood tests. We used the participants age at baseline assessment and the parents age at the onset of symptoms of Alzheimers disease to calculate the estimated years from expected symptom onset (age of the participant minus parents age at symptom onset). We conducted cross-sectional analyses of baseline data in relation to estimated years from expected symptom onset in order to determine the relative order and magnitude of pathophysiological changes. RESULTS Concentrations of amyloid-beta (Aβ)(42) in the CSF appeared to decline 25 years before expected symptom onset. Aβ deposition, as measured by positron-emission tomography with the use of Pittsburgh compound B, was detected 15 years before expected symptom onset. Increased concentrations of tau protein in the CSF and an increase in brain atrophy were detected 15 years before expected symptom onset. Cerebral hypometabolism and impaired episodic memory were observed 10 years before expected symptom onset. Global cognitive impairment, as measured by the Mini-Mental State Examination and the Clinical Dementia Rating scale, was detected 5 years before expected symptom onset, and patients met diagnostic criteria for dementia at an average of 3 years after expected symptom onset. CONCLUSIONS We found that autosomal dominant Alzheimers disease was associated with a series of pathophysiological changes over decades in CSF biochemical markers of Alzheimers disease, brain amyloid deposition, and brain metabolism as well as progressive cognitive impairment. Our results require confirmation with the use of longitudinal data and may not apply to patients with sporadic Alzheimers disease. (Funded by the National Institute on Aging and others; DIAN ClinicalTrials.gov number, NCT00869817.).

Advancing research diagnostic criteria for Alzheimer's disease: the IWG-2 criteria

In the past 8 years, both the International Working Group (IWG) and the US National Institute on Aging-Alzheimers Association have contributed criteria for the diagnosis of Alzheimers disease (AD) that better define clinical phenotypes and integrate biomarkers into the diagnostic process, covering the full staging of the disease. This Position Paper considers the strengths and limitations of the IWG research diagnostic criteria and proposes advances to improve the diagnostic framework. On the basis of these refinements, the diagnosis of AD can be simplified, requiring the presence of an appropriate clinical AD phenotype (typical or atypical) and a pathophysiological biomarker consistent with the presence of Alzheimers pathology. We propose that downstream topographical biomarkers of the disease, such as volumetric MRI and fluorodeoxyglucose PET, might better serve in the measurement and monitoring of the course of disease. This paper also elaborates on the specific diagnostic criteria for atypical forms of AD, for mixed AD, and for the preclinical states of AD.

Decreased Clearance of CNS β-Amyloid in Alzheimer’s Disease

Alzheimer’s disease is associated with reduced β-amyloid clearance from the brain Alzheimer’s disease is hypothesized to be caused by an imbalance between β-amyloid (Aβ) production and clearance that leads to Aβ accumulation in the central nervous system (CNS). Aβ 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 Aβ production or clearance in Alzheimer’s disease. By using metabolic labeling, we measured Aβ42 and Aβ40 production and clearance rates in the CNS of participants with Alzheimer’s disease and cognitively normal controls. Clearance rates for both Aβ42 and Aβ40 were impaired in Alzheimer’s disease compared with controls. On average, there were no differences in Aβ40 or Aβ42 production rates. Thus, the common late-onset form of Alzheimer’s disease is characterized by an overall impairment in Aβ clearance.

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

Alzheimer’s disease is associated with reduced β-amyloid clearance from the brain Alzheimer’s disease is hypothesized to be caused by an imbalance between β-amyloid (Aβ) production and clearance that leads to Aβ accumulation in the central nervous system (CNS). Aβ 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 Aβ production or clearance in Alzheimer’s disease. By using metabolic labeling, we measured Aβ42 and Aβ40 production and clearance rates in the CNS of participants with Alzheimer’s disease and cognitively normal controls. Clearance rates for both Aβ42 and Aβ40 were impaired in Alzheimer’s disease compared with controls. On average, there were no differences in Aβ40 or Aβ42 production rates. Thus, the common late-onset form of Alzheimer’s disease is characterized by an overall impairment in Aβ clearance.

Amyloid-β Dynamics are Regulated by Orexin and the Sleep-Wake Cycle

Sleep and Alzheimers Disease Accumulation of amyloid-β (Aβ) in the brain is thought to be the initiating event in the pathogenesis of Alzheimers disease (AD). Aβ is a peptide secreted in a soluble monomeric form predominantly by neurons and its aggregation into toxic forms is concentration dependent. Synaptic activity regulates the release of Aβ in vivo. However, how physiological and environmental processes are involved in regulation of Aβ levels is not understood. Kang et al. (p. 1005, published online 24 September), by performing sleep-wake studies in freely behaving animals concomitant with in vivo microdialysis, found that brain interstitial fluid levels of Aβ were significantly correlated with wakefulness and negatively correlated with sleep. Furthermore, relatively short-term (3 weeks) sleep deprivation markedly accelerated amyloid plaque deposition in amyloid precursor protein transgenic mice. Thus, sleep-wake behavior is linked to Aβ levels and abnormal sleep may be linked to AD pathogenesis. Sleep patterns can influence amyloid plaque formation in a mouse model of Alzheimer’s disease. Amyloid-β (Aβ) accumulation in the brain extracellular space is a hallmark of Alzheimer’s disease. The factors regulating this process are only partly understood. Aβ aggregation is a concentration-dependent process that is likely responsive to changes in brain interstitial fluid (ISF) levels of Aβ. Using in vivo microdialysis in mice, we found that the amount of ISF Aβ correlated with wakefulness. The amount of ISF Aβ also significantly increased during acute sleep deprivation and during orexin infusion, but decreased with infusion of a dual orexin receptor antagonist. Chronic sleep restriction significantly increased, and a dual orexin receptor antagonist decreased, Aβ plaque formation in amyloid precursor protein transgenic mice. Thus, the sleep-wake cycle and orexin may play a role in the pathogenesis of Alzheimer’s disease.

Human apoE isoforms differentially regulate brain amyloid-β peptide clearance

Human apoE4 increases the concentration of soluble Aβ in the brain by impairing its clearance. Clearing the Debris in Alzheimer’s Disease The strongest risk factor for developing the common sporadic form of Alzheimer’s disease (AD) that occurs in old age is the ε4 allele encoding apolipoprotein E4 (apoE4). Two ε4 alleles can lower the age of onset of AD by 10 to 15 years. In contrast, the ε2 allele decreases the risk of developing this neurodegenerative disorder. APOE is important for lipoprotein metabolism, but how it might be involved in AD has remained unclear. It has been suggested that the apoE4 isoform might somehow help to drive accumulation of the peptide amyloid-β (Aβ), which forms amyloid plaques in the brain that contribute to neuronal death and are the characteristic hallmark of AD. In a tour de force study in humans and mice, Holtzman and his team at Washington University in St. Louis now show that apoE4 contributes to Aβ accumulation in the brain not by affecting Aβ synthesis but by affecting its clearance. First, the authors looked at the Aβ concentration in the cerebrospinal fluid (CSF) of cognitively normal individuals under age 70 carrying different APOE genotypes. They found that those with the ε4/ε4 genotype had a much lower CSF Aβ concentration than did those with the protective ε2/ε3 genotype. A CSF Aβ concentration of less than 500 pg/ml is an indication that Aβ peptide is accumulating in the brain and thus is not moving into the CSF. Next, the researchers analyzed imaging data using a dye called Pittsburgh compound B that binds to amyloid plaques in the brain and showed that those individuals with the ε4/ε4 genotype bound more dye than did those with the other APOE genotypes. They then moved to a mouse model of AD in which the mice expressed one of the three human apoE isoforms. They measured Aβ concentrations in the interstitial fluid of these mice using in vivo microdialysis and then looked at stained hippocampal sections from these mice. They found greater Aβ concentrations in both interstitial fluid and the hippocampus in mice expressing the human apoE4 isoform than in animals expressing either the E3 or E2 isoforms. They discovered that this difference in Aβ concentration between the mice carrying different APOE genotypes was present in young as well as aged mice, suggesting that it predates the appearance of amyloid plaques. They then measured clearance of Aβ from the interstitial fluid of young mice and showed that those with the human apoE4 isoform were less able to clear Aβ than those with the apoE2 or apoE3 isoforms. The researchers showed that processing of the amyloid precursor protein and generation of the Aβ peptide did not vary according to genotype, lending credence to the hypothesis that apoE4 affects clearance of Aβ but not its synthesis. This thorough study sheds new light on how apoE4 is implicated in AD and highlights the Aβ clearance pathway as a new target for developing drugs to slow or even halt the accumulation of amyloid plaques in patients with AD. The apolipoprotein E (APOE) ε4 allele is the strongest genetic risk factor for late-onset, sporadic Alzheimer’s disease (AD). The APOE ε4 allele markedly increases AD risk and decreases age of onset, likely through its strong effect on the accumulation of amyloid-β (Aβ) peptide. In contrast, the APOE ε2 allele appears to decrease AD risk. Most rare, early-onset forms of familial AD are caused by autosomal dominant mutations that often lead to overproduction of Aβ42 peptide. However, the mechanism by which APOE alleles differentially modulate Aβ accumulation in sporadic, late-onset AD is less clear. In a cohort of cognitively normal individuals, we report that reliable molecular and neuroimaging biomarkers of cerebral Aβ deposition vary in an apoE isoform–dependent manner. We hypothesized that human apoE isoforms differentially affect Aβ clearance or synthesis in vivo, resulting in an apoE isoform–dependent pattern of Aβ accumulation later in life. Performing in vivo microdialysis in a mouse model of Aβ-amyloidosis expressing human apoE isoforms (PDAPP/TRE), we find that the concentration and clearance of soluble Aβ in the brain interstitial fluid depends on the isoform of apoE expressed. This pattern parallels the extent of Aβ deposition observed in aged PDAPP/TRE mice. ApoE isoform–dependent differences in soluble Aβ metabolism are observed not only in aged but also in young PDAPP/TRE mice well before the onset of Aβ deposition in amyloid plaques in the brain. Additionally, amyloidogenic processing of amyloid precursor protein and Aβ synthesis, as assessed by in vivo stable isotopic labeling kinetics, do not vary according to apoE isoform in young PDAPP/TRE mice. Our results suggest that APOE alleles contribute to AD risk by differentially regulating clearance of Aβ from the brain, suggesting that Aβ clearance pathways may be useful therapeutic targets for AD prevention.

Human amyloid-beta synthesis and clearance rates as measured in cerebrospinal fluid in vivo.

Certain disease states are characterized by disturbances in production, accumulation or clearance of protein. In Alzheimer disease, accumulation of amyloid-β (Aβ) in the brain and disease-causing mutations in amyloid precursor protein or in enzymes that produce Aβ indicate dysregulation of production or clearance of Aβ. Whether dysregulation of Aβ synthesis or clearance causes the most common form of Alzheimer disease (sporadic, >99% of cases), however, is not known. Here, we describe a method to determine the production and clearance rates of proteins within the human central nervous system (CNS). We report the first measurements of the fractional production and clearance rates of Aβ in vivo in the human CNS to be 7.6% per hour and 8.3% per hour, respectively. This method may be used to search for novel biomarkers of disease, to assess underlying differences in protein metabolism that contribute to disease and to evaluate treatments in terms of their pharmacodynamic effects on proposed disease-causing pathways.

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

Accumulation of amyloid‐β (Aβ) by overproduction or underclearance in the central nervous system (CNS) is hypothesized to be a necessary event in the pathogenesis of Alzheimers disease. However, previously, there has not been a method to determine drug effects on Aβ production or clearance in the human CNS. The objective of this study was to determine the effects of a γ‐secretase inhibitor on the production of Aβ in the human CNS.

Matrix metalloproteinase-9 degrades amyloid-β fibrils in vitro and compact plaques in situ

The pathological hallmark of Alzheimer disease is the senile plaque principally composed of tightly aggregated amyloid-β fibrils (fAβ), which are thought to be resistant to degradation and clearance. In this study, we explored whether proteases capable of degrading soluble Aβ (sAβ) could degrade fAβ as well. We demonstrate that matrix metalloproteinase-9 (MMP-9) can degrade fAβ and that this ability is not shared by other sAβ-degrading enzymes examined, including endothelin-converting enzyme, insulin-degrading enzyme, and neprilysin. fAβ was decreased in samples incubated with MMP-9 compared with other proteases, assessed using thioflavin-T. Furthermore, fAβ breakdown with MMP-9 but not with other proteases was demonstrated by transmission electron microscopy. Proteolytic digests of purified fAβ were analyzed with matrix-assisted laser desorption ionization time-of-flight mass spectrometry to identify sites of Aβ that are cleaved during its degradation. Only MMP-9 digests contained fragments (Aβ1-20 and Aβ1-30) from fAβ1-42 substrate; the corresponding cleavage sites are thought to be important for β-pleated sheet formation. To determine whether MMP-9 can degrade plaques formed in vivo, fresh brain slices from aged APP/PS1 mice were incubated with proteases. MMP-9 digestion resulted in a decrease in thioflavin-S (ThS) staining. Consistent with a role for endogenous MMP-9 in this process in vivo, MMP-9 immunoreactivity was detected in astrocytes surrounding amyloid plaques in the brains of aged APP/PS1 and APPsw mice, and increased MMP activity was selectively observed in compact ThS-positive plaques. These findings suggest that MMP-9 can degrade fAβ and may contribute to ongoing clearance of plaques from amyloid-laden brains.

Autosomal-dominant Alzheimer's disease: a review and proposal for the prevention of Alzheimer's disease

Autosomal-dominant Alzheimers disease has provided significant understanding of the pathophysiology of Alzheimers disease. The present review summarizes clinical, pathological, imaging, biochemical, and molecular studies of autosomal-dominant Alzheimers disease, highlighting the similarities and differences between the dominantly inherited form of Alzheimers disease and the more common sporadic form of Alzheimers disease. Current developments in autosomal-dominant Alzheimers disease are presented, including the international Dominantly Inherited Alzheimer Network and this networks initiative for clinical trials. Clinical trials in autosomal-dominant Alzheimers disease may test the amyloid hypothesis, determine the timing of treatment, and lead the way to Alzheimers disease prevention.