Natural oligomers of the amyloid-beta protein specifically disrupt cognitive function

A central unresolved problem in research on Alzheimer disease is the nature of the molecular entity causing dementia. Here we provide the first direct experimental evidence that a defined molecular species of the amyloid-beta protein interferes with cognitive function. Soluble oligomeric forms of amyloid-beta, including trimers and dimers, were both necessary and sufficient to disrupt learned behavior in a manner that was rapid, potent and transient; they produced impaired cognitive function without inducing permanent neurological deficits. Although beta-amyloidosis has long been hypothesized to affect cognition, the abnormally folded protein species associated with this or any other neurodegenerative disease has not previously been isolated, defined biochemically and then specifically characterized with regard to its effects on cognitive function. The biochemical isolation of discrete amyloid-beta moieties with pathophysiological properties sets the stage for a new approach to studying the molecular mechanisms of cognitive impairment in Alzheimer disease and related neurodegenerative disorders.

Gamma-secretase exists on the plasma membrane as an intact complex that accepts substrates and effects intramembrane cleavage

Research on Alzheimer's disease led to the identification of a novel proteolytic mechanism in all metazoans, the presenilin/gamma-secretase complex. This unique intramembrane-cleaving aspartyl protease is required for the normal processing of Notch, Jagged, beta-amyloid precursor protein (APP), E-cadherin, and many other receptor-like proteins. We recently provided indirect evidence of gamma-secretase activity at the cell surface in HeLa cells following inhibition of receptor-mediated endocytosis. Here, we directly identify and isolate gamma-secretase as an intact complex (Presenilin, Nicastrin, Aph-1, and Pen-2) from the plasma membrane, both in overexpressing cell lines and endogenously. Inhibition of its proteolytic activity allowed cell surface gamma-secretase to be captured in association with its plasma membrane-localized APP substrates (C83 and C99). Moreover, non-denaturing isolation of the intact enzyme complex revealed that cell surface gamma-secretase can specifically generate amyloid beta-protein from an APP substrate and similarly cleave a Notch substrate. These data directly establish the proteolytic function of gamma-secretase on the plasma membrane, independent of a hypothesized substrate trafficking role. We conclude that presenilin/gamma-secretase exists as a mature complex at the cell surface, where it interacts with and can cleave its substrates, consistent with an essential function in processing many adhesion molecules and receptors required for cell-cell interaction or intercellular signaling.

Deciphering the molecular basis of memory failure in Alzheimer's disease

Acutely developing lesions of the brain have been highly instructive in elucidating the neural systems underlying memory in humans and animal models. Much less has been learned from chronic neurodegenerative disorders that insidiously impair memory. But the advent of a detailed molecular hypothesis for the development of Alzheimer's disease and the creation of compelling mouse models thereof have begun to change this situation. Experiments in rodents suggest that soluble oligomers of the amyloid beta protein (Abeta) may discretely interfere with synaptic mechanisms mediating aspects of learning and memory, including long-term potentiation. In humans, memory impairment correlates strongly with cortical levels of soluble Abeta species, which include oligomers. Local inflammatory changes, neurofibrillary degeneration, and neurotransmitter deficits all contribute to memory impairment, but available evidence suggests that these develop as a consequence of early Abeta accumulation. Accordingly, attempts to slow memory and cognitive loss by decreasing cerebral Abeta levels have entered human trials.

Cell biology of protein misfolding: The examples of Alzheimer's and Parkinson's diseases

The salutary intersection of fundamental cell biology with the study of disease is well illustrated by the emerging elucidation of neurodegenerative disorders. Novel mechanisms in cell biology have been uncovered through disease-orientated research; for example, the discovery of presenilin as an intramembrane aspartyl protease that processes many diverse proteins within the lipid bilayer. A common theme has arisen in this field: normally-soluble proteins accumulate, misfold and oligomerize, inducing cytotoxic effects that are particularly devastating in the post-mitotic milieu of the neuron.

A seed for Alzheimer amyloid in the brain

A fundamental question about the early pathogenesis of Alzheimer's disease (AD) concerns how toxic aggregates of amyloid beta protein (Abeta) are formed from its nontoxic soluble form. We hypothesized previously that GM1 ganglioside-bound Abeta (GAbeta) is involved in the process. We now examined this possibility using a novel monoclonal antibody raised against GAbeta purified from an AD brain. Here, we report that GAbeta has a conformation distinct from that of soluble Abeta and initiates Abeta aggregation by acting as a seed. Furthermore, GAbeta generation in the brain was validated by both immunohistochemical and immunoprecipitation studies. These results imply a mechanism underlying the onset of AD and suggest that an endogenous seed can be a target of therapeutic strategy.

Alzheimer's disease abeta vaccine reduces central nervous system abeta levels in a non-human primate, the Caribbean vervet

Amyloid beta (Abeta) protein immunotherapy lowers cerebral Abeta and improves cognition in mouse models of Alzheimer's disease (AD). Here we show that Caribbean vervet monkeys (Chlorocebus aethiops, SK) develop cerebral Abeta plaques with aging and that these deposits are associated with gliosis and neuritic dystrophy. Five aged vervets were immunized with Abeta peptide over 10 months. Plasma and cerebral spinal fluid (CSF) samples were collected periodically from the immunized vervets and five aged controls; one monkey per group expired during the study. By Day 42, immunized animals generated plasma Abeta antibodies that labeled Abeta plaques in human, AD transgenic mouse and vervet brains; bound Abeta1-7; and recognized monomeric and oligomeric Abeta but not full-length amyloid precursor protein nor its C-terminal fragments. Low anti-Abeta titers were detected in CSF. Abetax-40 levels were elevated approximately 2- to 5-fold in plasma and decreased up to 64% in CSF in immunized vervets. Insoluble Abetax-42 was decreased by 66% in brain homogenates of the four immunized animals compared to archival tissues from 13 age-matched control vervets. Abeta42-immunoreactive plaques were detected in frontal cortex in 11 of the 13 control animals, but not in six brain regions examined in each of the four immunized vervets. No T cell response or inflammation was observed. Our study is the first to demonstrate age-related Abeta deposition in the vervet monkey as well as the lowering of cerebral Abeta by Abeta vaccination in a non-human primate. The findings further support Abeta immunotherapy as a potential prevention and treatment of AD.

Purification and Characterization of the Human γ-Secretase Complex†

Gamma-secretase is a member of an unusual class of proteases with intramembrane catalytic sites. This enzyme cleaves many type I membrane proteins, including the amyloid beta-protein (Abeta) precursor (APP) and the Notch receptor. Biochemical and genetic studies have identified four membrane proteins as components of gamma-secretase: heterodimeric presenilin (PS) composed of its N- and C-terminal fragments (PS-NTF/CTF), a mature glycosylated form of nicastrin (NCT), Aph-1, and Pen-2. Recent data from studies in Drosophila, mammalian, and yeast cells suggest that PS, NCT, Aph-1, and Pen-2 are necessary and sufficient to reconstitute gamma-secretase activity. However, many unresolved issues, in particular the possibility of other structural or regulatory components, would be resolved by actually purifying the enzyme. Here, we report a detailed, multistep purification procedure for active gamma-secretase and an initial characterization of the purified protease. Extensive mass spectrometry of the purified proteins strongly suggests that PS-NTF/CTF, mNCT, Aph-1, and Pen-2 are the components of active gamma-secretase. Using the purified gamma-secretase, we describe factors that modulate the production of specific Abeta species: (1) phosphatidylcholine and sphingomyelin dramatically improve activity without changing cleavage specificity within an APP substrate; (2) increasing CHAPSO concentrations from 0.1 to 0.25% yields a approximately 100% increase in Abeta42 production; (3) exposure of an APP-based recombinant substrate to 0.5% SDS modulates cleavage specificity from a disease-mimicking pattern (high Abeta42/43) to a physiological pattern (high Abeta40); and (4) sulindac sulfide directly and preferentially decreases Abeta42 cleavage within the purified complex. Taken together, our results define a procedure for purifying active gamma-secretase and suggest that the lipid-mediated conformation of both enzyme and substrate regulate the production of the potentially neurotoxic Abeta42 and Abeta43 peptides.

Amyloid β-protein induced electrophysiological changes are dependent on aggregation state: N-methyl-d-aspartate (NMDA) versus non-NMDA receptor/channel activation

Alzheimer's disease (AD) is a progressive neurodegenerative disease, however, the underlying mechanism driving this condition is unknown. Unexplored is the possibility that the time-dependent generation of different Abeta assemblies may underlie the pathogenic cascade with biophysically distinct structures interacting with unique biological targets. Thus, the presence of subtle alterations in synaptic function during the earliest clinical phase of AD may be mediated by diffusible assemblies of the amyloid beta-protein (Abeta). Using primary neocortical cultures, here we compare the synaptic responses induced by two different Abeta assemblies, protofibrils (PFs) and fibrils (FBs), and demonstrate for the first time that neuronal activation was selectively dependent on the assembly state of Abeta. PF-induced activity was specifically attenuated by the N-methyl-D-aspartate (NMDA) receptor antagonist, D-APV. In contrast, the non-NMDA glutamate receptor antagonist, NBQX, preferentially reduced FB-induced activity. In support, removal of Mg(2+) from the medium, which enhances NMDA channels, increased both PF- or FB-induced activation, but D-APV was more effective in attenuating PF-induced excitatory activity. These findings suggest that PFs may activate neurons differently than fibrils and lend support to the hypothesis that pre-fibrillar assemblies of Abeta may play an important role in the development of AD-type synaptic deficits.

Soluble Arctic amyloid beta protein inhibits hippocampal long-term potentiation in vivo

Mutations in the amyloid precursor protein that result in substitutions of glutamic acid at residue 22 of the amyloid beta protein (A beta) with glutamine (Q22, Dutch) or glycine (G22, Arctic) cause aggressive familial neurological diseases characterized by cerebrovascular haemorrhages or Alzheimer's-type dementia, respectively. The present study compared the ability of these peptides to block long-term potentiation (LTP) of glutamatergic transmission in the hippocampus in vivo. The effects of intracerebroventricular injection of wild-type, Q22 and G22 A beta(1-40) peptides were examined in the CA1 area of urethane-anaesthetized rats. Both mutant peptides were approximately 100-fold more potent than wild-type A beta at inhibiting LTP induced by high-frequency stimulation when solutions of A beta were freshly prepared. Fibrillar material, as determined by electron microscopy, was obvious in all these peptide solutions and exhibited appreciable Congo Red binding, particularly for A beta(1-40)G22 and A beta(1-40)Q22. A soluble fraction of A beta(1-40)G22, obtained following high-speed centrifugation, retained full activity of the peptide solution to inhibit LTP, providing strong evidence that in the case of the Arctic disease a soluble nonfibrillar form of A beta may represent the primary mediator of A beta-related cognitive deficits, particularly early in the disease. In contrast, nonfibrillar soluble A beta(1-40)Q22 supernatant solution was approximately 10-fold less potent at inhibiting LTP than A beta(1-40)G22, a finding consistent with fibrillar A beta contributing to the inhibition of LTP by the Dutch peptide.

Block of long-term potentiation by naturally secreted and synthetic amyloid beta-peptide in hippocampal slices is mediated via activation of the kinases c-Jun N-terminal kinase, cyclin-dependent kinase 5, and p38 mitogen-activated protein kinase as well as metabotropic glutamate receptor type 5

The mechanisms of action of human synthetic and naturally secreted cell-derived amyloid beta-peptide (Abeta)(1-42) on the induction of long-term potentiation (LTP) were investigated in the medial perforant path to dentate granule cell synapses in hippocampal slices. Synthetic and cell-derived Abeta strongly inhibited high-frequency stimulation (HFS)-induced LTP at peak HFS and 1 hr after HFS. Cell-derived Abeta was much more potent than synthetic Abeta at inhibiting LTP induction, with threshold concentrations of approximately 1 and 100-200 nm, respectively. The involvement of various kinases in Abeta-mediated inhibition of LTP induction was investigated by applying Abeta in the presence of inhibitors of these kinases. The c-Jun N-terminal kinase (JNK) inhibitor JNKI prevented the block of LTP induction by both synthetic and cell-derived Abeta. The block of LTP induced by synthetic Abeta was also prevented by the JNK inhibitor anthra[1,9-cd]pyrazol-6(2H)-one, the cyclin-dependent kinase 5 (Cdk5) inhibitors butyrolactone and roscovitine, and the p38 MAP kinase (MAPK) inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole but not by the p42-p44 MAP kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene. The group I-group II metabotropic glutamate receptor (mGluR) antagonist 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid and the mGluR5 antagonist methyl-6-(phenylethynyl)pyridine prevented the block of LTP induction by Abeta. However, thealpha7 nicotinic ACh receptor antagonist methylcaconatine did not prevent the inhibition of LTP induction by Abeta. These studies provide evidence that the Abeta-mediated inhibition of LTP induction involves stimulation of the kinases JNK, Cdk5, and p38 MAPK after the activation of both the Abeta receptor(s) and mGluR5.

Oligomers on the Brain: the Emerging Role of Soluble Protein Aggregates in Neurodegeneration

Extracellular fibrous amyloid deposits or intracellular inclusion bodies containing abnormal protein fibrils characterize many different neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), dementia with Lewy bodies, multiple system atrophy, Huntington's disease, and the transmissible 'prion' dementias. There is strong evidence from genetic, transgenic mouse and biochemical studies to support the idea that the accumulation of protein aggregates in the brain plays a seminal role in the pathogenesis of these diseases. How monomeric proteins ultimately convert to highly polymeric deposits is unknown. However, studies employing, synthetic, cell-derived and purified recombinant proteins suggest that amyloid proteins first come together to form soluble low n-oligomers. Further association of these oligomers results in higher molecular weight assemblies including so-called 'protofibrils' and 'ADDLs' and these eventually exceed solubility limits until, finally, they are deposited as amyloid fibrils. With particular reference to AD and PD, we review recent evidence that soluble oligomers are the principal pathogenic species that drive neuronal dysfunction.

Partial loss-of-function mutations in insulin-degrading enzyme that induce diabetes also impair degradation of amyloid beta-protein

The causes of cerebral accumulation of amyloid beta-protein (Abeta) in most cases of Alzheimer's disease (AD) remain unknown. We recently found that homozygous deletion of the insulin-degrading enzyme (IDE) gene in mice results in an early and marked elevation of cerebral Abeta. Both genetic linkage and allelic association in the IDE region of chromosome 10 have been reported in families with late-onset AD. For IDE to remain a valid candidate gene for late-onset AD on functional grounds, it must be shown that partial loss of function of IDE can still alter Abeta degradation, but without causing early, severe elevation of brain Abeta. Here, we show that naturally occurring IDE missense mutations in a well-characterized rat model of type 2 diabetes mellitus (DM2) result in decreased catalytic efficiency and a significant approximately 15 to 30% deficit in the degradation of both insulin and Abeta. Endogenously secreted Abeta(40) and Abeta(42) are significantly elevated in primary neuronal cultures from animals with the IDE mutations, but there is no increase in steady-state levels of rodent Abeta in the brain up to age 14 months. We conclude that naturally occurring, partial loss-of-function mutations in IDE sufficient to cause DM2 also impair neuronal regulation of Abeta levels, but the brain can apparently compensate for the partial deficit during the life span of the rat. Our findings have relevance for the emerging genetic evidence suggesting that IDE may be a late-onset AD-risk gene, and for the epidemiological relationships among hyperinsulinemia, DM2, and AD.

Aging, amyloid, and Alzheimer's disease: a perspective in honor of Carl Cotman

Deciphering the molecular basis of synaptic dysfuction in Alzheimer's disease (AD) has engaged the attention of scientists with diverse backgrounds and interests. The synthesis of experimental findings from neuropathology, biochemistry, genetics, animal modeling and even immunology, has provided a plausible model for the pathogenesis of the disorder. While not universally accepted, the so-called amyloid (or Abeta) hypothesis of AD is well supported scientifically and predicts several specific targets for therapeutic intervention. Some of these are now reaching the clinic, providing the final and most important test for this hypothetical mechanism of disease.

Detergent-Dependent Dissociation of Active γ-Secretase Reveals an Interaction between Pen-2 and PS1-NTF and Offers a Model for Subunit Organization within the Complex

Gamma-secretase is a member of a new class of proteases with an intramembrane catalytic site and cleaves numerous type I membrane proteins, including the amyloid beta-protein precursor (APP) and the Notch receptor. Biochemical and genetic studies have identified four membrane proteins as components of gamma-secretase: a heterodimeric form of presenilin (PS), composed of its N- and C-terminal fragments (PS-NTF and PS-CTF, respectively), a highly glycosylated, mature form of nicastrin (NCT), Aph-1, and Pen-2. However, it is unclear how these components interact physically with each other and assemble into functional complexes. We and others recently found that Aph-1 interacts with a less glycosylated, immature form of nicastrin as an intermediate toward full assembly of gamma-secretase. Here we show that (1) the detergent dodecyl beta-d-maltoside (DDM) mediates the dissociation and inactivation of active gamma-secretase in a concentration-dependent manner, (2) DDM-dependent dissociation of the active gamma-secretase complex generates two major inactive complexes (Pen-2-PS1-NTF and mNCT-Aph-1) and two minor inactive complexes (mNCT-Aph1-PS1-CTF and PS1-NTF-PS1-CTF), and (3) Pen-2 can also associate with the PS holoprotein in complexes devoid of NCT and Aph-1. Taken together, our results demonstrate that Pen-2 interacts with PS-NTF within active gamma-secretase and offer a model for how the components of active gamma-secretase interact physically with each other.

Folding proteins in fatal ways

Human diseases characterized by insoluble extracellular deposits of proteins have been recognized for almost two centuries. Such amyloidoses were once thought to represent arcane secondary phenomena of questionable pathogenic significance. But it is has now become clear that many different proteins can misfold and form extracellular or intracellular aggregates that initiate profound cellular dysfunction. Particularly challenging examples of such disorders occur in the post-mitotic environment of the neuron and include Alzheimer's and Parkinson's diseases. Understanding some of the principles of protein folding has helped to explain how such diseases arise, with attendant therapeutic insights.

Enhanced Proteolysis of β-Amyloid in APP Transgenic Mice Prevents Plaque Formation, Secondary Pathology, and Premature Death

Converging evidence suggests that the accumulation of cerebral amyloid beta-protein (Abeta) in Alzheimer's disease (AD) reflects an imbalance between the production and degradation of this self-aggregating peptide. Upregulation of proteases that degrade Abeta thus represents a novel therapeutic approach to lowering steady-state Abeta levels, but the consequences of sustained upregulation in vivo have not been studied. Here we show that transgenic overexpression of insulin-degrading enzyme (IDE) or neprilysin (NEP) in neurons significantly reduces brain Abeta levels, retards or completely prevents amyloid plaque formation and its associated cytopathology, and rescues the premature lethality present in amyloid precursor protein (APP) transgenic mice. Our findings demonstrate that chronic upregulation of Abeta-degrading proteases represents an efficacious therapeutic approach to combating Alzheimer-type pathology in vivo.

Functional γ-secretase complex assembly in Golgi/trans-Golgi network: interactions among presenilin, nicastrin, Aph1, Pen-2, and γ-secretase substrates

Gamma-secretase is a proteolytic complex whose substrates include Notch, beta-amyloid precursor protein (APP), and several other type I transmembrane proteins. Presenilin (PS) and nicastrin are known components of this high-molecular-weight complex, and recent genetic screens in invertebrates have identified two additional gene products, Aph1 and Pen-2, as key factors in gamma-secretase activity. Here, we examined the interaction of the components of the gamma-secretase complex in Chinese hamster ovary cells stably expressing human forms of APP, PS1, Aph1, and Pen-2. Subcellular fractionation of membrane vesicles and subsequent coimmunoprecipitation of individual gamma-secretase components revealed that interactions among all proteins occurred in the Golgi/trans-Golgi network (TGN) compartments. Furthermore, incubation of the Golgi/TGN-enriched vesicles resulted in de novo generation of amyloid beta-protein and APP intracellular domain. Immunofluorescent staining of the individual gamma-secretase components supported our biochemical evidence that the gamma-secretase components assemble into the proteolytically active gamma-secretase complex in the Golgi/TGN compartment.

Evidence for peripheral clearance of cerebral Abeta protein following chronic, active Abeta immunization in PSAPP mice

Immunization with amyloid-beta (Abeta) peptide in mouse models of Alzheimer's disease has been reported to decrease cerebral Abeta levels and improve behavioral deficits. Several mechanisms have been proposed, including antibody-induced phagocytosis of Abeta by cerebral microglia and increased efflux of Abeta from the brain to the periphery. The latter mechanism was suggested in mice undergoing acute, passive transfer of an Abeta monoclonal antibody. Here, PSAPP transgenic mice were actively immunized by a single intraperitoneal injection of synthetic Abeta followed by chronic intranasal administration of Abeta with the mucosal adjuvant, Escherichia coli heat-labile enterotoxin, LT, twice weekly for 8 weeks. Serum from Abeta-immunized mice had an average of 240 microg/ml of anti-Abeta-specific antibodies; these antibodies had epitope(s) within Abeta1-15 and were of immunoglobulin (Ig) isotypes IgG2b, IgG2a, and IgG1. Immunization led to a 75% decrease in plaque number (P < 0.0001) and a 58% decrease in Abetax-42 levels (P < 0.026) in brain, and gliosis and neuritic dystrophy were diminished. No pathological effects of the immunization were observed in kidney, spleen, or snout. Serum Abeta levels increased 28-fold in immunized mice (53.06 ng/ml) compared to controls (1.87 ng/ml). Most of the Abeta in the serum of the immunized mice was bound to antibodies. We conclude that following active immunization, anti-Abeta antibodies sequester serum Abeta and may increase central nervous system to serum Abeta clearance.