Interactions of beta-amyloids with the blood-brain barrier

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.

Peptides and aging: Their role in anorexia and memory

The rapid aging of the world's population has led to a need to increase our understanding of the pathophysiology of the factors leading to frailty and cognitive decline. Peptides have been shown to be involved in the pathophysiology of frailty and cognitive decline. Weight loss is a major component of frailty. In this review, we demonstrate a central role for both peripheral peptides (e.g., cholecystokinin and ghrelin) and neuropeptides (e.g., dynorphin and alpha-MSH) in the pathophysiology of the anorexia of aging. Similarly, peripheral peptides (e.g., ghrelin, glucagon-like peptide 1, and cholecystokinin) are modulators of memory. A number of centrally acting neuropeptides have also been shown to modulate cognitive processes. Amyloid-beta peptide in physiological levels is a memory enhancer, while in high (pathological) levels, it plays a key role in the development of Alzheimer's disease.

Aβ₁₋₄₂-RAGE interaction disrupts tight junctions of the blood-brain barrier via Ca²⁺-calcineurin signaling

The blood-brain barrier (BBB), which is formed by adherens and tight junctions (TJs) of endothelial cells, maintains homeostasis of the brain. Disrupted intracellular Ca²⁺ homeostasis and breakdown of the BBB have been implicated in the pathogenesis of Alzheimer's disease (AD). The receptor for advanced glycation end products (RAGE) is known to interact with amyloid β-peptide (Aβ) and mediate Aβ transport across the BBB, contributing to the deposition of Aβ in the brain. However, molecular mechanisms underlying Aβ-RAGE interaction-induced alterations in the BBB have not been identified. We found that Aβ₁₋₄₂ induces enhanced permeability, disruption of zonula occludin-1 (ZO-1) expression in the plasma membrane, and increased intracellular calcium and matrix metalloproteinase (MMP) secretion in cultured endothelial cells. Neutralizing antibodies against RAGE and inhibitors of calcineurin and MMPs prevented Aβ₁₋₄₂-induced changes in ZO-1, suggesting that Aβ-RAGE interactions alter TJ proteins through the Ca²⁺-calcineurin pathway. Consistent with these in vitro findings, we found disrupted microvessels near Aβ plaque-deposited areas, elevated RAGE expression, and enhanced MMP secretion in microvessels of the brains of 5XFAD mice, an animal model for AD. We have identified a potential molecular pathway underlying Aβ-RAGE interaction-induced breakage of BBB integrity. This pathway might play an important role in the pathogenesis of AD.

The profile of β-amyloid precursor protein expression of rats induced by aluminum

The environmental agent aluminum has been extensively investigated for a potential relationship with amyloid precursor protein (APP) expression. Despite many investigations, there is at present no definite proof from which to draw a conclusion. Since APP is an integral membrane protein expressed in different tissues and capable of fluxes across the blood-brain barrier (BBB), which may ultimately affect APP level in brain, it is necessary to assess the expression profile among vital body organs. The present study compared aluminum oxide and aluminum chloride injected rats with control rats (saline treated) to observe if aluminum affected APP expression patterns in different organs by immunohistochemistry (IHC). The expression of APP was observed in the brain of aluminum chloride treated rats and in the liver of aluminum oxide injected group. Results of double IHC staining showed that it is Kupffer cells, which are located in liver sinus and expressed APP after aluminum oxide treatment. Oxidative stress is suggested as the potential pathway that aluminum chloride exert effects in brain. These results suggest that different aluminum compounds may impact the expression of APP in brain and liver tissues. The mechanism that aluminum induced liver APP expression still needs further investigation.

Sink Hypothesis and Therapeutic Strategies for Attenuating Aβ Levels

Amyloid β (Aβ) plaque, comprised mainly by Aβ peptides, is an important pathology of Alzheimer’s brains. Major efforts have been devoted to targeting this neurotoxic Aβ peptide for discovering disease-modifying treatments for Alzheimer’s disease. Inasmuch as Aβ is found in both the brain and the periphery, it is hypothesized that there is some form of equilibrium for the Aβ in the brain and the periphery such that Aβ can be transported across the blood-brain barrier. By modulating the periphery Aβ levels, it is predicted that the brain Aβ levels will undergo concomitant changes, forming the basis of the “sink hypothesis” for Aβ lowering strategies. In this review, the significance and implication of this sink hypothesis as well as how the sink hypothesis may contribute to the recent Aβ-based drug discovery in AD are discussed. Ultimately, the validity of the sink hypothesis will be resolved when the appropriate Aβ agents are being tested in the clinic.

Oxidative modification to LDL receptor-related protein 1 in hippocampus from subjects with Alzheimer disease: implications for Aβ accumulation in AD brain

Alzheimer disease (AD) is a neurodegenerative disorder characterized histopathologically by the presence of senile plaques (SPs), neurofibrillary tangles, and synapse loss. The main component of SPs is amyloid-β peptide (Aβ), which has been associated with increased oxidative stress, leading to oxidative modification of proteins and consequently to neurotoxicity and neurodegeneration. Low-density lipoprotein receptor-related protein 1 (LRP1) is the primary moiety responsible for the efflux of Aβ from the brain to the blood across the blood-brain barrier. Impaired brain-to-blood transport of Aβ by LRP1 has been hypothesized to contribute to increased levels of Aβ in AD brain. The cause of LRP1 dysfunction is unknown, but we have hypothesized that Aβ oxidizes LRP1, thus damaging its own transporter. Consistent with this notion, we report in this study a significant increase in the levels of the lipid peroxidation product 4-hydroxy-2-nonenal bound to transmembrane LRP1 in AD hippocampus. In contrast, the levels of LRP1-resident 3-nitrotyrosine did not show a significant increase in AD hippocampus compared to age-matched controls. Based on this study, we propose that Aβ impairs its own efflux from the brain by oxidation of its transporter LRP1, leading to increased Aβ deposition in brain, thereby contributing to subsequent cognitive impairment in AD.

Aluminum complexing enhances amyloid β protein penetration of blood–brain barrier

A significant co-morbidity of Alzheimer's disease and cerebrovascular impairment suggests that cerebrovascular dysregulation is an important feature of dementia. Amyloid beta protein (Abeta), a relevant risk factor in Alzheimer's disease, has neurotoxic properties and is thought to play a critical role in the cognitive impairments. Previously, we demonstrated that the 42mer of Abeta (Abeta42) complexed with aluminum (Al-Abeta42) is much more cytotoxic than non-complexed Abeta42. The level of Abeta in the brain is a balance between synthesis, degradation, and fluxes across the blood-brain barrier (BBB). In the present paper, we determined whether complexing with aluminum affected the ability of radioactively iodinated Abeta to cross the in vivo BBB. We found that the rates of uptake of Al-Abeta42 and Abeta42 were similar, but that Al-Abeta42 was sequestered by brain endothelial cells much less than Abeta42 and so more readily entered the parenchymal space of the brain. Al-Abeta42 also had a longer half-life in blood and had increased permeation at the striatum and thalamus. Brain-to-blood transport was similar for Al-Abeta42 and Abeta42. In conclusion, complexing with aluminum affects some aspects of blood-to-brain permeability so that Al-Abeta42 would have more ready access to brain cells than Abeta42.

Efflux of human and mouse amyloid beta proteins 1-40 and 1-42 from brain: impairment in a mouse model of Alzheimer's disease

Brain to blood transport is believed to be a major determinant of the amount of amyloid beta protein (AbetaP) found in brain. Impaired efflux has been suggested as a mechanism by which AbetaP can accumulate in the CNS and so lead to Alzheimer's disease (AD). To date, however, no study of the efflux of the form of AbetaP most relevant to AD, AbetaP1-42, has been conducted, even though a single amino acid substitution in AbetaP can greatly alter efflux. Here, we examined the efflux of AbetaP mouse1-42, mouse1-40, human1-42, and human1-40 in young CD-1, young senesence accelerated mouse (SAM) P8, and aged SAMP8 mice. The SAMP8 mouse with aging spontaneously overproduces AbetaP and develops cognitive impairments reversed by AbetaP-directed antibody or phosphorothioate antisense oligonucleotide. CD-1 mice transported all forms of AbetaP, although mouse1-42 and human1-40 were transported faster than the other forms. There was a decrease in the saturable transport of mouse1-42 in SAMP8 mice regardless of age. Efflux of mouse1-40 and human1-42 was only by a non-saturable mechanism in young SAMP8 mice and their efflux was totally absent in aged SAMP8 mice. These differences in the efflux of the various forms of AbetaP among the three groups of mice supports the hypothesis that impaired efflux is an important factor in the accumulation of AbetaP in the CNS.

Neuroprotective approaches in experimental models of beta-amyloid neurotoxicity: relevance to Alzheimer's disease

1. beta-Amyloid peptides (A beta s) accumulate abundantly in the Alzheimer's disease (AD) brain in areas subserving information acquisition and processing, and memory formation. A beta fragments are produced in a process of abnormal proteolytic cleavage of their precursor, the amyloid precursor protein (APP). While conflicting data exist in the literature on the roles of A beta s in the brain, and particularly in AD, recent studies have provided firm experimental evidence for the direct neurotoxic properties of A beta. 2. Sequence analysis of A beta s revealed a high degree of evolutionary conservation and inter-species homology of the A beta amino acid sequence. In contrast, synthetic A beta fragments, even if modified fluorescent or isotope-labeled derivatives, are pharmacological candidates for in vitro and in vivo modeling of their cellular actions. During the past decade, acute injection, prolonged mini-osmotic brain perfusion approaches or A beta infusions into the blood circulation were developed in order to investigate the effects of synthetic A beta s, whereas transgenic models provided insight into the distinct molecular steps of pathological APP cleavage. 3. The hippocampus, caudate putamen, amygdala and neocortex all formed primary targets of acute neurotoxicity screening, but functional consequences of A beta infusions were primarily demonstrated following either intracerebroventricular or basal forebrain (medial septum or magnocellular basal nucleus (MBN)) infusions of A beta fragments. 4. In vivo investigations confirmed that, while the active core of A beta is located within the beta(25-35) sequence, the flanking peptide regions influence not only the folding properties of the A beta fragments, but also their in vivo neurotoxic potentials. 5. It has recently been established that A beta administration deranges neuron-glia signaling, affects the glial glutamate uptake and thereby induces noxious glutamatergic stimulation of nerve cells. In fact, a critical role for N-methyl-D-aspartate (NMDA) receptors was postulated in the neurotoxic processes. Additionally, A beta s might become internalized, either after their selective binding to cell-surface receptors or after membrane association in consequence of their highly lipophilic nature, and induce free radical generation and subsequent oxidative injury. Ca(2+)-mediated neurotoxic events and generation of oxygen free radicals may indeed potentiate each other, or even converge to the same neurotoxic events, leading to cell death. 6. Neuroprotection against A beta toxicity was achieved by both pre- and post-treatment with NMDA receptor channel antagonists. Moreover, direct radical-scavengers, such as vitamin E or vitamin C, attenuated A beta toxicity with high efficacy. Interestingly, combined drug treatments did not necessarily result in additive enhanced neuroprotection. 7. Similarly to the blockade of NMDA receptors, the neurotoxic action of A beta s could be markedly decreased by pharmacological manipulation of voltage-dependent Ca(2+)-channels, serotonergic IA or adenosine A1 receptors, and by drugs eliciting membrane hyperpolarization or indirect blockade of Ca(2+)-mediated intracellular consequences of intracerebral A beta infusions. 8. A beta neurotoxicity might be dose-dependently modulated by trace metals. In spite of the fact that zinc (Zn) may act as a potent inhibitor of the NMDA receptor channel, high Zn doses accelerate A beta fibril formation, stabilize the beta-sheet conformation and thereby potentiate A beta neurotoxicity. Combined trace element supplementation with Se, Mn, or Mg, which prevails over the expression of detoxifying enzymes or counteracts intracellular elevations of Ca2+, may reduce the neurotoxic impact of A beta s. 9. Alterations in the regulatory functions of the hypothalamo-pituitary-adrenal axis may contribute significantly to neurodegenerative changes in the brain. Furthermore, AD patients exhibit substantially increased circadia