Purpose of Review
To address the temporal sequencing of involvement of Aβ and tau in the pathogenesis of AD, and reconcile apparently conflicting neuropathologic and biomarker data.
While neuropathologic studies show that limbic system tau pathology occurs ubiquitously in middle aged individuals before the appearance of amyloid plaques, biomarker studies in living subjects suggest that Aβ pathology is the initiating event in AD and precedes CSF tau changes. Evidence from neuropathologic, biomarker, genetic, and cellular/mouse studies shows that tau accumulation in limbic regions occurs slowly with age and does not induce widespread neurodegeneration, but that Aβ interacts with tau in some way to accelerate neurofibrillary pathology and induce neurodegeneration.
Aβaggregation is the key initial trigger of AD pathologic changes, and interacts with tau to exacerbate age-related tauopathy and induce neurodegeneration.
tau; amyloid; biomarkers; neurofibrillary tangle; Aβ
Brain aging is associated with diminished circadian clock output and decreased expression of the core clock proteins, which regulate many aspects of cellular biochemistry and metabolism. The genes encoding clock proteins are expressed throughout the brain, though it is unknown whether these proteins modulate brain homeostasis. We observed that deletion of circadian clock transcriptional activators aryl hydrocarbon receptor nuclear translocator–like (Bmal1) alone, or circadian locomotor output cycles kaput (Clock) in combination with neuronal PAS domain protein 2 (Npas2), induced severe age-dependent astrogliosis in the cortex and hippocampus. Mice lacking the clock gene repressors period circadian clock 1 (Per1) and period circadian clock 2 (Per2) had no observed astrogliosis. Bmal1 deletion caused the degeneration of synaptic terminals and impaired cortical functional connectivity, as well as neuronal oxidative damage and impaired expression of several redox defense genes. Targeted deletion of Bmal1 in neurons and glia caused similar neuropathology, despite the retention of intact circadian behavioral and sleep-wake rhythms. Reduction of Bmal1 expression promoted neuronal death in primary cultures and in mice treated with a chemical inducer of oxidative injury and striatal neurodegeneration. Our findings indicate that BMAL1 in a complex with CLOCK or NPAS2 regulates cerebral redox homeostasis and connects impaired clock gene function to neurodegeneration.
Alzheimer’s disease is hypothesized to be caused by an over-production or reduced clearance of amyloid-beta (Aβ) peptide. Autosomal Dominant Alzheimer’s Disease (ADAD) caused by mutations in the presenilin (PSEN) gene have been postulated to result from increased production of Aβ42 compared to Aβ40 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 non-carriers to evaluate the pathophysiological effects of PSEN1 and PSEN2 mutations on the production and turnover of Aβ 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 Aβ42 to Aβ40 production rates were 24% higher in mutation carriers compared to non-carriers and this was independent of fibrillar amyloid deposits quantified by PET PiB imaging. The fractional turnover rate of soluble Aβ42 relative to Aβ40 was 65% faster in mutation carriers and correlated with amyloid deposition, consistent with increased deposition of Aβ42 into plaques leading to reduced recovery of Aβ42 in cerebrospinal fluid (CSF). Reversible exchange of Aβ42 peptides with pre-existing unlabeled peptide was observed in the presence of plaques. These findings support the hypothesis that Aβ42 is overproduced in the CNS of humans with presenilin mutations that cause AD, and demonstrate that soluble Aβ42 turnover and exchange processes are altered in the presence of amyloid plaques, causing a reduction in Aβ42 concentrations in the CSF.
The apolipoprotein E (APOE) genotype is the major genetic risk factor for Alzheimer's disease (AD). We have access to cerebrospinal fluid (CSF) and plasma APOE protein levels from 641 individuals and genome-wide genotyped data from 570 of these samples. The aim of this study was to test whether CSF or plasma APOE levels could be a useful endophenotype for AD and to identify genetic variants associated with APOE levels. We found that CSF (P = 8.15 × 10−4) but not plasma (P = 0.071) APOE protein levels are significantly associated with CSF Aβ42 levels. We used Mendelian randomization and genetic variants as instrumental variables to confirm that the association of CSF APOE with CSF Aβ42 levels and clinical dementia rating (CDR) is not because of a reverse causation or confounding effect. In addition the association of CSF APOE with Aβ42 levels was independent of the APOE ɛ4 genotype, suggesting that APOE levels in CSF may be a useful endophenotype for AD. We performed a genome-wide association study to identify genetic variants associated with CSF APOE levels: the APOE ɛ4 genotype was the strongest single-genetic factor associated with CSF APOE protein levels (P = 6.9 × 10−13). In aggregate, the Illumina chip single nucleotide polymorphisms explain 72% of the variability in CSF APOE protein levels, whereas the APOE ɛ4 genotype alone explains 8% of the variability. No other genetic variant reached the genome-wide significance threshold, but nine additional variants exhibited a P-value <10−6. Pathway mining analysis indicated that these nine additional loci are involved in lipid metabolism (P = 4.49 × 10−9).
Biomarkers are needed to improve the sensitivity and accuracy of diagnosis as well as prognosis in individuals with early Alzheimer disease (AD). Measures of brain structure and disease-related proteins in the cerebrospinal fluid (CSF) have been proposed as biomarkers, yet relatively little is known about the relationships between such measures. The present study was conducted to assess the relationship between CSF Aβ and tau protein levels and longitudinal measures of hippocampal structure in individuals with and without very mild dementia of the Alzheimer type.
A single CSF sample and longitudinal MR scans were collected. The CSF samples were assayed for tau, p-tau181, Aβ1–42 and Aβ1–40 by ELISA. Large-deformation diffeomorphic metric mapping was used to generate hippocampal surfaces, and a composite hippocampal surface (previously constructed from 86 healthy participants) was used as a structural reference.
Patients or Other Participants
13 participants with very mild AD (Clinical Dementia Rating, CDR 0.5) and 11 cognitively normal participants (CDR 0).
Main Outcome Measures
Initial and rate-of-change measures of total hippocampal volume and displacement of the hippocampal surface within zones overlying the CA1, subiculum and CA2-4+DG cellular subfields. Their correlations with initial CSF measures.
Lower CSF Aβ1–42 levels and higher tau/Aβ1–42 and p-tau181/Aβ1–42 ratios were strongly correlated with decreases in hippocampal volume and measure of progressive inward deformations of the CA1 subfield in participants with early AD, but not cognitively normal participants.
Despite small sample size, we found that Aβ1–42 and tau-related CSF measures were related to hippocampal degeneration in individuals with clinically diagnosed early AD, and may reflect an association with a common underlying disease mechanism.
Magnetic Resonance Imaging (MRI); Hippocampal subfields; β-Amyloid; Tau; P-Tau; biomarkers
Relations among antecedant biomarkers of AD were evaluated using causal modeling; although correlation cannot be equated to causation, causation does require correlation. Individuals aged 43 to 89 years (N = 220) enrolled as cognitively normal controls in longitudinal studies had clinical and psychometric assessment, structural magnetic resonance imaging (MRI), cerebrospinal fluid (CSF) biomarkers, and brain amyloid imaging via positron emission tomography with Pittsburgh Compound B (PIB) obtained within 1 year. CSF levels of Aβ42 and tau were minimally correlated, indicating they represent independent processes. Aβ42, tau, and their interaction explained 60% of the variance in PIB. Effects of APOE genotype and age on PIB were indirect, operating through CSF markers. Only spurious relations via their common relation with age were found between the biomarkers and regional brain volumes or cognition. Hence, at least two independent hypothesized processes, one reflected by CSF Aβ42 and one by CSF tau, contribute to the development of fibrillar amyloid plaques preclinically. The lack of correlation between these two processes and brain volume in the regions most often affected in AD suggests the operation of a third process related to brain atrophy.
preclinical Alzheimer disease; amyloid-β; tau; PIB; amyloid plaque; APOE; brain volumetry; memory; biomarkers; cerebrospinal fluid
The Dominantly Inherited Alzheimer Network (DIAN) is a collaborative effort of international Alzheimer disease (AD) centers that are conducting a multifaceted prospective biomarker study in individuals at-risk for autosomal dominant AD (ADAD). DIAN collects comprehensive information and tissue in accordance with standard protocols from asymptomatic and symptomatic ADAD mutation carriers and their non-carrier family members to determine the pathochronology of clinical, cognitive, neuroimaging, and fluid biomarkers of AD. This article describes the structure, implementation, and underlying principles of DIAN, as well as the demographic features of the initial DIAN cohort.
Alzheimer disease; autosomal dominant; biomarkers of Alzheimer disease; PSEN1; PSEN2; APP; amyloid-beta; preclinical Alzheimer disease
Aggregation of amyloid-β (Aβ) in the brain begins to occur years prior to the clinical onset of Alzheimer’s disease (AD). Prior to Aβ aggregation, levels of extracellular, soluble interstitial fluid (ISF) Aβ, which are regulated by neuronal activity and the sleep-wake cycle, correlate with the amount of Aβ deposition in the brain seen later. The amount and quality of sleep declines with aging and to a greater extent in AD. How sleep quality amount as well as the diurnal fluctuation in Aβ change with age and Aβ aggregation are not well understood. We report that a normal sleep-wake cycle and diurnal fluctuation of ISF Aβ is present in the brain of APPswe/PS1δE9 mice before Aβ plaque formation. Following plaque formation, the sleep-wake cycle markedly deteriorated and diurnal fluctuation of ISF Aβ dissipated. As in mice, diurnal fluctuation of cerebrospinal fluid (CSF) Aβ in young adult humans with presenilin mutations was also markedly attenuated with Aβ plaque formation. Virtual elimination of Aβ deposits in the mouse brain by active immunization with Aβ42 normalized the sleep-wake cycle and the diurnal fluctuation of ISF Aβ. These data suggest that Aβ aggregation disrupts the sleep-wake cycle and diurnal fluctuation of Aβ. Sleep-wake behavior and diurnal fluctuation of Aβ in the central nervous system appear to be functional and biochemical markers respectively of Aβ-associated pathology that should be explored in humans diagnostically prior to and following symptom onset and in response to treatment.
Recent epidemiologic studies suggest that caffeine may be protective against Alzheimer’s Disease (AD). Supportive of this premise, our previous studies have shown that moderate caffeine administration protects/restores cognitive function and suppresses brain β-amyloid (Aβ) production in AD transgenic mice. In the present study, we report that acute caffeine administration to both young adult and aged AD transgenic mice rapidly reduces Aβ levels in both brain interstitial fluid and plasma without affecting Aβ elimination. Long-term oral caffeine treatment to aged AD mice provided not only sustained reductions in plasma Aβ, but also decreases in both soluble and deposited Aβ in hippocampus and cortex. Irrespective of caffeine treatment, plasma Aβ levels did not correlate with brain Aβ levels or with cognitive performance in individual aged AD mice. Although higher plasma caffeine levels were strongly associated with lower plasma Aβ1-40 levels in aged AD mice, plasma caffeine levels were also not linked to cognitive performance. Plasma caffeine and theophylline levels were tightly correlated — both being associated with reduced inflammatory cytokine levels in hippocampus. Our conclusion is two-fold. First, that both plasma and brain Aβ levels are reduced by acute or chronic caffeine administration in several AD transgenic lines and ages, indicating a therapeutic value of caffeine against AD. Second, that plasma Aβ levels are not an accurate index of brain Aβ levels/deposition or cognitive performance in aged AD mice.
caffeine; Alzheimer’s Disease; transgenic mice; β-amyloid; plasma; brain interstitial fluid
The apolipoprotein E ε4 gene is the most important genetic risk factor for sporadic Alzheimer’s disease, but the link between this gene and neurodegeneration remains unclear. Using array tomography, we analysed >50 000 synapses in brains of 11 patients with Alzheimer’s disease and five non-demented control subjects and found that synapse loss around senile plaques in Alzheimer’s disease correlates with the burden of oligomeric amyloid-β in the neuropil and that this synaptotoxic oligomerized peptide is present at a subset of synapses. Further analysis reveals apolipoprotein E ε4 patients with Alzheimer’s disease have significantly higher oligomeric amyloid-β burden and exacerbated synapse loss around plaques compared with apolipoprotein E ε3 patients. Apolipoprotein E4 protein colocalizes with oligomeric amyloid-β and enhances synaptic localization of oligomeric amyloid-β by >5-fold. Biochemical characterization shows that the amyloid-β enriched at synapses by apolipoprotein E4 includes sodium dodecyl sulphate-stable dimers and trimers. In mouse primary neuronal culture, lipidated apolipoprotein E4 enhances oligomeric amyloid-β association with synapses via a mechanism involving apolipoprotein E receptors. Together, these data suggest that apolipoprotein E4 is a co-factor that enhances the toxicity of oligomeric amyloid-β both by increasing its levels and directing it to synapses, providing a link between apolipoprotein E ε4 genotype and synapse loss, a major correlate of cognitive decline in Alzheimer’s disease.
Alzheimer's disease; apolipoprotein E; synapse; array tomography; oligomeric amyloid-β
Over the last 15 years, cerebrospinal fluid (CSF) biomarkers have been shown to be useful for both the diagnosis as well as the prognosis in Alzheimer’s disease. It has been shown the CSF levels of Aβ42 are a very good marker for the presence of amyloid deposition in the brain regardless of clinical status and that total tau and phosphorylated forms of tau are useful in detection of neurodegeneration. When combined together, these CSF markers are useful not only in differential diagnosis but also in predicting conversion and rate of progression from mild cognitive impairment/very mild dementia to more severe impairment. The markers are also useful in predicting conversion from cognitive normalcy to very mild dementia. This field is briefly reviewed and recommendations for future studies in this area is provided.
Anti-ApoE antibody reduces amyloid deposition and enhances the microglial response to Aβ plaques in an Alzheimer’s disease mouse model.
The apolipoprotein E (APOE) ε4 allele is the strongest genetic risk factor for Alzheimer’s disease (AD). The influence of apoE on amyloid β (Aβ) accumulation may be the major mechanism by which apoE affects AD. ApoE interacts with Aβ and facilitates Aβ fibrillogenesis in vitro. In addition, apoE is one of the protein components in plaques. We hypothesized that certain anti-apoE antibodies, similar to certain anti-Aβ antibodies, may have antiamyloidogenic effects by binding to apoE in the plaques and activating microglia-mediated amyloid clearance. To test this hypothesis, we developed several monoclonal anti-apoE antibodies. Among them, we administered HJ6.3 antibody intraperitoneally to 4-mo-old male APPswe/PS1ΔE9 mice weekly for 14 wk. HJ6.3 dramatically decreased amyloid deposition by 60–80% and significantly reduced insoluble Aβ40 and Aβ42 levels. Short-term treatment with HJ6.3 resulted in strong changes in microglial responses around Aβ plaques. Collectively, these results suggest that anti-apoE immunization may represent a novel AD therapeutic strategy and that other proteins involved in Aβ binding and aggregation might also be a target for immunotherapy. Our data also have important broader implications for other amyloidosis. Immunotherapy to proteins tightly associated with misfolded proteins might open up a new treatment option for many protein misfolding diseases.
Alzheimer’s disease (AD) is the most common progressive neurodegenerative disorder causing dementia. Massive deposition of amyloid β peptide (Aβ) as senile plaques in the brain is the pathological hallmark of AD, but oligomeric, soluble forms of Aβ have been implicated as the synaptotoxic component. The apolipoprotein E epsilon 4 (apoE ε4) allele is known to be a genetic risk factor for developing AD. However it is still unknown how apoE impacts the process of Aβ oligomerization. Here, we found that the level of Aβ oligomers in APOEε4/ε4 AD patient brains is 2.7 times higher than those in APOEε3/ε3 AD patient brains, matched for total plaque burden, suggesting that apoE4 impacts the metabolism of Aβ oligomers. To test this hypothesis, we examined apoE’s effect on Aβ oligomer formation. Using both synthetic Aβ and a split-luciferase method for monitoring Aβ oligomers, we observed that apoE increased the level of Aβ oligomers in an isoform dependent manner (E2 < E3 < E4). This effect appears to be dependent on the ApoE carboxy-terminal domain. Moreover, these results were confirmed using endogenous apoE isolated from the TBS-soluble fraction of human brain, which increased the formation of Aβ oligomers. Taken together, these data show that lipidated apoE, especially apoE4, increases Aβ oligomers in the brain. Higher levels of Aβ oligomers in the brains of APOEε4/ε4 carriers compared to APOEε3/ε3 carriers may increase the loss of dendritic spines and accelerate memory impairments, leading to earlier cognitive decline in AD.
Axonal injury is believed to be a major determinant of adverse outcomes following traumatic brain injury. However, it has been difficult to assess acutely the severity of axonal injury in human traumatic brain injury patients. We hypothesized that microdialysis-based measurements of the brain extracellular fluid levels of tau and neurofilament light chain, two low molecular weight axonal proteins, could be helpful in this regard. To test this hypothesis, 100 kDa cut-off microdialysis catheters were placed in 16 patients with severe traumatic brain injury at two neurological/neurosurgical intensive care units. Tau levels in the microdialysis samples were highest early and fell over time in all patients. Initial tau levels were >3-fold higher in patients with microdialysis catheters placed in pericontusional regions than in patients in whom catheters were placed in normal-appearing right frontal lobe tissue (P = 0.005). Tau levels and neurofilament light-chain levels were positively correlated (r = 0.6, P = 0.013). Neurofilament light-chain levels were also higher in patients with pericontusional catheters (P = 0.04). Interestingly, initial tau levels were inversely correlated with initial amyloid-β levels measured in the same samples (r = −0.87, P = 0.000023). This could be due to reduced synaptic activity in areas with substantial axonal injury, as amyloid-β release is closely coupled with synaptic activity. Importantly, high initial tau levels correlated with worse clinical outcomes, as assessed using the Glasgow Outcome Scale 6 months after injury (r = −0.6, P = 0.018). Taken together, our data add support for the hypothesis that axonal injury may be related to long-term impairments following traumatic brain injury. Microdialysis-based measurement of tau levels in the brain extracellular space may be a useful way to assess the severity of axonal injury acutely in the intensive care unit. Further studies with larger numbers of patients will be required to assess the reproducibility of these findings and to determine whether this approach provides added value when combined with clinical and radiological information.
traumatic brain injury; microdialysis; amyloid-β; tau; neurofilament
Amyloid-β (Aβ) producing enzymes are key targets for disease-modifying Alzheimer’s disease (AD) therapies since Aβ 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 Aβ1-15 is produced by concerted β- and α-secretase cleavage of amyloid-β protein precursor (AβPP). Here, we test the hypothesis that this pathway is more engaged upon γ-secretase inhibition in humans and cerebrospinal fluid (CSF) levels of Aβ1-15/16 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 Aβ1-15/16, Aβx-38, Aβx-40, Aβx-42, sAβPPα and sAβPPβ. The CSF concentration of Aβ1-15/16 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 Aβx-38, Aβx-40 and Aβx-42 decreased the first 9 hours followed by increased concentrations after 36 hours relative to baseline. No significant changes were detected for CSF sAβPPα and sAβPPβ.Our data shows that CSF levels of Aβ1-15/16 increase during treatment with semagacestat supporting its feasibility as a pharmacodynamic biomarker for drug candidates aimed at inhibiting γ-secretase-mediated AβPP-processing.
Alzheimer’s; Aβ; Amyloid; γ-secretase
The bcl-x gene appears to play a critical role in regulating apoptosis in the developing and mature central nervous system (CNS) and following CNS injury. Two isoforms of Bcl-x are produced as a result of alternative pre-mRNA splicing: Bcl-xL (the long form) is anti-apoptotic, while Bcl-xS (short form) is pro-apoptotic. Despite the antagonistic activities of these two isoforms, little is known about how regulation of alternative splicing of bcl-x may mediate neural cell apoptosis. Here, we report that apoptotic stimuli (staurosporine or C2-ceramide) reciprocally altered Bcl-x splicing in neural cells, decreasing Bcl-xL while increasing Bcl-xS. Specific knockdown of Bcl-xS attenuated apoptosis. In order to further define regulatory elements that influenced Bcl-x splicing, a Bcl-x minigene was constructed. Deletional analysis revealed several consensus sequences within intron 2 that altered splicing. We found that the splicing factor, CUG-binding-protein-1 (CUGBP1), bound to a consensus sequence close to the Bcl-xL 5′ splice site, altering the Bcl-xL/Bcl-xS ratio and influencing cell death. In vivo, neonatal hypoxia-ischemia reciprocally altered Bcl-x pre-mRNA splicing, similar to the in vitro studies. Manipulation of the splice isoforms using viral gene transfer of Bcl-xS shRNA into the hippocampus of rats prior to neonatal hypoxia-ischemia decreased vulnerability to injury. Moreover, alterations in nuclear CUGBP1 preceded Bcl-x splicing changes. These results suggest that alternative pre-mRNA splicing may be an important regulatory mechanism for cell death after acute neurological injury, and may potentially provide novel targets for intervention.
Apolipoprotein E ε4 (APOE ε4) is the strongest genetic risk factor for Alzheimer’s disease (AD). Evidence suggests that the effect of apoE isoforms on amyloid-β (Aβ) accumulation in the brain plays a critical role in AD pathogenesis. Like in humans, apoE4 expression in animal models that develop Aβ-amyloidosis results in greater Aβ and amyloid deposition than with apoE3 expression. However, whether decreasing levels of apoE3 or apoE4 would promote or attenuate Aβ-related pathology has not been directly addressed. To determine the effect of decreasing human apoE levels on Aβ accumulation in vivo, we generated human APOE isoform haploinsufficient mouse models by crossing APPPS1-21 mice with APOE isoform knock-in mice. By genetically manipulating APOE gene dosage, we demonstrate that decreasing human apoE levels, regardless of isoform status, results in significantly decreased amyloid plaque deposition and microglial activation. This differences in amyloid load between apoE3 and apoE4 expressing mice were not due to apoE4 protein being present at lower levels than apoE3. These data suggest that current therapeutic strategies to increase apoE levels without altering its lipidation state may actually worsen Aβ amyloidosis, while increasing apoE degradation or inhibiting its synthesis may be a more effective treatment approach.
Apolipoprotein E; Haploinsufficiency; Aβ; Amyloid; Plaque; Alzheimer’s disease
The order and magnitude of pathologic processes in Alzheimer’s disease are not well understood, partly because the disease develops over many years. Autosomal dominant Alzheimer’s 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.
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 participant’s age at baseline assessment and the parent’s age at the onset of symptoms of Alzheimer’s disease to calculate the estimated years from expected symptom onset (age of the participant minus parent’s 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.
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.
We found that autosomal dominant Alzheimer’s disease was associated with a series of pathophysiological changes over decades in CSF biochemical markers of Alzheimer’s 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 Alzheimer’s disease. (Funded by the National Institute on Aging and others; DIAN ClinicalTrials.gov number, NCT00869817.)
APOE ε4 status has been associated with greater cortical amyloid deposition whereas exercise has been associated with less in cognitively normal adults. The primary objective here was to examine whether physical exercise moderates the association between APOE genotype and amyloid deposition in cognitively normal adults.
APOE genotyping and a questionnaire on physical exercise engagement over the last decade were obtained in conjunction with cerebrospinal fluid (CSF) samples and amyloid imaging with PET-PIB. Participants were classified as either low or high exercisers based on exercise guidelines of the American Heart Association.
201 cognitively normal adults (135 females) aged 45–88 were recruited from the Knight Alzheimer Disease Research Center at Washington University. CSF samples were collected from 165 participants. Amyloid imaging was performed on 163 participants.
APOE ε4 carriers evidenced higher PIB binding (p<.001) and lower CSF Aβ42 levels (p<.001) than non-carriers. Our previous findings of higher PIB binding (p=.005) and lower CSF Aβ42 levels (p=.009) in more sedentary individuals were replicated. Most importantly, we observed a novel interaction between APOE status and exercise engagement for PIB binding (p=.008) such that a more sedentary lifestyle was significantly associated with higher PIB binding for ε4 carriers (p=.013) but not for ε4 non-carriers (p=.208). All findings remained significant after controlling for age, gender, education, hypertension, body mass index, diabetes, heart problems, history of depression and interval between assessments.
Collectively, these results suggest that cognitively normal sedentary APOE ε4+ individuals may be at augmented risk for cerebral amyloid deposition.
Apolipoprotein E (ApoE) is a 299 amino acid protein encoded by the APOE gene. Three common polymorphisms in the APOE gene, ε2, ε3, and ε4, result in single amino changes in the ApoE protein. The APOEε2, ε3, and ε4 alleles strongly and dose-dependently alter the likelihood of developing Alzheimer’s disease (AD) and cerebral amyloid angiopathy (CAA). In particular, APOE ε4 is associated with increased risk for AD, whereas APOEε2 is associated with decreased risk. The effects of APOE genotype on AD and CAA risk are likely mediated, in large part, by differential effects of the ApoE protein on amyloid-β (Aβ) accumulation in the brain and cerebrovasculature. Recent data indicate that responses to AD treatments may differ according to APOE genotype. The APOE ε4 allele is also associated with poor outcome following traumatic brain injury and brain hemorrhage, though the mechanisms underlying these associations are unclear. Given the convincing body of literature tying APOE genotype to AD and CAA risk, APOE has also been studied in relation to other neurological diseases. While the possibility that APOE plays a role in these diseases is of great interest, convincing associations have not yet emerged.
Diagnostic challenges exist for differentiating HIV associated neurocognitive disorders (HAND) from symptomatic Alzheimer’s disease (AD) in HIV+ participants. Both disorders have cerebral amyloid containing plaques associated with abnormalities in amyloid beta protein 1–42 (Aβ42) metabolism. We evaluated if the amyloid-binding agent 11C-Pittsburgh compound B (11C-PiB) could discriminate AD from HAND in middle-aged HIV+ participants.
11C-PiB scanning, clinical assessment, and cerebrospinal fluid (CSF) analysis were performed. χ2 and t-tests assessed differences in clinical and demographic variables between HIV+ participants and community-living individuals followed by Alzheimer Disease Research Center (ADRC). An analysis of variance (ANOVA) assessed for regional differences in Aβ42 using 11C-PiB.
ADRC and HIV clinic
16 HIV+ participants (11 cognitively normal, 5 with HAND) and 19 ADRC participants (8 cognitively normal, 11 with symptomatic AD).
Main Outcome Measure(s)
Mean and regional 11C-PiB binding potentials
Symptomatic AD were older (p < 0.001), had lower CSF Aβ42 (p < 0.001), and had higher CSF tau levels (p < 0.001) than other groups. Regardless of degree of impairment, HIV+ participants did not have increased 11C-PiB. Mean and regional binding potentials were elevated for symptomatic AD participants (p <0.0001).
Middle-aged HIV+ participants, even with HAND, do not exhibit increased 11C-PiB while symptomatic AD individuals have increased fibrillar Aβ42 deposition in cortical and subcortical regions. Observed dissimilarities between HAND and AD may reflect differences in Aβ42 metabolism. 11C-PiB may provide a diagnostic biomarker for distinguishing symptomatic AD from HAND in middle-aged HIV+ participants. Future cross sectional and longitudinal studies are required to assess utility of 11C-PiB in older HAND individuals.
HIV; Pittsburgh compound B (PIB); amyloid; HIV associated neurocognitive disorders; Alzheimer’s disease
Alzheimer’s disease (AD) is the most common cause of dementia. Much is known concerning AD pathophysiology but our understanding of the disease at the systems level remains incomplete. Previous AD research has used resting state functional connectivity magnetic resonance imaging (rs-fcMRI) to assess the integrity of functional networks within the brain. Most studies have focused on the default-mode network (DMN), a primary locus of AD pathology. However, other brain regions are inevitably affected with disease progression. We studied rs-fcMRI in five functionally defined brain networks within a large cohort of human participants of either gender (n=510) that ranged in AD severity from unaffected (clinical dementia rating, CDR 0) to very mild (CDR 0.5) to mild AD (CDR 1). We observed loss of correlations within not only the DMN but other networks at CDR 0.5. Within the salience network (SAL), increases were seen between CDR 0 and CDR 0.5. However, at CDR 1, all networks, including SAL, exhibited reduced correlations. Specific networks were preferentially affected at certain CDR stages. In addition, cross-network relations were consistently lost with increasing AD severity. Our results demonstrate that AD is associated with widespread loss of both intra- and inter-network correlations. These results provide insight into AD pathophysiology and reinforce an integrative view of the brain’s functional organization.
Alzheimer’s disease; fMRI; resting state functional connectivity; BOLD; default mode network; salience network
The identification and characterization of amyloid-β (Aβ) and tau as the main pathological substrates of Alzheimer’s disease (AD) has driven many efforts in search for suitable biomarkers for AD. In the last decade, research in this area has focused on developing a better understanding of the principles that govern protein deposition, mechanisms that link aggregation to toxicity and neuronal death, and a better understanding of protein dynamics in brain tissue, interstitial fluid and CSF. While Aβ and tau represent the two key pathological mediators of disease, other aspects of this multifaceted disease (e.g. oxidative stress, calcium-mediated toxicity, and neuroinflammation) are being unraveled, with the hope to develop a more comprehensive approach in exploring disease mechanisms. This has not only expanded possible areas for disease-modifying therapies, but has also allowed the introduction of novel, and potentially useful, fluid and radiological markers for the presence and progression of AD pathology. There is no doubt that the identification of several fluid and imaging biomarkers that can reliably detect the early stages of AD will have great implications in the design of clinical trials, in the selection of homogenous research populations, and in the assessment of disease outcomes. Markers with good diagnostic specificity will aid researchers in differentiating individuals with preclinical and probable AD from individuals who do not have AD pathology or have other dementing disorders. Markers that change with disease progression may offer utility in assessing the rates of disease progression and the efficacy of potential therapeutic agents on AD pathology. For both of these purposes, CSF Aβ42, amyloid imaging, and CSF tau appear to be very good markers of the presence of AD pathology as well as predictive or who will progress from MCI to AD. Volumetric MRI is also good at separating individuals with MCI and AD from controls and is predictive of who will progress from MCI to AD. Perhaps the most important role biomarkers will have, and the most needed at this time, lies in the identification of individuals who are cognitively normal, and yet have evidence of AD pathology (i.e. preclinical AD). Such individuals, it appears, can be identified with CSF Aβ42, amyloid imaging, and CSF tau. Such individuals are the most likely to benefit from future disease modifying/prevention therapies as they become available, and therefore represent the population in which the field can make the biggest therapeutic impact.
Alzheimer’s disease; biomarkers; amyloid-β; tau; imaging; antecedent biomarkers; plaques
A major feature of Alzheimer’s disease (AD), a late-onset neurodegenerative disorder, is the ordered aggregation of the β-amyloid peptide (Aβ) into fibrils that comprise extracellular neuritic plaques found in the disease brain. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function. Here, we show by in situ atomic force microscopy (AFM) that astrocyte secreted lipoprotein particles (ASLPs) containing different isoforms of apolipoprotein E (apoE), of which the apoE4 allele is a major risk factor for the development of AD, can protect total brain lipid extract bilayers from Aβ1–40 induced disruption. The apoE4 allele was less effective in protecting lipid bilayers from disruption compared with apoE3. Size analysis of apoE-containing ASLPs and mechanical studies of bilayer properties revealed that apoE-containing ASLPs modulate the mechanical properties of bilayers by acquiring some bilayer components (most likely cholesterol and/or oxidatively damaged lipids). Measurement of bilayer mechanical properties was accomplished with scanning probe acceleration microscopy (SPAM). These measurements demonstrated that apoE4 was also less effective in modulating mechanical properties of bilayers in comparison with apoE3. This ability of apoE to alter the mechanical properties of lipid membranes may represent a potential mechanism for the suppression of Aβ1–40 induced bilayer disruption.
Apolipoprotein E; amyloid-β; lipid bilayer; cholesterol; atomic force microscopy; Alzheimer’s disease
A major feature of Alzheimer's disease (AD), a late-onset neurodegenerative disorder, is the ordered aggregation of the β-amyloid peptide (Aβ) into fibrils that comprise extracellular neuritic plaques found in the disease brain. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function. Here, we show by in situ atomic force microscopy (AFM) that astrocyte secreted lipoprotein particles (ASLPs) containing different isoforms of apolipoprotein E (apoE), of which the apoE4 allele is a major risk factor for the development of AD, can protect total brain lipid extract bilayers from Aβ1-40 induced disruption. The apoE4 allele was less effective in protecting lipid bilayers from disruption compared with apoE3. Size analysis of apoE-containing ASLPs and mechanical studies of bilayer properties revealed that apoE-containing ASLPs modulate the mechanical properties of bilayers by acquiring some bilayer components (most likely cholesterol and/or oxidatively damaged lipids). Measurement of bilayer mechanical properties was accomplished with scanning probe acceleration microscopy (SPAM). These measurements demonstrated that apoE4 was also less effective in modulating mechanical properties of bilayers in comparison with apoE3. This ability of apoE to alter the mechanical properties of lipid membranes may represent a potential mechanism for the suppression of Aβ1-40 induced bilayer disruption.
apolipoproteinE; Amyloid-β; lipid bilayer; cholesterol; atomic force microscopy; Alzheimer's disease