Functional analysis and purification of a Pen-2 fusion protein for γ-secretase structural studies

The 19-transmembrane, multisubunit γ-secretase complex generates the amyloid β-peptide (Aβ) of Alzheimer's disease (AD) by an unusual intramembrane proteolysis of the β-amyloid precursor protein. The complex, which similarly processes many other type 1 transmembrane substrates, is composed of presenilin, Aph1, nicastrin, and presenilin enhancer (Pen-2), all of which are necessary for proper complex maturation and enzymatic activity. Obtaining a high-resolution atomic structure of the intact complex would greatly aid the rational design of compounds to modulate activity but is a very difficult task. A complementary method is to generate structures for each individual subunit to allow one to build a model of the entire complex. Here, we describe a method by which recombinant human Pen-2 can be purified from bacteria to > 95% purity at milligram quantities per liter, utilizing a maltose binding protein tag to both increase solubility and facilitate purification. Expressing the same construct in mammalian cells, we show that the large N-terminal maltose binding protein tag on Pen-2 still permits incorporation into the complex and subsequent presenilin-1 endoproteolysis, nicastrin glycosylation and proteolytic activity. These new methods provide valuable tools to study the structure and function of Pen-2 and the γ-secretase complex.

The familial Alzheimer's disease APPV717I mutation alters APP processing and Tau expression in iPSC-derived neurons

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by extracellular plaques containing amyloid β (Aβ)-protein and intracellular tangles containing hyperphosphorylated Tau protein. Here, we describe the generation of inducible pluripotent stem cell lines from patients harboring the London familial AD (fAD) amyloid precursor protein (APP) mutation (V717I). We examine AD-relevant phenotypes following directed differentiation to forebrain neuronal fates vulnerable in AD. We observe that over differentiation time to mature neuronal fates, APP expression and levels of Aβ increase dramatically. In both immature and mature neuronal fates, the APPV717I mutation affects both β- and γ-secretase cleavage of APP. Although the mutation lies near the γ-secretase cleavage site in the transmembrane domain of APP, we find that β-secretase cleavage of APP is elevated leading to generation of increased levels of both APPsβ and Aβ. Furthermore, we find that this mutation alters the initial cleavage site of γ-secretase, resulting in an increased generation of both Aβ42 and Aβ38. In addition to altered APP processing, an increase in levels of total and phosphorylated Tau is observed in neurons with the APPV717I mutation. We show that treatment with Aβ-specific antibodies early in culture reverses the phenotype of increased total Tau levels, implicating altered Aβ production in fAD neurons in this phenotype. These studies use human neurons to reveal previously unrecognized effects of the most common fAD APP mutation and provide a model system for testing therapeutic strategies in the cell types most relevant to disease processes.

Unrecognized vitamin D3 deficiency is common in Parkinson disease: Harvard Biomarker Study

OBJECTIVE

To conclusively test for a specific association between the biological marker 25-hydroxy-vitamin D3, a transcriptionally active hormone produced in human skin and liver, and the prevalence and severity of Parkinson disease (PD).

METHODS

We used liquid chromatography/tandem mass spectrometry to establish an association specifically between deficiency of 25-hydroxy-vitamin D3 and PD in a cross-sectional and longitudinal case-control study of 388 patients (mean Hoehn and Yahr stage of 2.1 ± 0.6) and 283 control subjects free of neurologic disease nested in the Harvard Biomarker Study.

RESULTS

Plasma levels of 25-hydroxy-vitamin D3 were associated with PD in both univariate and multivariate analyses with p values = 0.0034 and 0.047, respectively. Total 25-hydroxy-vitamin D levels, the traditional composite measure of endogenous and exogenous vitamin D, were deficient in 17.6% of patients with PD compared with 9.3% of controls. Low 25-hydroxy-vitamin D3 as well as total 25-hydroxy-vitamin D levels were correlated with higher total Unified Parkinson's Disease Rating Scale scores at baseline and during follow-up.

CONCLUSIONS

Our study reveals an association between 25-hydroxy-vitamin D3 and PD and suggests that thousands of patients with PD in North America alone may be vitamin D-deficient. This finding has immediate relevance for individual patients at risk of falls as well as public health, and warrants further investigation into the mechanism underlying this association.

Neurotoxicity of amyloid β-protein: synaptic and network dysfunction

Evidence for an ever-expanding variety of molecular mediators of amyloid β-protein neurotoxicity (membrane lipids, receptor proteins, channel proteins, second messengers and related signaling cascades, cytoskeletal proteins, inflammatory mediators, etc.) has led to the notion that the binding of hydrophobic Aβ assemblies to cellular membranes triggers multiple effects affecting diverse pathways. It appears unlikely that there are only one or two cognate receptors for neurotoxic forms of Aβ and also that there are just one or two assembly forms of the peptide that induce neuronal dysfunction. Rather, various soluble (diffusible) oligomers of Aβ that may be in dynamic equilibrium with insoluble, fibrillar deposits (amyloid plaques) and that can bind to different components of neuronal and non-neuronal plasma membranes appear to induce complex patterns of synaptic dysfunction and network disorganization that underlie the intermittent but gradually progressive cognitive manifestations of the clinical disorder. Modern analyses of this problem utilize electrophysiology coupled with synaptic biochemistry and behavioral phenotyping of animal models to elucidate the affected circuits and assess the effects of potential therapeutic interventions.

Systematic evaluation of candidate ligands regulating ectodomain shedding of amyloid precursor protein

Despite intense interest in the proteolysis of the β-Amyloid Precursor Protein (APP) in Alzheimer's disease, how the normal processing of this type I receptor-like glycoprotein is physiologically regulated remains ill-defined. In recent years, several candidate protein ligands for APP, including F-spondin, Reelin, β1 Integrin, Contactins, Lingo-1, and Pancortin, have been reported. However, a cognate ligand for APP that regulates its processing by α- or β-secretase has yet to be widely confirmed in multiple laboratories. Here, we developed new assays in an effort to confirm a role for one or more of these candidate ligands in regulating APP ectodomain shedding in a biologically relevant context. A comprehensive quantification of APPsα and APPsβ, the immediate products of secretase processing, in both non-neuronal cell lines and primary neuronal cultures expressing endogenous APP yielded no evidence that any of these published candidate ligands stimulate ectodomain shedding. Rather, Reelin, Lingo-1, and Pancortin-1 emerged as the most consistent ligands for significantly inhibiting ectodomain shedding. These findings led us to conduct further detailed analyses of the interactions of Reelin and Lingo-1 with APP.

Environmental novelty activates β2-adrenergic signaling to prevent the impairment of hippocampal LTP by Aβ oligomers

A central question about human brain aging is whether cognitive enrichment slows the development of Alzheimer changes. Here, we show that prolonged exposure to an enriched environment (EE) facilitated signaling in the hippocampus of wild-type mice that promoted long-term potentiation. A key feature of the EE effect was activation of β2-adrenergic receptors and downstream cAMP/PKA signaling. This EE pathway prevented LTP inhibition by soluble oligomers of amyloid β-protein (Aβ) isolated from AD cortex. Protection by EE occurred in both young and middle-aged wild-type mice. Exposure to novelty afforded greater protection than did aerobic exercise. Mice chronically fed a β-adrenergic agonist without EE were protected from hippocampal impairment by Aβ oligomers. Thus, EE enhances hippocampal synaptic plasticity by activating β-adrenoceptor signaling and mitigating synaptotoxicity of human Aβ oligomers. These mechanistic insights support using prolonged exposure to cognitive novelty and/or oral β-adrenergic agonists to lessen the effects of Aβ accumulation during aging.

Impaired glutamate recycling and GluN2B-mediated neuronal calcium overload in mice lacking TGF-β1 in the CNS

Transforming growth factor β1 (TGF-β1) is a pleiotropic cytokine expressed throughout the CNS. Previous studies demonstrated that TGF-β1 contributes to maintain neuronal survival, but mechanistically this effect is not well understood. We generated a CNS-specific TGF-β1-deficient mouse model to investigate the functional consequences of TGF-β1-deficiency in the adult mouse brain. We found that depletion of TGF-β1 in the CNS resulted in a loss of the astrocyte glutamate transporter (GluT) proteins GLT-1 (EAAT2) and GLAST (EAAT1) and decreased glutamate uptake in the mouse hippocampus. Treatment with TGF-β1 induced the expression of GLAST and GLT-1 in cultured astrocytes and enhanced astroglial glutamate uptake. Similar to GLT-1-deficient mice, CNS-TGF-β1-deficient mice had reduced brain weight and neuronal loss in the CA1 hippocampal region. CNS-TGF-β1-deficient mice showed GluN2B-dependent aberrant synaptic plasticity in the CA1 area of the hippocampus similar to the glutamate transport inhibitor DL-TBOA and these mice were highly sensitive to excitotoxic injury. In addition, hippocampal neurons from TGF-β1-deficient mice had elevated GluN2B-mediated calcium signals in response to extrasynaptic glutamate receptor stimulation, whereas cells treated with TGF-β1 exhibited reduced GluN2B-mediated calcium signals. In summary, our study demonstrates a previously unrecognized function of TGF-β1 in the CNS to control extracellular glutamate homeostasis and GluN2B-mediated calcium responses in the mouse hippocampus.

New ELISAs with high specificity for soluble oligomers of amyloid β-protein detect natural Aβ oligomers in human brain but not CSF

BACKGROUND

Soluble oligomers of amyloid ß-protein (Aß) have been increasingly linked to synaptic dysfunction, tau alteration, and neuritic dystrophy in Alzheimer's disease (AD) and mouse models. There is a great need for assays that quantify Aß oligomers with high specificity and sensitivity.

METHODS

We designed and validated two oligomer-specific (o-) enzyme-linked immunoassays (ELISAs) using either an Aß aggregate-selective monoclonal for capture and a monoclonal to the free N-terminus for detection, or the latter antibody for both capture and detection.

RESULTS

The o-ELISAs specifically quantified pure oligomers of synthetic Aß with sizes from dimers up to much larger assemblies and over a wide dynamic range of concentrations, whereas Aß monomers were undetectable. Natural Aß oligomers of similarly wide size and concentration ranges were measured in extracts of AD and control brains, revealing >1000-fold higher concentrations of Aß oligomers than monomers in the soluble fraction of AD cortex. The assays quantified the age-related rise in oligomers in hAPP transgenic mice. Unexpectedly, none of 90 human cerebrospinal fluid (CSF) samples gave a specific signal in either o-ELISA.

CONCLUSIONS

These new o-ELISAs with rigorously confirmed specificity can quantify oligomer burden in human and mouse brains for diagnostic and mechanistic studies and for AD biomarker development. However, our data raise the likelihood that the hydrophobicity of Aß oligomers makes them very low in number or absent in aqueous CSF.

Alzheimer disease in 2020

Remarkable advances in unraveling the biological underpinnings of Alzheimer disease (AD) have occurred during the last 25 years. Despite this, we have made only the smallest of dents in the development of truly disease-modifying treatments. What will change over the next 10 years? While the answer is not clear, we make several predictions on the state of the field in 2020, based on the rich knowledge described in the other contributions in this collection. As such, our predictions represent some of the principal unresolved questions that we believe deserve special investigative attention in the coming decade.

Pancortins interact with amyloid precursor protein and modulate cortical cell migration

Neuronal precursor cell migration in the developing mammalian brain is a complex process requiring the coordinated interaction of numerous proteins. We have recently shown that amyloid precursor protein (APP) plays a role in migration into the cortical plate through its interaction with two cytosolic signaling proteins, disabled 1 (DAB1) and disrupted in schizophrenia 1 (DISC1). In order to identify extracellular factors that may signal through APP to regulate migration, we performed an unbiased mass spectrometry-based screen for factors that bind to the extracellular domain of APP in the rodent brain. Through this screen, we identified an interaction between APP and pancortins, proteins expressed throughout the developing and mature cerebral cortex. Via co-immunoprecipitation, we show that APP interacts with all four of the mammalian pancortin isoforms (AMY, AMZ, BMY, BMZ). We demonstrate that the BMZ and BMY isoforms of pancortin can specifically reduce β-secretase- but not α-secretase-mediated cleavage of endogenous APP in cell culture, suggesting a biochemical consequence of the association between pancortins and APP. Using in utero electroporation to overexpress and knock down specific pancortin isoforms, we reveal a novel role for pancortins in migration into the cortical plate. Interestingly, we observe opposing roles for alternate pancortin isoforms, with AMY overexpression and BMZ knock down both preventing proper migration of neuronal precursor cells. Finally, we show that BMZ can partially rescue a loss of APP expression and that APP can rescue effects of AMY overexpression, suggesting that pancortins act in conjunction with APP to regulate entry into the cortical plate. Taken together, these results suggest a biochemical and functional interaction between APP and pancortins, and reveal a previously unidentified role for pancortins in mammalian cortical development.

Plasma amyloid-β as a predictor of dementia and cognitive decline: a systematic review and meta-analysis

BACKGROUND

Preclinical prediction of Alzheimer disease (AD) is important and critical to effective intervention. Plasma levels of amyloid-β (Aβ) peptides have been a principal focus of the growing literature on blood-based biomarkers, but studies to date have varied in design, assay methods, and sample size, making it difficult to readily interpret the overall data.

OBJECTIVE

To conduct a systematic review and meta-analysis of relevant prospective studies to determine whether plasma amyloid-β levels may predict development of dementia, AD, and cognitive decline.

DESIGN

We searched prospective studies published between 1995 and 2011 indexed in the MEDLINE, EMBASE, and PsycINFO databases. Selected studies included those measuring at least 1 relevant plasma amyloid-β species (Aβ(40), Aβ(42), or Aβ(42):Aβ(40) ratio) and reporting an effect estimate for dementia, AD, or cognitive change.

MAIN OUTCOME MEASURES

Using a standardized extraction form, appropriate study parameters on subject information, exposure, and outcome were extracted. Random effects models were used to generate summary risk ratios and 95% confidence intervals comparing the bottom vs top quantiles for each plasma measure.

RESULTS

Thirteen studies with a total of 10 303 subjects met inclusion criteria for meta-analysis. Lower Aβ(42):Aβ(40) ratios were significantly associated with development of AD (summary risk ratio, 1.60; 95% CI, 1.04-2.46; P = .03) and dementia (risk ratio, 1.67; 95% CI, 1.02-2.75; P = .04). Significant heterogeneity was found for both summary estimates, which could not be explained by participants' age, sex distribution, the study's follow-up time, or year of publication. Plasma levels of Aβ(40) and Aβ(42) alone were not significantly associated with either outcome.

CONCLUSIONS

Overall, the literature indicates that plasma Aβ(42):Aβ(40) ratios predict development of AD and dementia. However, significant heterogeneity in the meta-analysis underlines the need for substantial further investigation of plasma amyloid-β levels as a preclinical biomarker.

Biochemistry of amyloid β-protein and amyloid deposits in Alzheimer disease

Progressive cerebral deposition of the amyloid β-protein (Aβ) in brain regions serving memory and cognition is an invariant and defining feature of Alzheimer disease. A highly similar but less robust process accompanies brain aging in many nondemented humans, lower primates, and some other mammals. The discovery of Aβ as the subunit of the amyloid fibrils in meningocerebral blood vessels and parenchymal plaques has led to innumerable studies of its biochemistry and potential cytotoxic properties. Here we will review the discovery of Aβ, numerous aspects of its complex biochemistry, and current attempts to understand how a range of Aβ assemblies, including soluble oligomers and insoluble fibrils, may precipitate and promote neuronal and glial alterations that underlie the development of dementia. Although the role of Aβ as a key molecular factor in the etiology of Alzheimer disease remains controversial, clinical trials of amyloid-lowering agents, reviewed elsewhere in this book, are poised to resolve the question of its pathogenic primacy.

Effects of membrane lipids on the activity and processivity of purified γ-secretase

The 19-transmembrane multisubunit γ-secretase complex generates the amyloid β-peptide (Aβ) of Alzheimer's disease (AD) by intramembrane proteolysis of the β-amyloid precursor protein (APP). Despite substantial advances in elucidating how this protein complex functions, the effect of the local membrane lipid microenvironment on γ-secretase cleavage of substrates is still poorly understood. Using detergent-free proteoliposomes to reconstitute purified human γ-secretase, we examined the effects of fatty acyl (FA) chain length, saturation and double-bond isomerization, and membrane lipid polar headgroups on γ-secretase function. We analyzed γ-secretase activity and processivity [i.e., sequential cleavages in the APP transmembrane domain that convert longer Aβ species (e.g., Aβ(46)) into shorter ones (e.g., Aβ(40))] by quantifying the APP intracellular domain (AICD) and various Aβ peptides, including via a bicine/urea gel system that detects multiple Aβ lengths. These assays revealed several trends. (1) Switching from a cis to a trans isomer of a monounsaturated FA chain in phosphatidylcholine (PC) increased γ-activity, did not affect Aβ(42):Aβ(40) ratios, but decreased the ratio of long (≥42) versus short (≤41) Aβ peptides. (2) Increasing the FA carbon chain length (14, 16, 18, and 20) increased γ-activity, reduced longer Aβ species, and reduced the Aβ(42):Aβ(40) ratio. (3) Shifting the position of the double bond in 18:1(Δ9-cis) PC to the Δ6 position substantially reduced activity. (4) Gangliosides increased γ-activity but decreased processivity, thus elevating the Aβ(42):Aβ(40) ratio. (5) Phosphatidylserine decreased γ-activity but increased processivity. (6) Phosphatidylinositol strongly inhibited γ-activity. Overall, our results show that subtle changes in membrane lipid composition can greatly influence γ-secretase activity and processivity, suggesting that relatively small changes in lipid membrane composition may affect the risk of AD at least as much as presenilin or APP mutations do.

Aβ oligomers inhibit synapse remodelling necessary for memory consolidation

Extensive research has implicated the amyloid-β protein (Aβ) in the aetiology of Alzheimer's disease (AD). This protein has been shown to produce memory deficits when injected into rodent brain and in mouse models of AD Aβ production is associated with impaired learning and/or recall. Here we examined the effects of cell-derived SDS-stable 7PA2-derived soluble Aβ oligomers on consolidation of avoidance learning. At 0, 3, 6, 9 or 12h after training, animals received an intracerebroventricular injection of Aβ-containing or control media and recall was tested at 24 and 48 h. Immediately after 48 h recall animals were transcardially perfused and the brain removed for sectioning and EM analysis. Rats receiving injections of Aβ at 6 or 9h post-training showed a significant impairment in memory consolidation at 48 h. Importantly, impaired animals injected at 9h had significantly fewer synapses in the dentate gyrus. These data suggest that Aβ low-n oligomers target specific temporal facets of consolidation-associated synaptic remodelling whereby loss of functional synapses results in impaired consolidation.

Cholesterol level and statin use in Alzheimer disease: II. Review of human trials and recommendations

Substantial evidence has accumulated in support of the hypothesis that elevated cholesterol levels increase the risk of developing Alzheimer disease (AD). As a result, much work has investigated the potential use of lipid-lowering agents, particularly statins, as preventive or therapeutic agents for AD. Although epidemiology and preclinical statin research (described in part I of this review) have generally supported an adverse role of high cholesterol levels regarding AD, human studies of statins (reviewed herein) show highly variable outcomes, making it difficult to draw firm conclusions. We identify several confounding factors among the human studies, including differing blood-brain barrier permeabilities among statins, the stage in AD at which statins were administered, and the drugs' pleiotropic metabolic effects, all of which contribute to the substantial variability observed to date. We recommend that future human studies of this important therapeutic topic (1) take the blood-brain barrier permeabilities of statins into account when analyzing results, (2) include specific analyses of the effects on low- and high-density lipoprotein cholesterol, and, most important, (3) conduct statin treatment trials solely in patients with mild AD, who have the best chance for disease modification.

Α-synuclein neuropathology is controlled by nuclear hormone receptors and enhanced by docosahexaenoic acid in a mouse model for Parkinson's disease

α-Synuclein (α-Syn) is a neuronal protein that accumulates progressively in Parkinson's disease (PD) and related synucleinopathies. Attempting to identify cellular factors that affect α-Syn neuropathology, we previously reported that polyunsaturated fatty acids (PUFAs) promote α-Syn oligomerization and aggregation in cultured cells. We now report that docosahexaenoic acid (DHA), a 22:6 PUFA, affects α-Syn oligomerization by activating retinoic X receptor (RXR) and peroxisome proliferator-activated receptor γ2 (PPARγ2). In addition, we show that dietary changes in brain DHA levels affect α-Syn cytopathology in mice transgenic for the PD-causing A53T mutation in human α-Syn. A diet enriched in DHA, an activating ligand of RXR, increased the accumulation of soluble and insoluble neuronal α-Syn, neuritic injury and astrocytosis. Conversely, abnormal accumulations of α-Syn and its deleterious effects were significantly attenuated by low dietary DHA levels. Our results suggest a role for activated RXR/PPARγ 2, obtained by elevated brain PUFA levels, in α-Syn neuropathology.

Cholesterol level and statin use in Alzheimer disease: I. Review of epidemiological and preclinical studies

During the last 2 decades, evidence has accumulated that a high cholesterol level may increase the risk of developing Alzheimer disease (AD). With the global use of statins to treat hypercholesterolemia, this finding has led to the anticipation that statins could prove useful in treating or preventing AD. However, the results of work on this topic are inconsistent: some studies find beneficial effects, but other studies do not. In this first segment of a 2-part review, we examine the complex preclinical and clinical literature on cholesterol level and AD. First, we review epidemiological research on cholesterol level and the risk of AD and discuss the relevance of discrepancies among studies with regard to participants' age and clinical status. Second, we assess studies correlating cholesterol level with neuropathological AD type. The potential molecular mechanisms for the apparent adverse effects of cholesterol on the development of AD are then discussed. Third, we review preclinical studies of statin use and AD. Therefore, this first part of our review provides the background and rationale for investigating statins as potential therapeutic agents in patients with AD, the subject of the second part.

Association of SNCA with Parkinson: replication in the Harvard NeuroDiscovery Center Biomarker Study

BACKGROUND

Mutations in the α-synuclein gene (SNCA) cause autosomal dominant forms of Parkinson's disease, but the substantial risk conferred by this locus to the common sporadic disease has only recently emerged from genome-wide association studies.

METHODS

We genotyped a prioritized noncoding variant in SNCA intron 4 in 344 patients with Parkinson's disease and 275 controls from the longitudinal Harvard NeuroDiscovery Center Biomarker Study.

RESULTS

The common minor allele of rs2736990 was associated with elevated disease susceptibility (odds ratio, 1.40; P = .0032).

CONCLUSIONS

This result increases confidence in the notion that in many clinically well-characterized patients, genetic variation in SNCA contributes to "sporadic" disease.

Alzheimer's disease

Over the last three decades, advances in biochemical pathology and human genetics have illuminated one of the most enigmatic subjects in biomedicine--neurodegeneration. Eponymic diseases of the nervous system such as Alzheimer's, Parkinson's, and Huntington's diseases that were long characterized by mechanistic ignorance have yielded striking progress in our understanding of their molecular underpinnings. A central theme in these and related disorders is the concept that certain normally soluble neuronal proteins can misfold and aggregate into oligomers and amyloid fibrils which can confer profound cytotoxicity. Perhaps the foremost example, both in terms of its societal impact and how far knowledge has moved toward the clinic, is that of Alzheimer's disease (AD). Here, we will review the classical protein lesions of the disorder that have provided a road map to etiology and pathogenesis. We will discuss how elucidating the genotype-to-phenotype relationships of familial forms of Alzheimer's disease has highlighted the importance of the misfolding and altered proteostasis of two otherwise soluble proteins, amyloid β-protein and tau, suggesting mechanism-based therapeutic targets that have led to clinical trials.

Isolation of low-n amyloid β-protein oligomers from cultured cells, CSF, and brain

Recent data suggest that soluble, non-fibrillar assemblies of the amyloid β-protein (Aβ) may mediate the synaptic deficits that characterize the early stages of Alzheimer's disease. Consequently, much effort has been expended in isolating and studying a variety of different Aβ assemblies. Here, we describe the use of immunoprecipitation/western blotting and size exclusion chromatography/western blotting to characterize Aβ present in conditioned medium from cultured cells, human cerebrospinal fluid, and human cortex extracted with aqueous buffer, detergent, and formic acid.