Clinicopathological studies suggest that Alzheimer's disease (AD) pathology begins ∼10-15 years before the resulting cognitive impairment draws medical attention. Biomarkers that can detect AD ...pathology in its early stages and predict dementia onset would, therefore, be invaluable for patient care and efficient clinical trial design. We utilized a targeted proteomics approach to discover novel cerebrospinal fluid (CSF) biomarkers that can augment the diagnostic and prognostic accuracy of current leading CSF biomarkers (Aβ42, tau, p-tau181).
Using a multiplexed Luminex platform, 190 analytes were measured in 333 CSF samples from cognitively normal (Clinical Dementia Rating CDR 0), very mildly demented (CDR 0.5), and mildly demented (CDR 1) individuals. Mean levels of 37 analytes (12 after Bonferroni correction) were found to differ between CDR 0 and CDR>0 groups. Receiver-operating characteristic curve analyses revealed that small combinations of a subset of these markers (cystatin C, VEGF, TRAIL-R3, PAI-1, PP, NT-proBNP, MMP-10, MIF, GRO-α, fibrinogen, FAS, eotaxin-3) enhanced the ability of the best-performing established CSF biomarker, the tau/Aβ42 ratio, to discriminate CDR>0 from CDR 0 individuals. Multiple machine learning algorithms likewise showed that the novel biomarker panels improved the diagnostic performance of the current leading biomarkers. Importantly, most of the markers that best discriminated CDR 0 from CDR>0 individuals in the more targeted ROC analyses were also identified as top predictors in the machine learning models, reconfirming their potential as biomarkers for early-stage AD. Cox proportional hazards models demonstrated that an optimal panel of markers for predicting risk of developing cognitive impairment (CDR 0 to CDR>0 conversion) consisted of calbindin, Aβ42, and age.
Using a targeted proteomic screen, we identified novel candidate biomarkers that complement the best current CSF biomarkers for distinguishing very mildly/mildly demented from cognitively normal individuals. Additionally, we identified a novel biomarker (calbindin) with significant prognostic potential.
Despite the important role of apolipoprotein E (apoE) secretion from astrocytes in brain lipid metabolism and the strong association of apoE4, one of the human apoE isoforms, with sporadic and late ...onset forms of Alzheimer's disease (AD) little is known about the regulation of astrocytic apoE. Utilizing annotated chemical libraries and a phenotypic screening strategy that measured apoE secretion from a human astrocytoma cell line, inhibition of pan class I histone deacetylases (HDACs) was identified as a mechanism to increase apoE secretion. Knocking down select HDAC family members alone or in combination revealed that inhibition of the class I HDAC family was responsible for enhancing apoE secretion. Knocking down LXRα and LXRβ genes revealed that the increase in astrocytic apoE in response to HDAC inhibition occurred via an LXR-independent pathway. Collectively, these data suggest that pan class I HDAC inhibition is a novel pathway for regulating astrocytic apoE secretion.
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 Aβ(42) 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.
Brain lipid homoeostasis is critical during neurodevelopment, repair after traumatic brain injury and for the maintenance of efficient neurotransmission. Several neurodegenerative disorders occur as ...a direct result of neuronal lipid dysfunction and underlying disease processes that are associated with Alzheimer's disease (AD) also appear to be related to an imbalance in brain lipid homeostasis. In support of this latter hypothesis, recent genome wide association studies have confirmed and extended the now widely reproduced association between the ɛ4 allele of the apolipoprotein E gene (APOE) and late onset AD. Even in populations with low APOE ɛ4 allele frequency, gene dosage of APOE ɛ4 increases the age-adjusted relative risk for developing the more common late onset form of AD. A major role for apolipoprotein E (apoE) in the brain is to maintain a constant supply of neuronal lipids for rapid and dynamic membrane synthesis thus ensuring efficient neurotransmitter release and the propagation of action potentials. Additionally, apoE synthesized primarily by glia is critical for the elimination of toxic brain-derived Aβ peptides. In addition to apoE isoform, the overall levels of apoE appear to be important determinants for brain Aβ clearance. Susceptibility to AD in APOE ɛ4 carriers may occur early since brain activity and the accumulation of Aβ in brain parenchyma both appear well in advance of disease onset. Given the pivotal role apoE plays in maintaining neuronal membrane homeostasis, elevating the levels of apoE in brain may be a viable therapeutic strategy for the prevention and/or treatment of AD.
•APOE4 is strongly associated with late onset Alzheimer’s disease.•APOE4 influences b-amyloid accumulation and neurofibrillary tangle burden.•Novel biology surrounding APOE function in brain is ...revealing new therapeutic opportunities.•Several clinical trials are currently underway enriching for APOE4 and/or targeting the APOE pathway.
The ε4 allele of the apolipoprotein E gene represents the most widely reproduced and robust susceptibility loci for the most common late onset and sporadic forms of Alzheimer’s disease. While the discovery of this now widely replicated association was reported more than 25 years ago, few therapeutic interventions that specifically target the apolipoprotein pathway in brain have emerged. Here we discuss our current understanding of apolipoprotein E biology in brain, its relationship to the pathogenesis of Alzheimer’s disease and present potential future avenues for exploration that may be amenable to drug development.
To investigate the role of human apolipoprotein E (apoE) on Abeta deposition in vivo, we crossed PDAPP mice lacking mouse Apoe to targeted replacement mice expressing human apoE (PDAPP/TRE2, ...PDAPP/TRE3, or PDAPP/TRE4). We then measured the levels of apoE protein and Abeta peptides in plasma, CSF, and brain homogenates in these mice at different ages. We also quantified the amount of brain Abeta and amyloid burden in 18-month-old mice. In young PDAPP/TRE4 mice that were analyzed at an age before brain Abeta deposition, we observed a significant decrease in the levels of apoE in CSF and brain when compared with age-matched mice expressing either human E2 or E3. The brain levels of Abeta42 in PDAPP/TRE4 mice were substantially elevated even at this very early time point. In older PDAPP/TRE4 mice, the levels of insoluble apoE protein increased in parallel to the dramatic rise in brain Abeta burden, and the majority of apoE was associated with Abeta. In TRE4 only mice, we also observed a significant decrease in the level of apoE in brain homogenates. Since the relative level of apoE mRNA was equivalent in PDAPP/TRE and TRE only mice, it appears that post-translational mechanisms influence the levels of apoE protein in brain (E4 < E3 << E2), resulting in early and dramatic apoE isoform-dependent effects on brain Abeta levels (E4 >> E3 > E2) that increase with age. Therapeutic strategies aimed at increasing the soluble levels of apoE protein, regardless of isoform, may effectively prevent and (or) treat Alzheimer's disease.
The exponential growth in the world's aged population has increased pressure on drug discovery efforts to identify innovative therapies for Alzheimer's disease (AD). The long and uncertain clinical ...trial path utilized to test the potential efficacy of these novel agents is challenging. For these and other reasons, there has been an explosion in the generation and availability of transgenic mouse models that mimic some, but not all aspects of AD. The largely overwhelmingly positive results obtained when testing potential clinical agents in these same animal models have failed to translate into similar positive clinical outcomes.
This review discusses the value and limitations associated with currently available transgenic mouse models of AD. Furthermore, the article proposes ways in which researchers can better characterize pharmacodynamic and pharmacokinetic endpoints to increase the success rate for novel therapies advancing into clinical development. Lastly, the author discusses ways in which researchers can supplement, expand and improve transgenic mouse models used in AD drug discovery.
The use of transgenic mouse models that recapitulate various aspects of AD has expanded our knowledge and understanding of disease pathogenesis immensely. Further success in testing and translating novel therapies from animal models into bona fide medicines would be enhanced by i) the availability of better models that more fully recapitulate the disease spectrum, ii) defining and measuring standardized endpoints that display a pharmacodynamic range, iii) building and including translatable biomarkers and iv) including novel endpoints that would be expected to translate into clinically beneficial outcomes.
Aggregation of the amyloid-β (Aβ) peptide in the extracellular space of the brain is central to Alzheimer's disease pathogenesis. Aβ aggregation is concentration dependent and brain region specific. ...Utilizing in vivo microdialysis concurrently with field potential recordings, we demonstrate that Aβ levels in the brain interstitial fluid are dynamically and directly influenced by synaptic activity on a timescale of minutes to hours. Using an acute brain slice model, we show that the rapid effects of synaptic activity on Aβ levels are primarily related to synaptic vesicle exocytosis. These results suggest that synaptic activity may modulate a neurodegenerative disease process, in this case by influencing Aβ metabolism and ultimately region-specific Aβ deposition. The findings also have important implications for treatment development.
sorLA (sorting protein-related receptor) is a type-1 membrane protein of unknown function that is expressed in neurons. Its homology to sorting receptors that shuttle between the plasma membrane, ...endosomes, and the Golgi suggests a related function in neuronal trafficking processes. Because expression of sorLA is reduced in the brain of patients with Alzheimer's disease (AD), we tested involvement of this receptor in intracellular transport and processing of the amyloid precursor protein (APP) to the amyloid β-peptide (Aβ), the principal component of senile plaques. We demonstrate that sorLA interacts with APP in vitro and in living cells and that both proteins colocalize in endosomal and Golgi compartments. Overexpression of sorLA in neurons causes redistribution of APP to the Golgi and decreased processing to Aβ, whereas ablation of sorLA expression in knockout mice results in increased levels of Aβ in the brain similar to the situation in AD patients. Thus, sorLA acts as a sorting receptor that protects APP from processing into Aβ and thereby reduces the burden of amyloidogenic peptide formation. Consequently, reduced receptor expression in the human brain may increase Aβ production and plaque formation and promote spontaneous AD.
APOE genotype is a major genetic risk factor for late-onset Alzheimer disease (AD). ABCA1, a member of the ATP-binding cassette family of active transporters, lipidates apoE in the CNS. Abca1(-/-) ...mice have decreased lipid associated with apoE and increased amyloid deposition in several AD mouse models. We hypothesized that mice overexpressing ABCA1 in the brain would have increased lipidation of apoE-containing lipoproteins and decreased amyloid deposition. To address these hypotheses, we created PrP-mAbca1 Tg mice that overexpress mouse Abca1 throughout the brain under the control of the mouse prion promoter. We bred the PrP-mAbca1 mice to the PDAPP AD mouse model, a transgenic line overexpressing a mutant human amyloid precursor protein. PDAPP/Abca1 Tg mice developed a phenotype remarkably similar to that seen in PDAPP/Apoe(-/-) mice: there was significantly less amyloid beta-peptide (Abeta) deposition, a redistribution of Abeta to the hilus of the dentate gyrus in the hippocampus, and an almost complete absence of thioflavine S-positive amyloid plaques. Analyses of CSF from PrP-mAbca1 Tg mice and media conditioned by PrP-mAbca1 Tg primary astrocytes demonstrated increased lipidation of apoE-containing particles. These data support the conclusions that increased ABCA1-mediated lipidation of apoE in the CNS can reduce amyloid burden and that increasing ABCA1 function may have a therapeutic effect on AD.
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