Receptor-mediated transport of human amyloid beta-protein 1-40 and 1-42 at the blood-brain barrier

A Chronic Increase in Blood-Brain Barrier Permeability Facilitates Intraneuronal Deposition of Exogenous Bloodborne Amyloid-Beta1-42 Peptide in the Brain and Leads to Alzheimer's Disease-Relevant Cognitive Changes in a Mouse Model

Background

Increased blood-brain barrier (BBB) permeability and amyloid-β (Aβ) peptides (especially Aβ1-42) (Aβ42) have been linked to Alzheimer's disease (AD) pathogenesis, but the nature of their involvement in AD-related neuropathological changes leading to cognitive changes remains poorly understood.

Objective

To test the hypothesis that chronic extravasation of bloodborne Aβ42 peptide and brain-reactive autoantibodies and their entry into the brain parenchyma via a permeable BBB contribute to AD-related pathological changes and cognitive changes in a mouse model.

Methods

The BBB was rendered chronically permeable through repeated injections of Pertussis toxin (PT), and soluble monomeric, fluorescein isothiocyanate (FITC)-labeled or unlabeled Aβ42 was injected into the tail-vein of 10-month-old male CD1 mice at designated intervals spanning ∼3 months. Acquisition of learned behaviors and long-term retention were assessed via a battery of cognitive and behavioral tests and linked to neuropathological changes.

Results

Mice injected with both PT and Aβ42 demonstrated a preferential deficit in the capacity for long-term retention and an increased susceptibility to interference in selective attention compared to mice exposed to PT or saline only. Immunohistochemical analyses revealed increased BBB permeability and entry of bloodborne Aβ42 and immunoglobulin G (IgG) into the brain parenchyma, selective neuronal binding of IgG and neuronal accumulation of Aβ42 in animals injected with both PT and Aβ42 compared to controls.

Conclusion

Results highlight the potential synergistic role of BBB compromise and the influx of bloodborne Aβ42 into the brain in both the initiation and progression of neuropathologic and cognitive changes associated with AD.

Gut Microbiota and Immunotherapy for Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that eventually leads to dementia and death of the patient. Currently, no effective treatment is available that can slow or halt the progression of the disease. The gut microbiota can modulate the host immune system in the peripheral and central nervous system through the microbiota-gut-brain axis. Growing evidence indicates that gut microbiota dysbiosis plays an important role in the pathogenesis of AD, and modulation of the gut microbiota may represent a new avenue for treating AD. Immunotherapy targeting Aβ and tau has emerged as the most promising disease-modifying therapy for the treatment of AD. However, the underlying mechanism of AD immunotherapy is not known. Importantly, preclinical and clinical studies have highlighted that the gut microbiota exerts a major influence on the efficacy of cancer immunotherapy. However, the role of the gut microbiota in AD immunotherapy has not been explored. We found that immunotherapy targeting tau can modulate the gut microbiota in an AD mouse model. In this article, we focused on the crosstalk between the gut microbiota, immunity, and AD immunotherapy. We speculate that modulation of the gut microbiota induced by AD immunotherapy may partially underlie the efficacy of the treatment.

Semimechanistic Population Pharmacokinetic Modeling to Investigate Amyloid Beta Trafficking and Accumulation at the BBB Endothelium

Elevated exposure to toxic amyloid beta (Aβ) peptides and consequent blood-brain barrier (BBB) dysfunction are believed to promote vasculopathy in Alzheimer's disease (AD). However, the accumulation kinetics of different Aβ isoforms within the BBB endothelium and how it drives BBB dysfunction are not clearly characterized. Using single positron emission computed tomography (SPECT)-computed tomography (CT) dynamic imaging coupled with population pharmacokinetic modeling, we investigated the accumulation kinetics of Aβ40 and Aβ42 in the BBB endothelium. Brain clearance was quantified after intracerebral administration of ¹²⁵I-Aβ, and BBB-mediated transport was shown to account for 54% of ¹²⁵I-Aβ40 total clearance. A brain influx study demonstrated lower values of both maximal rate (V_max) and Michaelis constant (K_m) for ¹²⁵I-Aβ42 compared to ¹²⁵I-Aβ40. Validated by a transcytosis study in polarized human BBB endothelial cell (hCMEC/D3) monolayers, model simulations demonstrated impaired exocytosis was responsible for inefficient permeability and enhanced accumulation of Aβ42 in the BBB endothelium. Further, both isoforms were shown to disrupt the exocytosis machinery of BBB endothelial cells so that a vicious cycle could be generated. The validated model was able to capture changes in Aβ steady-state levels in plasma as well as the brain during AD progression and allowed us to predict the kinetics of Aβ accumulation in the BBB endothelium.

Infusion of blood from mice displaying cerebral amyloidosis accelerates amyloid pathology in animal models of Alzheimer's disease

Previous studies showed that injection of tissue extracts containing amyloid-β (Aβ) aggregates accelerate amyloid deposition in the brain of mouse models of Alzheimer’s disease (AD) through prion-like mechanisms. In this study, we evaluated whether brain amyloidosis could be accelerated by blood infusions, procedures that have been shown to transmit prion diseases in animals and humans. Young transgenic mice infused with whole blood or plasma from old animals with extensive Aβ deposition in their brains developed significantly higher levels brain amyloidosis and neuroinflammation compared to untreated animals or mice infused with wild type blood. Similarly, intra-venous injection of purified Aβ aggregates accelerated amyloid pathology, supporting the concept that Aβ seeds present in blood can reach the brain to promote neuropathological alterations in the brain of treated animals. However, an amyloid-enhancing effect of other factors present in the blood of donors cannot be discarded. Our results may help to understand the role of peripheral (amyloid-dependent or -independent) factors implicated in the development of AD and uncover new strategies for disease intervention.

Efficacy and Safety of Plasma Exchange with 5% Albumin to Modify Cerebrospinal Fluid and Plasma Amyloid-β Concentrations and Cognition Outcomes in Alzheimer's Disease Patients: A Multicenter, Randomized, Controlled Clinical Trial

Background: Studies conducted in animal models and humans suggest the presence of a dynamic equilibrium of amyloid-β (Aβ) peptide between cerebrospinal fluid (CSF) and plasma compartments.

Objective: To determine whether plasma exchange (PE) with albumin replacement was able to modify Aβ concentrations in CSF and plasma as well as to improve cognition in patients with mild-moderate Alzheimer’s disease (AD).

Methods: In a multicenter, randomized, patient- and rater-blind, controlled, parallel-group, phase II study, 42 AD patients were assigned (1 : 1) to PE treatment or control (sham) groups. Treated patients received a maximum of 18 PE with 5% albumin (Albutein^®, Grifols) with three different schedules: two PE/weekly (three weeks), one PE/weekly (six weeks), and one PE/bi- weekly (12 weeks), plus a six-month follow-up period. Plasma and CSF Aβ_1–40 and Aβ_1–42 levels, as well as cognitive, functional, and behavioral measures were determined.

Results: CSF Aβ_1–42 levels after the last PE compared to baseline were marginally higher in PE-treated group versus controls (adjusted means of variation: 75.3 versus –45.5 pg/mL; 95% CI: –19.8, 170.5 versus 135.1, 44.2; p = 0.072). Plasma Aβ_1–42 levels were lower in the PE-treated group after each treatment period (p < 0.05). Plasma Aβ_1–40 levels showed a saw-tooth pattern variation associated with PE. PE-treated patients scored better in the Boston Naming Test and Semantic Verbal Fluency (p < 0.05) throughout the study. Neuropsychiatric Inventory scores were higher in controls during the PE phase (p < 0.05).

Conclusion: PE with human albumin modified CSF and plasma Aβ_1–42 levels. Patients treated with PE showed improvement in memory and language functions, which persisted after PE was discontinued.

In vivo Differential Brain Clearance and Catabolism of Monomeric and Oligomeric Alzheimer's Aβ protein

Amyloid β (Aβ) is the major constituent of the brain deposits found in parenchymal plaques and cerebral blood vessels of patients with Alzheimer's disease (AD). Several lines of investigation support the notion that synaptic pathology, one of the strongest correlates to cognitive impairment, is related to the progressive accumulation of neurotoxic Aβ oligomers. Since the process of oligomerization/fibrillization is concentration-dependent, it is highly reliant on the homeostatic mechanisms that regulate the steady state levels of Aβ influencing the delicate balance between rate of synthesis, dynamics of aggregation, and clearance kinetics. Emerging new data suggest that reduced Aβ clearance, particularly in the aging brain, plays a critical role in the process of amyloid formation and AD pathogenesis. Using well-defined monomeric and low molecular mass oligomeric Aβ1-40 species stereotaxically injected into the brain of C57BL/6 wild-type mice in combination with biochemical and mass spectrometric analyses in CSF, our data clearly demonstrate that Aβ physiologic removal is extremely fast and involves local proteolytic degradation leading to the generation of heterogeneous C-terminally cleaved proteolytic products, while providing clear indication of the detrimental role of oligomerization for brain Aβ efflux. Immunofluorescence confocal microscopy studies provide insight into the cellular pathways involved in the brain removal and cellular uptake of Aβ. The findings indicate that clearance from brain interstitial fluid follows local and systemic paths and that in addition to the blood-brain barrier, local enzymatic degradation and the bulk flow transport through the choroid plexus into the CSF play significant roles. Our studies highlight the diverse factors influencing brain clearance and the participation of various routes of elimination opening up new research opportunities for the understanding of altered mechanisms triggering AD pathology and for the potential design of combined therapeutic strategies.

The Blood-Brain Barrier: Regulatory Roles in Wakefulness and Sleep

Sleep and its disorders are known to affect the functions of essential organs and systems in the body. However, very little is known about how the blood-brain barrier (BBB) is regulated. A few years ago, we launched a project to determine the impact of sleep fragmentation and chronic sleep restriction on BBB functions, including permeability to fluorescent tracers, tight junction protein expression and distribution, glucose and other solute transporter activities, and mediation of cellular mechanisms. Recent publications and relevant literature allow us to summarize here the sleep-BBB interactions in five sections: (1) the structural basis enabling the BBB to serve as a huge regulatory interface; (2) BBB transport and permeation of substances participating in sleep-wake regulation; (3) the circadian rhythm of BBB function; (4) the effect of experimental sleep disruption maneuvers on BBB activities, including regional heterogeneity, possible threshold effect, and reversibility; and (5) implications of sleep disruption-induced BBB dysfunction in neurodegeneration and CNS autoimmune diseases. After reading the review, the general audience should be convinced that the BBB is an important mediating interface for sleep-wake regulation and a crucial relay station of mind-body crosstalk. The pharmaceutical industry should take into consideration that sleep disruption alters the pharmacokinetics of BBB permeation and CNS drug delivery, being attentive to the chrono timing and activation of co-transporters in subjects with sleep disorders.

A survey of peptides with effective therapeutic potential in Alzheimer's disease rodent models or in human clinical studies

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder and the most common cause of dementia. Today, only palliative therapies are available. The pathological hallmarks of AD are the presence of neurofibrillary tangles and amyloid plaques, mainly composed of the amyloid-β peptide (Aβ), in the brains of the patients. Several lines of evidence suggest that the increased production and/or decreased cleavage of Aβ and subsequent accumulation of Aβ oligomers and aggregates play a fundamental role in the disease progress. Therefore, substances which bind to Aβ and influence aggregation thereof are of great interest. A wide range of Aβ binding peptides were investigated to date for therapeutic purposes. Only very few were shown to be effective in rodent AD models or in clinical studies. Here, we review those peptides and discuss their possible mechanisms of action.

Initial fate of prions upon peripheral infection: half-life, distribution, clearance, and tissue uptake

Prion diseases are infectious neurodegenerative disorders associated with the misfolded prion protein (PrP(Sc)), which appears to be the sole component of the infectious agent (termed prion). To produce disease, prions have to be absorbed into the body and reach sufficient quantities in the brain. Very little is known about the biological mechanisms controlling the initial fate of prions. Here, we studied the systemic pharmacokinetics and biodistribution of PrP(Sc) in vivo. After an intravenous injection of highly purified radiolabeled or native unlabeled PrP(Sc), the protein was eliminated rapidly from the serum (half-life of 3.24 h), mostly through tissue uptake. The quantity of intact PrP(Sc) reaching the brain was ∼ 0.2% of the injected dose per gram of brain tissue (ID/g). The highest levels were found in liver (∼ 20% ID/g), spleen (∼ 13% ID/g), and kidney (∼ 7.4% ID/g). Cell surface PrP(C) does not appear to play a role in PrP(Sc) pharmacokinetics, since the infectious protein distributed similarly in wild-type and PrP-null mice. To measure tissue uptake kinetics and biodistribution accurately, vascular space in tissues was measured with radioactively labeled albumin coinjected with radioactively labeled PrP(Sc). Our results provide a fundamental pharmacokinetic characterization of PrP(Sc) in vivo, which may be relevant to estimate tissue risks and mechanisms of prion neuroinvasion and to identify novel therapeutic strategies.

Western diet consumption and cognitive impairment: links to hippocampal dysfunction and obesity

Intake of saturated fats and simple carbohydrates, two of the primary components of a modern Western diet, is linked with the development of obesity and Alzheimer's Disease. The present paper summarizes research showing that Western diet intake is associated with cognitive impairment, with a specific emphasis on learning and memory functions that are dependent on the integrity of the hippocampus. The paper then considers evidence that saturated fat and simple carbohydrate intake is correlated with neurobiological changes in the hippocampus that may be related to the ability of these dietary components to impair cognitive function. Finally, a model is described proposing that Western diet consumption contributes to the development of excessive food intake and obesity, in part, by interfering with a type of hippocampal-dependent memory inhibition that is critical in the ability of animals to refrain from responding to environmental cues associated with food, and ultimately from consuming energy intake in excess of that driven solely by caloric need.

Cerebral amyloid angiopathy: a systematic review

Cerebral amyloid angiopathy (CAA) is a disorder characterized by amyloid deposition in the walls of leptomeningeal and cortical arteries, arterioles, and less often capillaries and veins of the central nervous system. CAA occurs mostly as a sporadic condition in the elderly, its incidence associating with advancing age. All sporadic CAA cases are due to deposition of amyloid-β, originating from proteolytic cleavage of the Amyloid Precursor Protein. Hereditary forms of CAA are generally familial (and therefore rare in the general population), more severe and earlier in onset. CAA-related lobar intracerebral hemorrhage is the most well-studied clinical condition associated with brain amyloid deposition. Despite ever increasing understanding of CAA pathogenesis and availability of reliable clinical and diagnostic tools, preventive and therapeutic options remain very limited. Further research efforts are required in order to identify biological targets for novel CAA treatment strategies. We present a systematic review of existing evidence regarding the epidemiology, genetics, pathogenesis, diagnosis and clinical management of CAA.

Genetics and molecular pathogenesis of sporadic and hereditary cerebral amyloid angiopathies

In cerebral amyloid angiopathy (CAA), amyloid fibrils deposit in walls of arteries, arterioles and less frequently in veins and capillaries of the central nervous system, often resulting in secondary degenerative vascular changes. Although the amyloid-beta peptide is by far the commonest amyloid subunit implicated in sporadic and rarely in hereditary forms of CAA, a number of other proteins may also be involved in rare familial diseases in which CAA is also a characteristic morphological feature. These latter proteins include the ABri and ADan subunits in familial British dementia and familial Danish dementia, respectively, which are also known under the umbrella term BRI2 gene-related dementias, variant cystatin C in hereditary cerebral haemorrhage with amyloidosis of Icelandic-type, variant transthyretins in meningo-vascular amyloidosis, disease-associated prion protein (PrP(Sc)) in hereditary prion disease with premature stop codon mutations and mutated gelsolin (AGel) in familial amyloidosis of Finnish type. In this review, the characteristic morphological features of the different CAAs is described and the implication of the biochemical, genetic and transgenic animal data for the pathogenesis of CAA is discussed.

CNS delivery via adsorptive transcytosis

Adsorptive-mediated transcytosis (AMT) provides a means for brain delivery of medicines across the blood-brain barrier (BBB). The BBB is readily equipped for the AMT process: it provides both the potential for binding and uptake of cationic molecules to the luminal surface of endothelial cells, and then for exocytosis at the abluminal surface. The transcytotic pathways present at the BBB and its morphological and enzymatic properties provide the means for movement of the molecules through the endothelial cytoplasm. AMT-based drug delivery to the brain was performed using cationic proteins and cell-penetrating peptides (CPPs). Protein cationization using either synthetic or natural polyamines is discussed and some examples of diamine/polyamine modified proteins that cross BBB are described. Two main families of CPPs belonging to the Tat-derived peptides and Syn-B vectors have been extensively used in CPP vector-mediated strategies allowing delivery of a large variety of small molecules as well as proteins across cell membranes in vitro and the BBB in vivo. CPP strategy suffers from several limitations such as toxicity and immunogenicity--like the cationization strategy--as well as the instability of peptide vectors in biological media. The review concludes by stressing the need to improve the understanding of AMT mechanisms at BBB and the effectiveness of cationized proteins and CPP-vectorized proteins as neurotherapeutics.

Hippocampal RAGE immunoreactivity in early and advanced Alzheimer's disease

Microvascular accumulation and neuronal overproduction of amyloid-beta peptide (Abeta) are pathologic features of Alzheimer's disease (AD). In this study, we examined the receptor for advanced glycation endproducts (RAGE), a multi-ligand receptor found in both neurons and cerebral microvascular endothelia that binds Abeta. RAGE expression was assessed in aged controls (n = 6), patients with early AD-like pathology (n = 6), and severe, Braak V-VI AD (n = 6). Human hippocampi were stained with a specific polyclonal antibody directed against RAGE (Research Diagnostics, Flanders, NJ). Immunoreactivity was localized in both neurons and cerebral endothelial cells. Quantitative image-analyses were performed on grayscale images to assess the total surface area of endothelial RAGE immunoreaction product in cross sections of cerebral microvessels (5-20 microm). Confocal images were acquired for confirmation of RAGE immunoreactivity in both microvessels and neurons by coupling RAGE with CD-31 and neurofilament, respectively. A significant increase in endothelial RAGE immunoreactivity was found in severe Braak V-VI AD patients when compared to aged controls (p < 0.001), and when compared to patients with early AD pathology (p = 0.0125). In addition, a significant increase in endothelial RAGE immunoreactivity was witnessed when comparing aged controls having no reported AD pathology with patients having early AD-like pathology (p = 0.038). Our data suggest that microvascular RAGE levels increase in conjunction with the onset of AD, and continue to increase linearly as a function of AD pathologic severity (p < 0.0001).

Apolipoprotein E, amyloid-beta, and blood-brain barrier permeability in Alzheimer disease

There is increasing evidence for blood-brain barrier (BBB) compromise in Alzheimer disease (AD). The presence of the epsilon4 allele of the apolipoprotein E (apoE) gene is a risk factor for sporadic AD. Apolipoprotein E is essential both for maintenance of BBB integrity and for the deposition of fibrillar amyloid-beta (Abeta) that leads to the development of Abeta plaques in AD and to cerebral amyloid angiopathy. This review investigates the relationships between apoE, Abeta, and the BBB in AD. Alterations in the expression and distribution of the BBB Abeta transporters receptor for advanced glycation end-products and low-density lipoprotein receptor-related protein 1 in AD and the potential roles of apoE4 expression in adversely influencing Abeta burden and BBB permeability are also examined. Because both apoE and Abeta are ligands for low-density lipoprotein receptor-related protein 1, all 3 molecules are present in AD plaques, and most AD plaques are located close to the cerebral microvasculature. The interactions of these molecules at the BBB likely influence metabolism and clearance of Abeta and contribute to AD pathogenesis. Therapeutic alternatives targeting apoE/Abeta and sealing a compromised BBB are under development for the treatment of AD.

Pharmacokinetics and Amyloid Plaque Targeting Ability of a Novel Peptide-Based Magnetic Resonance Contrast Agent in Wild-Type and Alzheimer's Disease Transgenic Mice

A novel magnetic resonance (MR) imaging contrast agent based on a derivative of human amyloid beta (Abeta) peptide, Gd[N-4ab/Q-4ab]Abeta 30, was previously shown to cross the blood-brain barrier (BBB) and bind to amyloid plaques in Alzheimer's disease (AD) transgenic mouse (APP/PS1) brain. We now report extensive plasma and brain pharmacokinetics of this contrast agent in wild-type (WT) and in APP/PS1 mice along with a quantitative summary of various physiological factors that govern its efficacy. Upon i.v. bolus administration, (125)I-Gd[N-4ab/Q-4ab]Abeta 30 was rapidly eliminated from the plasma following a three-exponential disposition, which is saturable at higher concentrations. Nevertheless, the contrast agent exhibited rapid and nonsaturable absorption at the BBB. The brain pharmacokinetic profile of (125)I-Gd[N-4ab/Q-4ab]Abeta 30 showed a rapid absorption phase followed by a slower elimination phase. No significant differences were observed in the plasma or brain kinetics of WT and APP/PS1 animals. Emulsion autoradiography studies conducted on WT and APP/PS1 mouse brain after an i.v. bolus administration of (125)I-Gd[N-4ab/Q-4ab]Abeta 30 in vivo confirmed the brain pharmacokinetic data and also demonstrated the preferential localization of the contrast agent on the plaques for an extended period of time. These attributes of the contrast agent are extremely useful in providing an excellent signal/noise ratio during longer MR scans, which may be essential for obtaining a high resolution image. In conclusion, this study documents the successful plaque targeting of Gd[N-4ab/Q-4ab]Abeta 30 and provides crucial pharmacokinetic information to determine the dose, mode of administration, and scan times for future in vivo MR imaging of amyloid plaques in AD transgenic mice.

In vivo targeting of antibody fragments to the nervous system for Alzheimer’s disease immunotherapy and molecular imaging of amyloid plaques

Targeting therapeutic or diagnostic proteins to the nervous system is limited by the presence of the blood-brain barrier. We report that a F(ab')(2) fragment of a monoclonal antibody against fibrillar human Abeta42 that is polyamine (p)-modified has increased permeability at the blood-brain barrier, comparable binding to the antigen, and comparable in vitro binding to amyloid plaques in Alzheimer's disease (AD) transgenic mouse brain sections. Intravenous injection of the pF(ab')(2)4.1 in the AD transgenic mouse demonstrated efficient targeting to amyloid plaques throughout the brain, whereas the unmodified fragment did not. Removal of the Fc portion of this antibody derivative will minimize the inflammatory response and cerebral hemorrhaging associated with passive immunization and provide increased therapeutic potential for treating AD. Coupling contrast agents/radioisotopes might facilitate the molecular imaging of amyloid plaques with magnetic resonance imaging/positron emission tomography. The efficient delivery of immunoglobulin G fragments may also have important applications to other neurodegenerative disorders or for the generalized targeting of nervous system antigens.

Macromolecules involved in production and metabolism of beta-amyloid at the brain barriers

One of the notable features of Alzheimer's disease (AD) is the overabundance of beta-amyloid peptides in brain fluids, leading to the formation and deposition of insoluble amyloid plaques. Previous work in this lab demonstrates that the normal choroid plexus, a primary component of the blood-cerebrospinal fluid barrier, has the capacity to remove beta-amyloid from the cerebrospinal fluid, potentially preventing the formation of beta-amyloid plaques. The purpose of this work was to determine whether the choroid plexus and/or the brain capillaries, a primary component of the blood-brain barrier, possessed the capacity to produce or degrade beta-amyloid peptides. Using quantitative real-time RT-PCR, immunodetection and enzyme activity assays, we demonstrated the presence in brain barriers of several key enzymes involved in beta-amyloid production, namely, amyloid precursor protein and beta-secretase, and in beta-amyloid metabolism and alternate processing, such as insulin degrading enzyme, endothelin-converting enzyme-1, neprilysin and alpha-secretase. Furthermore, beta-amyloid presence, in the absence of its application in culture media, was detected in an immortalized choroidal epithelial cell line, known as Z310 cells. The ability of the choroid plexus to produce and degrade beta-amyloid, in addition to its transport function, suggests a vital role of this tissue in maintaining beta-amyloid homeostasis. Disruption of this homeostasis due to aging, injury or toxicant exposure may contribute to accumulation of beta-amyloid peptides in the brain fluids, leading to AD.

Fibrillar and Oligomeric beta-Amyloid as Distinct Local Biomarkers for Alzheimer's Disease

Beta-amyloid is a key component of Alzheimer's disease (AD) pathology. Researchers in both academic and industry are actively pursuing the development of imaging tracers and techniques to noninvasively measure local levels of beta-amyloid in the Alzheimer's brain. This presentation summarizes recent data and discusses the opportunities and challenges of imaging plaques containing fibrillar beta-amyloid for the early diagnosis and therapeutic monitoring of amyloid targeted therapies. Further, the value and feasibility of measuring the recently described soluble oligomeric form of beta-amyloid as an alternative noninvasive biomarker is also discussed.

Abeta peptides can enter the brain through a defective blood-brain barrier and bind selectively to neurons

We have investigated the possibility that soluble, blood-borne amyloid beta (Abeta) peptides can cross a defective blood-brain barrier (BBB) and interact with neurons in the brain. Immunohistochemical analyses revealed extravasated plasma components, including Abeta42 in 19 of 21 AD brains, but in only 3 of 13 age-matched control brains, suggesting that a defective BBB is common in AD. To more directly test whether blood-borne Abeta peptides can cross a defective BBB, we tracked the fate of fluorescein isothiocyanate (FITC)-labeled Abeta42 and Abeta40 introduced via tail vein injection into mice with a BBB rendered permeable by treatment with pertussis toxin. Both Abeta40 and Abeta42 readily crossed the permeabilized BBB and bound selectively to certain neuronal subtypes, but not glial cells. By 48 h post-injection, Abeta42-positive neurons were widespread in the brain. In the cerebral cortex, small fluorescent, Abeta42-positive granules were found in the perinuclear cytoplasm of pyramidal neurons, suggesting that these cells can internalize exogenous Abeta42. An intact BBB (saline-injected controls) blocked entry of blood-borne Abeta peptides into the brain. The neuronal subtype selectivity of Abeta42 and Abeta40 was most evident in mouse brains subjected to direct intracranial stereotaxic injection into the hippocampal region, thereby bypassing the BBB. Abeta40 was found to preferentially bind to a distinct subset of neurons positioned at the inner face of the dentate gyrus, whereas Abeta42 bound selectively to the population of large neurons in the hilus region of the dentate gyrus. Our results suggest that the blood may serve as a major, chronic source of soluble, exogenous Abeta peptides that can bind selectively to certain subtypes of neurons and accumulate within these cells.

RAGE, LRP-1, and amyloid-beta protein in Alzheimer's disease

The receptor for advanced glycation end products (RAGE) is thought to be a primary transporter of beta-amyloid across the blood-brain barrier (BBB) into the brain from the systemic circulation, while the low-density lipoprotein receptor-related protein (LRP)-1 mediates transport of beta-amyloid out of the brain. To determine whether there are Alzheimer's disease (AD)-related changes in these BBB-associated beta-amyloid receptors, we studied RAGE, LRP-1, and beta-amyloid in human elderly control and AD hippocampi. In control hippocampi, there was robust RAGE immunoreactivity in neurons, whereas microvascular staining was barely detectable. LRP-1 staining, in contrast, was clearly evident within microvessels but only weakly stained neurons. In AD cases, neuronal RAGE immunoreactivity was significantly decreased. An unexpected finding was the strongly positive microvascular RAGE immunoreactivity. No evidence for colocalization of RAGE and beta-amyloid was seen within either microvessels or senile plaques. A reversed pattern was evident for LRP-1 in AD. There was very strong staining for LRP-1 in neurons, with minimal microvascular staining. Unlike RAGE, colocalization of LRP-1 and beta-amyloid was clearly present within senile plaques but not microvessels. Western blot analysis revealed a much higher concentration of RAGE protein in AD hippocampi as compared with controls. Concentration of LRP-1 was increased in AD hippocampi, likely secondary to its colocalization with senile plaques. These data confirm that AD is associated with changes in the relative distribution of RAGE and LRP-1 receptors in human hippocampus. They also suggest that the proportion of amyloid within the brains of AD patients that is derived from the systemic circulation may be significant.

Physiological and biophysical factors that influence Alzheimer's disease amyloid plaque targeting of native and putrescine modified human amyloid beta40

Amyloid beta40 (Abeta40) and its derivatives are being developed as probes for the ante-mortem diagnosis of Alzheimer's disease. Putrescine-Abeta40 (PUT-Abeta40) showed better plaque targeting than the native Abeta40, which was not solely explained by the differences in their blood-brain-barrier (BBB) permeabilities. The objective of this study was to elucidate the physiological and biophysical factors influencing the differential targeting of Abeta40 and PUT-Abeta40. Despite better plaque-targeting ability 125I-PUT-Abeta40 was more rapidly cleared from the systemic circulation than amyloid beta40 labeled with 125I (125I-Abeta40) after i.v. administration in mice. The BBB permeability of both compounds was inhibited by circulating peripheral Abeta40 levels. 125I-Abeta40 but not 125I-PUT-Abeta40 was actively taken up by the mouse brain slices in vitro. Only fluorescein-Abeta40, not fluorescein-PUT-Abeta40, was localized in the brain parenchymal cells in vitro. The metabolism of 125I-Abeta40 in the brain slices was twice as great as 125I-PUT-Abeta40. 125I-Abeta40 efflux from the brain slices was saturable and found to be 5 times greater than that of 125I-PUT-Abeta40. Thioflavin-T fibrillogenesis assay demonstrated that PUT-Abeta40 has a greater propensity to form insoluble fibrils compared with Abeta40, most likely due to the ability of PUT-Abeta40 to form beta sheet structure more readily than Abeta40. These results demonstrate that the inadequate plaque targeting of Abeta40 is due to cellular uptake, metabolism, and efflux from the brain parenchyma. Despite better plaque targeting of PUTAbeta40, its propensity to form fibrils may render it less suitable for human use and thus allow increased focus on the development of novel derivatives of Abeta with improved characteristics.

Brain insulin impairs amyloid-beta(1-40) clearance from the brain

Cerebral amyloid-beta peptide (Abeta) clearance plays a key role in determining the brain level of Abeta; however, its mechanism remains unclear. In this study, we investigated cerebral Abeta clearance across the blood-brain barrier (BBB) by using the Brain Efflux Index method. [125I]Abeta(1-40) was eliminated from rat brain to circulating blood with a half-life of 48.8 min and a half-saturation concentration of 8.15 nm. The Abeta(1-40) elimination rate was reduced by 30.5% in 23-month-old rats compared with 7-week-old rats. The intact form of Abeta(1-40) was detected in plasma after intracerebral administration, indicating the occurrence of efflux transport of intact Abeta(1-40). The Abeta(1-40) elimination rate was significantly inhibited by coadministration of 100 microg/ml insulin and 1 mm thiorphan by 44.6 and 34.0%, respectively. The level of intact [125I]Abeta(1-40) in the brain was increased by coadministration of insulin. Among insulin-degrading enzyme inhibitors, bacitracin inhibited the elimination rate, whereas N-ethylmaleimide and metal chelators had no effect. Receptor-associated protein, fucoidan, 3-bromo-5-t-butyl-4-hydroxy-benzylidenemalonitrile, anti-IGF-I receptor antibody, and l-tyrosine did not affect the Abeta(1-40) elimination rate, suggesting that the relevant receptors or transporters are not likely to be involved in the clearance. In conclusion, the present study has demonstrated the involvement of a proteolytic degradation process and an insulin-sensitive process in cerebral Abeta(1-40) clearance in the rat.

Pharmacokinetic Analysis of the Blood-Brain Barrier Transport of 125I-Amyloid β Protein 40 in Wild-Type and Alzheimer's Disease Transgenic Mice (APP,PS1) and Its Implications for Amyloid Plaque Formation

Amyloid plaques are formed in the extracellular space of Alzheimer's disease (AD) brain due to the accumulation of amyloid beta (Abeta) proteins such as Abeta40. The relationship between Abeta40 pharmacokinetics and its accumulation within and clearance from the brain in both wild-type (WT) and AD transgenic mice (APP,PS1) was studied to understand the mechanism of amyloid plaque formation and the potential use of Abeta40 as a probe to target and detect amyloid plaques. In both WT and APP,PS1 mice, the (125)I-Abeta40 tracer exhibited biexponential disposition in plasma with very short first and second phase half-lives. The (125)I-Abeta40 was significantly metabolized in the liver kidney > spleen. Coadministration of exogenous Abeta40 inhibited the plasma clearance and the uptake of (125)I-Abeta40 at the blood-brain barrier (BBB) in WT animals but did not affect its elimination from the brain. The (125)I-Abeta40 was shown to be metabolized within and effluxed from the brain parenchyma. The rate of efflux from APP,PS1 brain slices was substantially lower compared with WT brain slices. Since the Abeta40 receptor at the BBB can be easily saturated, the blood-to-brain transport of Abeta40 is less likely to be a primary contributor to the amyloid plaque formation in APP,PS1 mice. The decreased elimination of Abeta40 from the brain is most likely responsible for the amyloid plaque formation in the brain of APP,PS1 mice. Furthermore, inadequate targeting of Abeta40 to amyloid plaques, despite its high BBB permeability, is due to the saturability of Abeta40 transporter at the BBB and its metabolism and efflux from the brain.

β-Amyloid (Aβ) causes detachment of N1E-115 neuroblastoma cells by acting as a scaffold for cell-associated plasminogen activation

A major component of neuritic plaques in brain tissue of Alzheimer's disease patients is the beta-amyloid peptide (Abeta). Accumulation of Abeta has been associated with increased neuronal cell death and cognitive decline. We have previously shown that amyloid peptides like Abeta bind tissue-type plasminogen activator (tPA) and stimulate plasmin production. Here we investigated how Abeta regulates plasmin formation by N1E-115 neuroblastoma cells and the effects of Abeta-mediated plasmin formation on cell attachment and cell survival. We find that Abeta induces excessive cell-associated plasmin generation that causes cell detachment. Cell detachment is inhibited by carboxypeptidase B (CPB), an enzyme that blocks plasmin formation by cleaving off C-terminal lysine residues. Plasmin and CPB control Abeta-induced cell detachment independently of direct effects on cell viability. Abeta40 as well as oligomeric and fibrillar forms of Abeta42 stimulated tPA-mediated plasminogen activation and cell detachment. Our results suggest that plasmin-mediated cell detachment could contribute to the pathological effects of Abeta in diseased brain.

Pathogenic factors in vascular dementia and Alzheimer's disease. Multiple actions of heparin that probably are beneficial

The following areas are discussed in this review: atherogenesis; cerebrovascular factors; hypoperfusion; beta-amyloid production; beta-amyloid fibril formation; beta-sheets; metal cations; reactive oxygen species/free radicals; chronic inflammatory factors; endogenous plasma heparin; lipoprotein lipase; polyamines; protein kinase C; casein kinases; phospholipase A2; serine proteases; myeloperoxidase; cyclooxygenase 2; cysteine proteases; caspases; proprotein convertases; aspartic proteases; cyclin proteinases; thrombin; tau hyperphosphorylation; advanced glycosylation end products; activator protein 1; calcium; apolipoprotein E epsilon4; histamine; blood-brain barrier; glutamate; transglutaminase; insulin-like growth factor 1.

The protective role of DL-?-lipoic acid in the oxidative vulnerability triggered by A?-amyloid vaccination in mice

Recent reports indicate that beta-amyloid peptide (Abeta) vaccine based therapy for Alzheimer's disease (AD) may be on the horizon. There are however, concerns about the safety of this approach. Immunization with Abeta has several disadvantages, because it crosses the blood brain barrier and cause inflammation and neurotoxicity. The present work is aimed to study the protective effective of alpha-lipoic acid (LA) in the oxidative vulnerability of beta-amyloid in plasma, liver, spleen and brain, when Abeta fibrils are given intraperitoneally in inflammation induced mice. Result shows that reactive oxygen species (ROS) in the astrocytes of inflammation induced mice along with Abeta (IA) has shown 2.5-fold increase when compared with LA treated mice. The increased level of lipid peroxidase (LPO) (p < 0.05) and decreased antioxidant status (p < 0.05) were observed in the plasma, liver, spleen and brain of LA induced mice when compared with LA treated mice. Data shows that there were no significant changes observed between the control and LA treated mice. Our biochemical and histological results highlight that significant oxidative vulnerability was observed in IA treated mice, which was prevented by LA therapy. Our findings suggest that the antioxidant effect of LA when induced with Abeta may serve as a potent therapeutic tool for inflammatory AD models.

A Computational Positron Emission Tomography Simulation Model for Imaging ?-Amyloid in Mice

PURPOSE

We aimed to develop a computational simulation model for beta-amyloid (Abeta) positron emission tomography (PET) imaging.

PROCEDURES

Model parameters were set to reproduce levels of Abeta within the PDAPP mouse. Pharmacokinetic curves of virtual tracers were computed and a PET detector simulator was configured for a commercially available preclinical PET-imaging system.

RESULTS

We modeled the effects of Abeta therapy and tracer affinity on the ability to differentiate Abeta levels by PET. Varying affinity had a significant effect on the ability to quantitate Abeta. Further, PET tracers for Abeta monomers were more sensitive to the therapeutic reduction in Abeta levels than total brain amyloid. Following therapy, the decrease in total brain Abeta corresponded to the slow rate of change in total amyloid load as expected.

CONCLUSIONS

We have developed a first proof-of-concept Abeta-PET simulation model that will be a useful tool in the interpretation of preclinical Abeta imaging data and tracer development.

Amyloid β(1–42) and its β(25–35) fragment induce activation and membrane translocation of cytosolic phospholipase A2 in bovine retina capillary pericytes

We investigated changes in cytosolic phospholipase A(2) (cPLA(2)) and calcium-independent PLA(2) (iPLA(2)) activities in bovine retina capillary pericytes after stimulation with 50 microM amyloid-beta (Abeta) (1-42) and its (25-35) fragment, over 24 h (mild, sublethal model of cell damage). In the presence of Abeta peptides, we found that cPLA(2) activity was increased and translocated from the cytosolic fraction to the membrane system, particularly in the nuclear region. Reversed-sequence Abeta(35-25) peptide did not stimulate or induce cPLA(2) translocation. Exposure to both Abeta peptides had no significant effect on cPLA(2) protein content as tested by Western immunoblot analysis. The addition of Abetas to quiescent pericytes was followed by phosphorylation of cPLA(2) and arachidonic acid release. Treatment with inhibitors (AACOCF(3), staurosporine and cycloheximide) resulted in a sharp decrease in basal and stimulated cPLA(2) activity. Inactivating effects of bromoenol lactone (BEL), inhibitor of iPLA(2), demonstrated that the stimulation of total PLA(2) activity by Abetas was mediated by both PLA(2) enzymes. Taken together with our previous observations that both Abeta peptides may induce hydrolysis of phosphatidylcholine, the present results provide evidence that this process is cooperatively mediated by cPLA(2) activation/translocation and iPLA(2) activation. The effect is very likely triggered by a mild prooxidant mechanism which was not able to divert the cell to degeneration. The data confirm the hypothesis that pericytes could be a target of potential vascular damage and reactivity during processes involving amyloid accumulation.

Immunotherapy for Alzheimer's disease

The utility of vaccine strategies to treat neurodegenerative diseases such as Alzheimer's disease (AD) may still hold promise. Both active and passive immunization strategies reduced AD-like pathology and restored cognitive deficits in transgenic mice. These results were initially met with considerable optimism; however, phase IIa clinical trials were halted because of a small but significant occurrence of meningoencephalitis. Knowledge gained from studies on amyloid-beta peptide (A beta) immunotherapy will allow optimization of new-generation vaccines, targeting highly specific epitopes while reducing undesired side effects. In harnessing and steering the immune system, an effective response can be generated against A beta. If this proves successful, A beta vaccination could provide the first definitive treatment for AD.

A synthetic peptide blocking the apolipoprotein E/beta-amyloid binding mitigates beta-amyloid toxicity and fibril formation in vitro and reduces beta-amyloid plaques in transgenic mice

Alzheimer's disease (AD) is associated with accumulation of beta-amyloid (Abeta). A major genetic risk factor for sporadic AD is inheritance of the apolipoprotein (apo) E4 allele. ApoE can act as a pathological chaperone of Abeta, promoting its conformational transformation from soluble Abeta into toxic aggregates. We determined if blocking the apoE/Abeta interaction reduces Abeta load in transgenic (Tg) AD mice. The binding site of apoE on Abeta corresponds to residues 12 to 28. To block binding, we synthesized a peptide containing these residues, but substituted valine at position 18 to proline (Abeta12-28P). This changed the peptide's properties, making it non-fibrillogenic and non-toxic. Abeta12-28P competitively blocks binding of full-length Abeta to apoE (IC50 = 36.7 nmol). Furthermore, Abeta12-28P reduces Abeta fibrillogenesis in the presence of apoE, and Abeta/apoE toxicity in cell culture. Abeta12-28P is blood-brain barrier-permeable and in AD Tg mice inhibits Abeta deposition. Tg mice treated with Abeta12-28P for 1 month had a 63.3% reduction in Abeta load in the cortex (P = 0.0043) and a 59.5% (P = 0.0087) reduction in the hippocampus comparing to age-matched control Tg mice. Antibodies against Abeta were not detected in sera of treated mice; therefore the observed therapeutic effect of Abeta12-28P cannot be attributed to an antibody clearance response. Our experiments demonstrate that compounds blocking the interaction between Abeta and its pathological chaperones may be beneficial for treatment of beta-amyloid deposition in AD.

Design and chemical synthesis of a magnetic resonance contrast agent with enhanced in vitro binding, high blood-brain barrier permeability, and in vivo targeting to Alzheimer's disease amyloid plaques

Molecular imaging is an important new direction in medical diagnosis; however, its success is dependent upon molecular probes that demonstrate selective tissue targeting. We report the design and chemical synthesis of a derivative of human amyloid-beta (Abeta) peptide that is capable of selectively targeting individual amyloid plaques in the brain of Alzheimer's disease transgenic mice after being intravenously injected. This derivative is based on the sequence of the first 30 amino acid residues of Abeta with asparagyl/glutamyl-4-aminobutane residues (N-4ab/Q-4ab) substituted at unique Asp and Glu positions and with Gd-DTPA-aminohexanoic acid covalently attached at the N-terminal Asp. The Gd[N-4ab/Q-4ab]Abeta30 peptide was homogeneous as shown by high-resolution analytical techniques with a mass of +/-4385 Da determined by electrospray ionization mass spectrometry. This diamine- and gadolinium-substituted derivative of Abeta is shown to have enhanced in vitro binding to Alzheimer's disease (AD) amyloid plaques and increased in vivo permeability at the blood-brain barrier because of the unique Asp/Glu substitutions. In addition, specific in vivo targeting to AD amyloid plaques is demonstrated throughout the brain of an APP, PS1 transgenic mouse after intravenous injection. Because of the magnetic resonance (MR) imaging contrast enhancement provided by gadolinium, this derivative should enable the in vivo MR imaging of individual amyloid plaques in the brains of AD animals or patients to allow for early diagnosis and also provide a direct measure of the efficacy of anti-amyloid therapies currently being developed.

Systemic catabolism of Alzheimer's Abeta40 and Abeta42

To better understand the physiologic excretion and/or catabolism of circulating peripheral amyloid beta (Abeta), we labeled human Abeta40 (monomeric, with predominant unordered structure) and Abeta42 (mixture of monomers and oligomers in approximately 50:50 ratio, rich in beta-sheet conformation) with either Na(125)I or (125)I-tyramine cellobiose, also known as the cell-trapping ligand procedure, testing their blood clearance and organ uptake in B6SJLF1/J mice. Irrespective of the labeling protocol, the peptide conformation, and the degree of oligomerization, both Abeta40 and Abeta42 showed a short half-life of 2.5-3.0 min. The liver was the major organ responsible for plasma clearance, accounting for >60% of the peptide uptake, followed by the kidney. In vivo, hepatocytes captured >90% of the radiolabeled peptides which, after endocytosis, were preferentially catabolized and excreted into the bile. Biliary excretion of intact as well as partially degraded Abeta species became obviously relevant at doses above 10 microg. The use of biotin-labeled Abeta allowed the visualization of the interaction with HepG2 cells in culture, whereas competitive inhibition experiments with unlabeled Abeta demonstrated the specificity of the binding. The capability of the liver to uptake, catabolize, and excrete large doses of Abeta, several orders of magnitude above its physiologic concentration, may explain not only the femtomolar plasma levels of Abeta but the little fluctuation observed with age and disease stages.

Studies of Aβ Pharmacodynamics in the Brain, Cerebrospinal Fluid, and Plasma in Young (Plaque-Free) Tg2576 Mice Using the γ-Secretase Inhibitor N2-[(2S)-2-(3,5-Difluorophenyl)-2-hydroxyethanoyl]-N1-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-L-alaninamide (LY-411575)

A previous study by us suggests the utility of cerebrospinal fluid (CSF) and plasma Abeta as biomarkers of beta- or gamma-secretase inhibition. The present study characterized further Abeta pharmacodynamics in these tissues from Tg2576 mice and examined their correlation with brain Abeta after acute treatment with a potent gamma-secretase inhibitor, N(2)-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N(1)-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-l-alaninamide (LY-411575). A single dose of LY-411575 dose-dependently (0.1-10 mg/kg p.o.) reduced Abeta(1-40) and Abeta(1-42) in the CSF and the brain. In contrast, plasma Abeta levels were increased by 0.1 mg/kg LY-411575 and were followed by a dose-dependent reduction at higher doses. The time courses of Abeta reduction and recovery were distinct for the three tissues: maximal declines in Abeta levels were evident by 3 h in the CSF and plasma but not until 9 h in the brain. A recovery in Abeta levels was underway in the CSF by 9 h and nearly completed by 24 h in all tissues. The differential time courses in the three compartments do not seem to be due to pharmacokinetic factors. Five days of twice-daily treatment with LY-411575 not only sustained the Abeta reductions in all tissues but also significantly augmented the efficacy in the brain and plasma. The increased efficacy occurred in the absence of compound accumulation and was consistent with the recovery rates in each compartment. Overall, Abeta in the CSF and not plasma seems to be a better biomarker of brain Abeta reduction; however, the time course of Abeta changes needs to be established in clinical studies.

Anti-A?: The good, the bad, and the unforeseen

Alzheimer's disease (AD) is characterized in part by the deposition of amyloid beta protein (Abeta) in compact fibrillar plaques. These structures can induce an innate immune response in the brain, which triggers progressive inflammation, neuronal loss, and further acceleration of Abeta plaque formation. Compared with the case in normal individuals, the T and B lymphocytes in AD patients and murine models are hyporesponsive to Abeta. However, depending on the route of delivery, tolerance can be overcome by vaccination, with the induction of an anti-Abeta-mediated immune response. Through mechanisms that are incompletely understood, immunized APP transgenic animals show markedly reduced Abeta deposition, preservation of normal neuronal architecture, and improved performance in memory and spatial learning tasks. In human trials, Abeta vaccination stabilized cognition and slowed the progression of dementia. Neuropathologic examination of a vaccinated subject showed reduced cortical Abeta without changes in other AD-associated pathology. However, in some patients, vaccination induced severe meningoencephalitis, causing the trial to be terminated. Thus, vaccination appears to activate both beneficial and deleterious anti-Abeta immunity, suggesting that the vaccine can have potent clinical utility if an appropriate immunologic response can be generated.

Is beta-amyloid fibrillogenesis a strict process of deposition inherently interactive in molecular terms?

Amyloid fibrillogenesis as a process of interactive molecular processes of deposition in Alzheimer's disease might function as a phenomenon that transforms intracellular amyloid segregation to a state of equilibration with extracellular deposition. beta-Amyloidosis might dynamically implicate loss of viability of vascular tunica media myofibers as a strict reflection of loss of viability of neurons in such an overall system of equilibration between intracellular and extracellular amyloid fibrillogenesis. In terms beyond simple concepts of strict biophysical equilibration, deposition of beta-amyloid in Alzheimer's disease might constitute a phenomenon of congophilic angiopathy as a strict pathobiologic index of activity of the Alzheimer process; such a correlate would perhaps involve a quantitative index that would qualitatively characterize the Alzheimer process as an interactive series of reactions between the intracellular and extracellular microenvironment.

Detection of Alzheimer's amyloid in transgenic mice using magnetic resonance microimaging

The presence of amyloid-beta (Abeta) plaques in the brain is a hallmark pathological feature of Alzheimer's disease (AD). Transgenic mice overexpressing mutant amyloid precursor protein (APP), or both mutant APP and presenilin-1 (APP/PS1), develop Abeta plaques similar to those in AD patients, and have been proposed as animal models in which to test experimental therapeutic approaches for the clearance of Abeta. However, at present there is no in vivo whole-brain imaging method to detect Abeta plaques in mice or men. A novel method is presented to detect Abeta plaques in the brains of transgenic mice by magnetic resonance microimaging (muMRI). This method uses Abeta1-40 peptide, known for its high binding affinity to Abeta, magnetically labeled with either gadolinium (Gd) or monocrystalline iron oxide nanoparticles (MION). Intraarterial injection of magnetically labeled Abeta1-40, with mannitol to transiently open the blood-brain barrier (BBB), enabled the detection of many Abeta plaques. Furthermore, the numerical density of Abeta plaques detected by muMRI and by immunohistochemistry showed excellent correlation. This approach provides an in vivo method to detect Abeta in AD transgenic mice, and suggests that diagnostic MRI methods to detect Abeta in AD patients may ultimately be feasible.

RAGE mediates amyloid-beta peptide transport across the blood-brain barrier and accumulation in brain

Amyloid-beta peptide (Abeta) interacts with the vasculature to influence Abeta levels in the brain and cerebral blood flow, providing a means of amplifying the Abeta-induced cellular stress underlying neuronal dysfunction and dementia. Systemic Abeta infusion and studies in genetically manipulated mice show that Abeta interaction with receptor for advanced glycation end products (RAGE)-bearing cells in the vessel wall results in transport of Abeta across the blood-brain barrier (BBB) and expression of proinflammatory cytokines and endothelin-1 (ET-1), the latter mediating Abeta-induced vasoconstriction. Inhibition of RAGE-ligand interaction suppresses accumulation of Abeta in brain parenchyma in a mouse transgenic model. These findings suggest that vascular RAGE is a target for inhibiting pathogenic consequences of Abeta-vascular interactions, including development of cerebral amyloidosis.

Immunotherapy for Alzheimer's disease

Recent studies in murine models of Alzheimer's disease (AD) have found that active immunisation with amyloid-beta peptide (Abeta) or passive immunisation with Abeta antibodies can lessen the severity of Abeta-induced neuritic plaque pathology through the activation of microglia. These antibodies can be detected in the serum and CSF. Whether they slow down or speed up the development and progression of AD has not been determined. Furthermore, the conditions that induce formation of such antibodies are unknown, or how specific they are to AD. However, the evidence suggests at least a potential beneficial role for some features of neuroinflammation in AD. A clinical phase II study of an active immunisation approach with AN1792 was started in 2001, but was recently suspended after some patients developed serious adverse events. These were most likely caused by the activation of the proinflammatory cascade. Immunotherapy approaches represent fascinating ways to test the amyloid hypothesis and may offer genuine opportunities to modify disease progression. This review focuses on immunisation strategies and details of the pathways involved in antibody clearance of Abeta.

Abeta immunization and anti-Abeta antibodies: potential therapies for the prevention and treatment of Alzheimer's disease

Amyloid-beta (Abeta) is a normally soluble 39-43 amino peptide. Genetic and biochemical data strongly suggest that the conversion of Abeta from soluble to insoluble forms with high beta-sheet content and its buildup in the brain is a key step in the pathogenesis of Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). Prevention and/or reversal of this process may serve as a treatment. Methods to prevent or reverse Abeta deposition and its toxic effects would include decreasing its production, preventing its conversion to insoluble forms (e.g. inhibit beta-sheet formation) or in changing the dynamics of extracellular brain Abeta, either locally within the brain or by altering net flux of Abeta between the central nervous system (CNS) and plasma compartment. Transgenic mouse models of AD that develop age-dependent Abeta deposition, damage to the neuropil, and behavioral deficits have enabled researchers to test whether different manipulations can influence these AD-like changes. Recently, active immunization with different forms of the Abeta peptide has been shown to decrease brain Abeta deposition and improve cognitive performance in mouse models of AD. Certain peripherally administered anti-Abeta antibodies have similar effects. The mechanism(s) by which anti-Abeta antibodies result in these effects is just beginning to be elucidated. Abeta-related immune therapies in humans are an exciting new area of AD research. Understanding their detailed mechanism(s) of action and their potential usefulness awaits the results of future animal and human studies.

Vascular disorder in Alzheimer's disease: role in pathogenesis of dementia and therapeutic targets

It is not clear whether Alzheimer's Disease (AD) is primarily a neurodegenerative disorder or not. A body of evidence suggests that vascular disorder in brains of individuals with AD contributes to the extremes of this disease. This raises a question whether Alzheimer's dementia is secondary to vascular dysfunction in the central nervous system (CNS) and, therefore, the neurodegeneration that follows is a consequence of inadequate cerebral blood flow, altered brain metabolism and failure in physiological functions of brain endothelium which represents a site at the blood-brain barrier (BBB). In this paper the evidence for a primary role of the CNS vascular system in pathogenesis of Alzheimer's dementia is reviewed to show how alterations in transport across the BBB contribute to development of cerebral beta-amyloidosis in AD. In addition, vascularly-based therapeutic strategies to limit the development of beta-amyloidosis and to remove amyloid and plaques from the CNS of AD individuals are discussed.

A safer vaccine for Alzheimer's disease?

Recent reports indicate that amyloid-beta (Abeta) vaccine-based therapy for Alzheimer's disease (AD) may be on the horizon. There are, however, concerns about the safety of this approach. Immunization with Abeta1-42 may not be appropriate in humans because it crosses the blood-brain barrier, can seed fibril formation, and is highly fibrillogenic. Abeta1-42 fibrils can in turn cause inflammation and neurotoxicity. This issue is of a particular concern in the elderly who often do not mount an adequate immune response to vaccines. Our findings show that vaccination with nonamyloidogenic/nontoxic Abeta derivative may be a safer therapeutic approach to impede the progression of Abeta-related histopathology in AD. Although the site of action of the anti-Abeta antibodies has been suggested to be within the brain, peripheral clearance of Abeta may have a greater role in reducing cerebral amyloid plaques in these animals and eventually in AD patients. Antibodies in general are predominantly found outside the central nervous system (CNS) and will, therefore, primarily clear systemic Abeta compared to brain Abeta. This disruption of the equilibrium between central and peripheral Abeta should then result in efflux of Abeta out of the brain, and subsequent removal of plaques. Abeta therapy can be targeted to the periphery, which may result in fewer CNS side effects, such as inflammation. Future Abeta derived vaccines should include T(h) epitopes, carriers and/or lipid moieties to enhance antibody production in the elderly, the population predominantly affected by AD.

Immunization treatment approaches in Alzheimer's and prion diseases

There is growing realization that many neurodegenerative conditions have the same underlying pathogenetic mechanism: a change in protein conformation, where the beta-sheet content is increased. In Alzheimer's disease (AD), amyloid deposition in the form of neuritic plaques and congophilic angiopathy is driven by the conversion of normal soluble amyloid beta (sAbeta) to Abeta plaques, whereas in the prionoses the critical event is the conversion of normal prion protein, PrP(C), to PrP(Sc). This common theme in the pathogenesis of these disorders and the extracellular localization of the accumulating abnormal protein make them highly amenable to therapeutic approaches based on experimental manipulation of protein conformation and clearance. Different approaches under development include drugs that affect the processing of the precursor proteins, enhance clearance of the amyloidogenic protein, and inhibit or prevent the conformation change. Particularly interesting are recent studies of immune system activation, which appear to increase the clearance of the disease-associated protein. These immunologically based approaches are highly effective in animal models of these disorders, and in these model systems are associated with no obvious side effects. In transgenic mice with AD-related pathology, immunization has also been shown to prevent age-related cognitive impairment. However, the first clinical trial of this approach in AD patients was associated with unacceptable toxicity. These immune-based treatment approaches have great potential as rational therapies for this devastating group of disorders, but additional development is needed before they can be safely applied to humans.

Plaque-associated disruption of CSF and plasma amyloid-beta (Abeta) equilibrium in a mouse model of Alzheimer's disease

To better understand amyloid-beta (Abeta) metabolism in vivo, we assessed the concentration of Abeta in the CSF and plasma of APP(V717F) (PDAPP) transgenic mice, a model that develops age-dependent Alzheimer's disease (AD)-like pathology. In 3-month-old mice, prior to the development of Abeta deposition in the brain, there was a highly significant correlation between Abeta levels in CSF and plasma. In 9-month-old-mice, an age at which some but not all mice have developed Abeta deposition, there was also a significant correlation between CSF and plasma Abeta; however, the correlation was not as strong as that present in young mice. In further exploring CSF and plasma Abeta levels in 9-month-old mice, levels of CSF Abeta were found to correlate highly with Abeta burden. Analysis of the CSF: plasma Abeta ratio revealed a selective two-fold increase in plaque versus non-plaque bearing mice, strongly suggesting a plaque-mediated sequestration of soluble Abeta in brain. Interestingly, in 9-month-old mice, a significant correlation between CNS and plasma Abeta was limited to mice lacking Abeta deposition. These findings suggest that there is a dynamic equilibrium between CNS and plasma Abeta, and that plaques create a new equilibrium because soluble CNS Abeta not only enters the plasma but also deposits onto amyloid plaques in the CNS.

Transient appearance of amyloid precursor protein plaques in the brain of thymectomized rats after human leptomeningeal cell grafts

Cells cultured from Alzheimer disease leptomeninges or skin were grafted into the cortex of adult thymectomized rats. At 3 days post-implant, plaque-like aggregates were found in the cortex, corpus callosum, septum and caudate nucleus. These structures were immunopositive for human amyloid precursor protein (APP), human amyloid beta peptide (Abeta), cathepsin D, apolipoprotein E and ubiquitin. Aberrant tau+ neurites, reactive astrocytes and microglia were associated with many aggregates. Although birefringent amyloid occupied the central area of most aggregates, these structures had disappeared by l month post-implant. Abeta and APP produced by grafted non-neural human cells can penetrate rat brain and form plaque-like structures, which can be effectively cleared by the rat.