Soluble aggregates of the amyloid-beta protein selectively stimulate permeability in human brain microvascular endothelial monolayers

Human in vitro blood barrier models: architectures and applications

Blood barriers serve as key points of transport for essential molecules as well as lines of defense to protect against toxins. In vitro modeling of these barriers is common practice in the study of their physiology and related diseases. This review describes a common method of using an adaptable, low cost, semipermeable, suspended membrane to experimentally model three blood barriers in the human body: the blood-brain barrier (BBB), the gut-blood barrier (GBB), and the air-blood barrier (ABB). The GBB and ABB both protect from the outside environment, while the BBB protects the central nervous system from potential neurotoxic agents in the blood. These barriers share several commonalities, including the formation of tight junctions, polarized cellular monolayers, and circulatory system contact. Cell architectures used to mimic barrier anatomy as well as applications to study function, dysfunction, and response provide an overview of the versatility enabled by these cultural systems.

Patterning amyloid-β aggregation under the effect of acetylcholinesterase using a biological nanopore - an in vitro study

Aggregation of amyloid-β peptide (Aβ) is hypothesized to be the primary cause of Alzheimer's disease (AD) progression. Aβ aggregation has been widely studied using conventional sensing tools like emission fluorescence, electron microscopy, mass spectroscopy, and circular dichroism. However, none of these techniques can provide cost-efficient, highly sensitive quantification of Aβ aggregation kinetics at the molecular level. Among the influences on Aβ aggregation of interest to disease progression is the acceleration of Aβ aggregation by acetylcholinesterase (AChE), which is present in the brain and inflicts the fast progression of disease due to its direct interaction with Aβ. In this work, we demonstrate the ability of a biological nanopore to map and quantify AChE accelerated aggregation of Aβ monomers to mixed oligomers and small soluble aggregates with single-molecule precision. This method will allow future work on testing direct and indirect effects of therapeutic drugs on AChE accelerated Aβ aggregation as well as disease prognosis.

Microglia and the Blood-Brain Barrier: An External Player in Acute and Chronic Neuroinflammatory Conditions

Microglia are the resident immune cells of the central nervous system that guarantee immune surveillance and exert also a modulating role on neuronal synaptic development and function. Upon injury, microglia get activated and modify their morphology acquiring an ameboid phenotype and pro- or anti-inflammatory features. The active role of microglia in blood-brain barrier (BBB) function and their interaction with different cellular components of the BBB-endothelial cells, astrocytes and pericytes-are described. Here, we report the specific crosstalk of microglia with all the BBB cell types focusing in particular on the involvement of microglia in the modulation of BBB function in neuroinflammatory conditions that occur in conjunction with an acute event, such as a stroke, or in a slow neurodegenerative disease, such as Alzheimer's disease. The potential of microglia to exert a dual role, either protective or detrimental, depending on disease stages and environmental conditioning factors is also discussed.

Oleuropein-Rich Olive Leaf Extract Attenuates Neuroinflammation in the Alzheimer's Disease Mouse Model

Alzheimer's disease (AD) is the most common form of dementia among several neurodegenerative disorders afflicting the elderly. AD is characterized by the deposition of extracellular amyloid-β (Aβ) plaques, disrupted blood-brain barrier (BBB), and neuroinflammation. Several studies have demonstrated the health benefits of olive oil and olive leaf extract (OLE) due to their polyphenolic content. The main phenolic compound in OLE is glycosylated oleuropein (OLG), while the aglycon form of oleuropein (OLA) exists in much lower amounts. This work aimed to evaluate the effect of a low dose of OLG-rich OLE and the mechanism(s) that contributed to the observed beneficial effects against Aβ pathology in the homozygous 5xFAD mouse model. Mice were fed with OLE-enriched diet (695 μg/kg body weight/day) for 3 months, starting at 3 months old. Overall findings demonstrated that OLE reduced neuroinflammation by inhibiting the NF-κB pathway and suppressing the activation of NLRP3 inflammasomes and RAGE/HMGB1 pathways. In addition, OLE reduced total Aβ brain levels due to increased clearance and reduced production of Aβ and enhanced BBB integrity and function, which collectively improved the memory function. Thus, the consumption of OLE as a dietary supplement is expected to stop and/or slow the progression of AD.

Blood-Brain Barrier Breakdown: An Emerging Biomarker of Cognitive Impairment in Normal Aging and Dementia

The blood–brain barrier (BBB) plays a vital role in maintaining the specialized microenvironment of the neural tissue. It separates the peripheral circulatory system from the brain parenchyma while facilitating communication. Alterations in the distinct physiological properties of the BBB lead to BBB breakdown associated with normal aging and various neurodegenerative diseases. In this review, we first briefly discuss the aging process, then review the phenotypes and mechanisms of BBB breakdown associated with normal aging that further cause neurodegeneration and cognitive impairments. We also summarize dementia such as Alzheimer's disease (AD) and vascular dementia (VaD) and subsequently discuss the phenotypes and mechanisms of BBB disruption in dementia correlated with cognition decline. Overlaps between AD and VaD are also discussed. Techniques that could identify biomarkers associated with BBB breakdown are briefly summarized. Finally, we concluded that BBB breakdown could be used as an emerging biomarker to assist to diagnose cognitive impairment associated with normal aging and dementia.

In Vitro Biosensing of β-Amyloid Peptide Aggregation Dynamics using a Biological Nanopore

Alzheimer's disease and other neurodegenerative disorders are becoming more prevalent as advances in technology and medicine increase living standards and life expectancy. Alzheimer's disease is thought to initiate development early in the patient's life and progresses continuously into old age. This process is characterized molecularly by the amyloid hypothesis, which asserts that self-aggregating amyloid peptides are core to the pathophysiology in Alzheimer's progression. Precise quantification of amyloid peptides in human bodily fluid samples (i.e. cerebrospinal fluid, blood) may inform diagnosis and prognosis, and has been studied using established biosensing technologies like liquid chromatography, mass spectrometry, and immunoassays. However, existing methods are challenged to provide single molecule, quantitative analysis of the disease-causing aggregation process. Ultra-sensitive nanopore biosensors can step in to fill this role as a dynamic mapping tool. The work in this paper establishes characteristic signals of β-amyloid 40 monomers, oligomers, and soluble aggregates, as well as a proof-of-concept foundation where a biological nanopore biosensor is used to monitor the extent of in vitro β-amyloid 40 peptide aggregation at the single molecule level. This foundation allows for future work to expand in drug screening, diagnostics, and aggregation dynamic experiments.

Effects of Alzheimer's Disease-Related Proteins on the Chirality of Brain Endothelial Cells

Introduction

Cell chirality is an intrinsic cellular property that determines the directionality of cellular polarization along the left-right axis. We recently show that endothelial cell chirality can influence intercellular junction formation and alter trans-endothelial permeability, depending on the uniformity of the chirality of adjacent cells, which suggests a potential role for cell chirality in neurodegenerative diseases with blood-brain barrier (BBB) dysfunctions, such as Alzheimer's disease (AD). In this study, we determined the effects of AD-related proteins amyloid-β (Aβ), tau, and apolipoprotein E4 (ApoE4) on the chiral bias of the endothelial cell component in BBB.

Methods

We first examined the chiral bias and effects of protein kinase C (PKC)-mediated chiral alterations of human brain microvascular endothelial cells (hBMECs) using the ring micropattern chirality assay. We then investigated the effects of Aβ, tau, and ApoE4 on hBMEC chirality using chirality assay and biased organelle positions.

Results

The hBMECs have a strong clockwise chiral bias, which can be reversed by protein kinase C (PKC) activation. Treatment with tau significantly disrupted the chiral bias of hBMECs with altered cellular polarization. In contrast, neither ApoE4 nor Aβ-42 caused significant changes in cell chirality.

Conclusions

We conclude that tau might cause BBB dysfunction by disrupting cell polarization and chiral morphogenesis, while the effects of ApoE4 and Aβ-42 on BBB integrity might be chirality-independent. The potential involvement of chiral morphogenesis in tau-mediated BBB dysfunction in AD provides a novel perspective in vascular dysfunction in tauopathies such as AD, chronic traumatic encephalopathy, progressive supranuclear palsy, and frontotemporal dementia.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00669-w.

DPepH3, an Improved Peptide Shuttle for Receptor-independent Transport Across the Blood-Brain Barrier

Background:

The use of peptides as drug carriers across the blood-brain barrier (BBB) has increased significantly during the last decades. PepH3, a seven residue sequence (AGILKRW) derived from the α-helical domain of the dengue virus type-2 capsid protein, translocates across the BBB with very low toxicity. Somehow predictably from its size and sequence, PepH3 is degraded in serum relatively fast. Among strategies to increase peptide half-life (t1/2), the use of the enantiomer (wholly made of D-amino acid residues) can be quite successful if the peptide interacts with a target in non-stereospecific fashion.

Methods:

The goal of this work was the development of a more proteolytic-resistant peptide, while keeping the translocation properties. The serum stability, cytotoxicity, in vitro BBB translocation, and internalization mechanism of DPepH3 was assessed and compared to the native peptide.

Results:

DPepH3 demonstrates a much longer t1/2 compared to PepH3. We also confirm that BBB translocation is receptor-independent, which fully validates the enantiomer strategy chosen. In fact, we demonstrate that internalization occurs trough macropinocytosis. In addition, the enantiomer demonstrates to be non-cytotoxic towards endothelial cells as PepH3.

Conclusion:

DPepH3 shows excellent translocation and internalization properties, safety, and improved stability. Taken together, our results place DPepH3 at the forefront of the second generation of BBB shuttles.

Fish oil protects the blood-brain barrier integrity in a mouse model of Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is ranked as the most prevalent neurodegenerative disease. However, the exact molecular mechanisms underlying pathophysiological alterations in AD remain unclear, especially at the prodromal stage. The decreased proteolytic degradation of Aβ, blood-brain barrier (BBB) disruption, and neuroinflammation are considered to play key roles in the course of AD.

METHODS

Male APPswe/PS1dE9 C57BL/6 J double-transgenic (APP/PS1) mice in the age range from 1 month to 6 months and age-matched wild type mice were used in this study, intending to investigate the expression profiles of Aβ-degrading enzymes for Aβ degradation activities and zonula occludens-1 (zo-1) for BBB integrity at the prodromal stage.

RESULTS

Our results showed that there were no significant genotype-related alterations in mRNA expression levels of 4 well-characterized Aβ-degrading enzymes in APP/PS1 mice within the ages of 6 months. Interestingly, a significant decrease in zo-1 expression was observed in APP/PS1 mice starting from the age of 5 months, suggesting that BBB disrupt occurs at an early stage. Moreover, treatment of fish oil (FO) for 4 weeks remarkably increased zo-1 expression and significantly inhibited the glial activation and NF-κB activation in APP/PS1 mice.

CONCLUSION

The results of our study suggest that FO supplement could be a potential therapeutic early intervention for AD through protecting the BBB integrity and suppressing glial and NF-κB activation.

Astrocytes Modify Migration of PBMCs Induced by β-Amyloid in a Blood-Brain Barrier in vitro Model

Background

The brain is protected by the blood-brain barrier (BBB), constituted by endothelial cells supported by pericytes and astrocytes. In Alzheimer’s disease a dysregulation of the BBB occurs since the early phases of the disease leading to an increased access of solutes and immune cells that can participate to the central inflammatory response. Here we investigated whether astrocytes may influence endothelial-leukocytes interaction in the presence of amyloid-β (Aβ).

Methods

We used an in vitro BBB model, where endothelial cells, cultured alone or with astrocytes were exposed for 5 h to Aβ, both under resting or inflammatory conditions (TNFα and IFNγ), to evaluate endothelial barrier properties, as well as transendothelial migration of peripheral blood mononuclear cells (PBMCs).

Results

In the co-culture model, barrier permeability to solutes was increased by all treatments, but migration was only observed in inflammatory conditions and was prevented by Aβ treatment. On the contrary, in endothelial monocultures, Aβ induced leukocytes migration under resting conditions and did not modify that induced by inflammatory cytokines. In endothelial astrocyte co-cultures, a low molecular weight (MW) isoform of the adhesion molecule ICAM-1, important to allow interaction with PBMCs, was increased after 5 h exposure to inflammatory cytokines, an effect that was prevented by Aβ. This modulation by Aβ was not observed in endothelial monocultures. In addition, endothelial expression of β-1,4-N-acetylglucosaminyltransferase III (Gnt-III), responsible for the formation of the low MW ICAM-1 isoform, was enhanced in inflammatory conditions, but negatively modulated by Aβ only in the co-culture model. miR-200b, increased in astrocytes following Aβ treatment and may represent one of the factors involved in the control of Gnt-III expression.

Conclusion

These data point out that, at least in the early phases of Aβ exposure, astrocytes play a role in the modulation of leukocytes migration through the endothelial layer.

Hyperoside protects the blood-brain barrier from neurotoxicity of amyloid beta 1-42

Mounting evidence indicates that amyloid β protein (Aβ) exerts neurotoxicity by disrupting the blood-brain barrier (BBB) in Alzheimer's disease. Hyperoside has neuroprotective effects both in vitro and in vivo against Aβ. Our previous study found that hyperoside suppressed Aβ_1–42-induced leakage of the BBB, however, the mechanism remains unclear. In this study, bEnd.3 cells were pretreated with 50, 200, or 500 µM hyperoside for 2 hours, and then exposed to Aβ_1–42 for 24 hours. Cell viability was determined using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide assay. Flow cytometry and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay were used to analyze cell apoptosis. Western blot assay was carried out to analyze expression levels of Bax, Bcl-2, cytochrome c, caspase-3, caspse-8, caspase-9, caspase-12, occludin, claudin-5, zonula occludens-1, matrix metalloproteinase-2 (MMP-2), and MMP-9. Exposure to Aβ_1–42 alone remarkably induced bEnd.3 cell apoptosis; increased ratios of cleaved caspase-9/caspase-9, Bax/Bcl-2, cleaved caspase-8/caspase-8, and cleaved caspase-12/caspase-12; increased expression of cytochrome c and activity of caspase-3; diminished levels of zonula occludens-1, claudin-5, and occludin; and increased levels of MMP-2 and MMP-9. However, hyperoside pretreatment reversed these changes in a dose-dependent manner. Our findings confirm that hyperoside alleviates fibrillar Aβ_1–42-induced BBB disruption, thus offering a feasible therapeutic application in Alzheimer's disease.

A New Noncanonical Anionic Peptide That Translocates a Cellular Blood-Brain Barrier Model

The capacity to transport therapeutic molecules across the blood–brain barrier (BBB) represents a breakthrough in the development of tools for the treatment of many central nervous system (CNS)-associated diseases. The BBB, while being protective against infectious agents, hinders the brain uptake of many drugs. Hence, finding safe shuttles able to overcome the BBB is of utmost importance. Herein, we identify a new BBB-translocating peptide with unique properties. For years it was thought that cationic sequences were mandatory for a cell-penetrating peptide (CPP) to achieve cellular internalization. Despite being anionic at physiological pH, PepNeg (sequence (SGTQEEY) is an efficient BBB translocator that is able to carry a large cargo (27 kDa), while maintaining BBB integrity. In addition, PepNeg is able to use two distinct methods of translocation, energy-dependent and -independent, suggesting that direct penetration might occur when low concentrations of peptide are presented to cells. The discovery of this new anionic trans-BBB peptide allows the development of new delivery systems to the CNS and contributes to the need to rethink the role of electrostatic attraction in BBB-translocation.

Crocus sativus Extract Tightens the Blood-Brain Barrier, Reduces Amyloid β Load and Related Toxicity in 5XFAD Mice

Crocus sativus, commonly known as saffron or Kesar, is used in Ayurveda and other folk medicines for various purposes as an aphrodisiac, antispasmodic, and expectorant. Previous evidence suggested that Crocus sativus is linked to improving cognitive function in Alzheimer's disease (AD) patients. The aim of this study was to in vitro and in vivo investigate the mechanism(s) by which Crocus sativus exerts its positive effect against AD. The effect of Crocus sativus extract on Aβ load and related toxicity was evaluated. In vitro results showed that Crocus sativus extract increases the tightness of a cell-based blood-brain barrier (BBB) model and enhances transport of Aβ. Further in vivo studies confirmed the effect of Crocus sativus extract (50 mg/kg/day, added to mice diet) on the BBB tightness and function that was associated with reduced Aβ load and related pathological changes in 5XFAD mice used as an AD model. Reduced Aβ load could be explained, at least in part, by Crocus sativus extract effect to enhance Aβ clearance pathways including BBB clearance, enzymatic degradation and ApoE clearance pathway. Furthermore, Crocus sativus extract upregulated synaptic proteins and reduced neuroinflammation associated with Aβ pathology in the brains of 5XFAD mice. Crocin, a major active constituent of Crocus sativus and known for its antioxidant and anti-inflammatory effect, was also tested separately in vivo in 5XFAD mice. Crocin (10 mg/kg/day) was able to reduce Aβ load but to a lesser extent when compared to Crocus sativus extract. Collectively, findings from this study support the positive effect of Crocus sativus against AD by reducing Aβ pathological manifestations.

Astrocytes contribute to Aβ-induced blood-brain barrier damage through activation of endothelial MMP9

The blood-brain barrier (BBB) plays an important role in the maintenance of the brain homeostasis, and its proper functions are warranted by the interplay between different cellular components (endothelial cells, astrocytes and pericytes). BBB dysfunctions in pathological conditions, and particularly in Alzheimer's disease, have been documented. Here, using an in vitroBBB model, the interaction between endothelial cells and astrocytes exposed to Aβ1-42 was investigated. Human endothelial cells, cultured in monolayer or co-cultured with astrocytes, were exposed to Aβ1-42 (2 μM for 18 h). Aβ induced dysfunction of endothelial barrier, as assessed by enhanced permeability to FITC-conjugated dextran and reduced expression of claudin-5; these modifications were observed in the co-culture model, but not in endothelial cells cultured in monolayer. Similarly, Aβ-induced damage at the barrier was observed when endothelial cells were challenged in the presence of conditioned medium generated by astrocytes previously exposed to Aβ (ACM Aβ). Endothelial barrier damages were associated with enhanced matrix metalloprotease 9 (MMP9) activity, known to mediate claudin-5 disruption. These events were not related to the direct effects played by Aβ on endothelial cells, but they were rather the consequence of Aβ-induced vascular endothelial growth factor (VEGF) expression in astrocytes. Indeed, when vascular endothelial growth factor expression was down-regulated in astrocytes, neither barrier properties or MMP9 expression in endothelial cells were affected after Aβ exposure both in the co-culture model or in the presence of ACM Aβ. These data point out the importance of astrocytes' mediation in inducing endothelial sensitivity to Aβ1-42.

Matrix Metalloproteinase in Blood-Brain Barrier Breakdown in Dementia

The neurovascular unit, which consists of astrocytic end-feet, neurons, pericytes, and endothelial cells, plays a key role in maintaining brain homeostasis by forming the blood-brain barrier and carefully controlling local cerebral blood flow. When the blood-brain barrier is disrupted, blood components can leak into the brain, damage the surrounding tissue and lead to cognitive impairment. This disruption in the blood-brain barrier and subsequent impairment in cognition are common after stroke and during cerebral amyloid angiopathy and Alzheimer's disease. Matrix metalloproteinases are proteases that degrade the extracellular matrix as well as tight junctions between endothelial cells and have been implicated in blood-brain barrier breakdown in neurodegenerative diseases. This review will focus on the roles of MMP2 and MMP9 in dementia, primarily post-stroke events that lead to dementia, cerebral amyloid angiopathy, and Alzheimer's disease.

High-Throughput Screening for Identification of Blood-Brain Barrier Integrity Enhancers: A Drug Repurposing Opportunity to Rectify Vascular Amyloid Toxicity

The blood-brain barrier (BBB) is a dynamic interface that maintains brain homeostasis and protects it from free entry of chemicals, toxins, and drugs. The barrier function of the BBB is maintained mainly by capillary endothelial cells that physically separate brain from blood. Several neurological diseases, such as Alzheimer's disease (AD), are known to disrupt BBB integrity. In this study, a high-throughput screening (HTS) was developed to identify drugs that rectify/protect BBB integrity from vascular amyloid toxicity associated with AD progression. Assessing Lucifer Yellow permeation across in-vitro BBB model composed from mouse brain endothelial cells (bEnd3) grown on 96-well plate inserts was used to screen 1280 compounds of Sigma LOPAC®1280 library for modulators of bEnd3 monolayer integrity. HTS identified 62 compounds as disruptors, and 50 compounds as enhancers of the endothelial barrier integrity. From these 50 enhancers, 7 FDA approved drugs were identified with EC50 values ranging from 0.76-4.56 μM. Of these 7 drugs, 5 were able to protect bEnd3-based BBB model integrity against amyloid toxicity. Furthermore, to test the translational potential to humans, the 7 drugs were tested for their ability to rectify the disruptive effect of Aβ in the human endothelial cell line hCMEC/D3. Only 3 (etodolac, granisetron, and beclomethasone) out of the 5 effective drugs in the bEnd3-based BBB model demonstrated a promising effect to protect the hCMEC/D3-based BBB model integrity. These drugs are compelling candidates for repurposing as therapeutic agents that could rectify dysfunctional BBB associated with AD.

Decreased Plasma Aβ in Hyperlipidemic APPSL Transgenic Mice Is Associated with BBB Dysfunction

Besides the continued focus on Aβ and Tau in Alzheimer's disease (AD), it is increasingly evident that other pathologic characteristics, such as vascular alterations or inflammation, are associated with AD. Whether these changes are an initial cause for the onset of AD or occur as a result of the disease in late stages is still under debate. In the present study, the impact of the high-fat diet (HFD) induced vascular risk factor hyperlipidemia on Aβ levels and clearance as well as cerebral vasculature and blood-brain barrier (BBB) integrity was examined in mice. For this purpose, human APP transgenic (APPSL) and wildtype (WT) mice were fed a HFD for 12 weeks. Plasma and tissues were subsequently investigated for Aβ distribution and concentrations of several vascular markers. Decreased plasma Aβ together with increased levels of insoluble Aβ and amyloid plaques in the brains of HFD fed APPSL mice point toward impaired Aβ clearance due to HFD. Additionally, HFD induced manifold alterations in the cerebral vasculature and BBB integrity exclusively in human APP overexpressing mice but not in wildtype mice. Therefore, HFD appears to enhance Aβ dependent vascular/BBB dysfunction in combination with an increased proportion of cerebral to plasma Aβ in APPSL mice.

Cerebral Small Vessel Disease: Targeting Oxidative Stress as a Novel Therapeutic Strategy?

Cerebral small vessel disease (SVD) is a major contributor to stroke, and a leading cause of cognitive impairment and dementia. Despite the devastating effects of cerebral SVD, the pathogenesis of cerebral SVD is still not completely understood. Moreover, there are no specific pharmacological strategies for its prevention or treatment. Cerebral SVD is characterized by marked functional and structural abnormalities of the cerebral microcirculation. The clinical manifestations of these pathological changes include lacunar infarcts, white matter hyperintensities, and cerebral microbleeds. The main purpose of this review is to discuss evidence implicating oxidative stress in the arteriopathy of both non-amyloid and amyloid (cerebral amyloid angiopathy) forms of cerebral SVD and its most important risk factors (hypertension and aging), as well as its contribution to cerebral SVD-related brain injury and cognitive impairment. We also highlight current evidence of the involvement of the NADPH oxidases in the development of oxidative stress, enzymes that are a major source of reactive oxygen species in the cerebral vasculature. Lastly, we discuss potential pharmacological strategies for oxidative stress in cerebral SVD, including some of the historical and emerging NADPH oxidase inhibitors.

Pinocembrin protects human brain microvascular endothelial cells against fibrillar amyloid-β(1-40) injury by suppressing the MAPK/NF-κB inflammatory pathways

Cerebrovascular accumulation of amyloid-β (Aβ) peptides in Alzheimer's disease (AD) may contribute to disease progression through Aβ-induced microvascular endothelial pathogenesis. Pinocembrin has been shown to have therapeutic effects in AD models. These effects correlate with preservation of microvascular function, but the effect on endothelial cells under Aβ-damaged conditions is unclear. The present study focuses on the in vitro protective effect of pinocembrin on fibrillar Aβ₁₋₄₀ (fAβ₁₋₄₀) injured human brain microvascular endothelial cells (hBMECs) and explores potential mechanisms. The results demonstrate that fAβ₁₋₄₀-induced cytotoxicity in hBMECs can be rescued by pinocembrin treatment. Pinocembrin increases cell viability, reduces the release of LDH, and relieves nuclear condensation. The mechanisms of this reversal from Aβ may be associated with the inhibition of inflammatory response, involving inhibition of MAPK activation, downregulation of phosphor-IKK level, relief of IκBα degradation, blockage of NF-κB p65 nuclear translocation, and reduction of the release of proinflammatory cytokines. Pinocembrin does not show obvious effects on regulating the redox imbalance after exposure to fAβ₁₋₄₀. Together, the suppression of MAPK and the NF-κB signaling pathways play a significant role in the anti-inflammation of pinocembrin in hBMECs subjected to fAβ₁₋₄₀. This may serve as a therapeutic agent for BMEC protection in Alzheimer's-related deficits.

Mixed oligomers and monomeric amyloid-β disrupts endothelial cells integrity and reduces monomeric amyloid-β transport across hCMEC/D3 cell line as an in vitro blood-brain barrier model

Senile amyloid plaques are one of the diagnostic hallmarks of Alzheimer's disease (AD). However, the severity of clinical symptoms of AD is weakly correlated with the plaque load. AD symptoms severity is reported to be more strongly correlated with the level of soluble amyloid-β (Aβ) assemblies. Formation of soluble Aβ assemblies is stimulated by monomeric Aβ accumulation in the brain, which has been related to its faulty cerebral clearance. Studies tend to focus on the neurotoxicity of specific Aβ species. There are relatively few studies investigating toxic effects of Aβ on the endothelial cells of the blood-brain barrier (BBB). We hypothesized that a soluble Aβ pool more closely resembling the in vivo situation composed of a mixture of Aβ40 monomer and Aβ42 oligomer would exert higher toxicity against hCMEC/D3 cells as an in vitro BBB model than either component alone. We observed that, in addition to a disruptive effect on the endothelial cells integrity due to enhancement of the paracellular permeability of the hCMEC/D3 monolayer, the Aβ mixture significantly decreased monomeric Aβ transport across the cell culture model. Consistent with its effect on Aβ transport, Aβ mixture treatment for 24h resulted in LRP1 down-regulation and RAGE up-regulation in hCMEC/D3 cells. The individual Aβ species separately failed to alter Aβ clearance or the cell-based BBB model integrity. Our study offers, for the first time, evidence that a mixture of soluble Aβ species, at nanomolar concentrations, disrupts endothelial cells integrity and its own transport across an in vitro model of the BBB.

Capillary electrophoresis for the analysis of the effect of sample preparation on early stages of Aβ1-40 aggregation

Aggregation of the amyloid-β protein (Aβ) contributes to the neurodegeneration characteristic of Alzheimer's disease. Of particular importance are the early stages of aggregation, which involve the formation of soluble oligomers and protofibrils. In these studies, we demonstrate the potential for CE with UV detection using a polyethylene oxide separation matrix to identify the evolution of various oligomeric species of Aβ1-40 . To demonstrate the efficacy of this technique, UV-CE was utilized to compare two methods commonly used to prepare Aβ for aggregation experiments and their effect on the formation of early aggregates. SEC-purified Aβ1-40 initially contained more small species, including monomer, than did freshly dissolved Aβ1-40 pretreated with hexafluoroisopropanol. Strikingly, the lag time to oligomer formation for SEC-isolated Aβ1-40 samples was ∼23 h shorter compared to freshly dissolved Aβ1-40 samples. Furthermore, oligomers formed from the aggregation of SEC-purified Aβ1-40 persisted within solution for a longer period of time. These results indicate that the initial sample preparation has a drastic influence on the early stages of Aβ1-40 aggregation. This is the first report of the use of UV-CE with a separation matrix to study the effect of sample preparation on early aggregation of Aβ1-40 . UV-CE was also used in parallel with dot blot analysis and inhibitory compounds to discern structural characteristics of individual oligomer peaks, demonstrating the capacity of UV-CE as a complimentary technique to further understand the aggregation process.

High-density lipoproteins and cerebrovascular integrity in Alzheimer's disease

Cerebrovascular dysfunction significantly contributes to the clinical presentation and pathoetiology of Alzheimer's disease (AD). Deposition and aggregation of β-amyloid (Aβ) within vascular smooth muscle cells leads to inflammation, oxidative stress, impaired vasorelaxation, and disruption of blood-brain barrier integrity. Midlife vascular risk factors, such as hypertension, cardiovascular disease, diabetes, and dyslipidemia, increase the relative risk for AD. These comorbidities are all characterized by low and/or dysfunctional high-density lipoproteins (HDL), which itself is a risk factor for AD. HDL performs a wide variety of critical functions in the periphery and CNS. In addition to lipid transport, HDL regulates vascular health via mediating vasorelaxation, inflammation, and oxidative stress and promotes endothelial cell survival and integrity. Here, we summarize clinical and preclinical data examining the involvement of HDL, originating from the circulation and from within the CNS, on AD and hypothesize potential synergistic actions between the two lipoprotein pools.

Blood-brain barrier dysfunction as a cause and consequence of Alzheimer's disease

The blood-brain barrier (BBB) plays critical roles in the maintenance of central nervous system (CNS) homeostasis. Dysfunction of the BBB occurs in a number of CNS diseases, including Alzheimer's disease (AD). A prevailing hypothesis in the AD field is the amyloid cascade hypothesis that states that amyloid-β (Aβ) deposition in the CNS initiates a cascade of molecular events that cause neurodegeneration, leading to AD onset and progression. In this review, the participation of the BBB in the amyloid cascade and in other mechanisms of AD neurodegeneration will be discussed. We will specifically focus on three aspects of BBB dysfunction: disruption, perturbation of transporters, and secretion of neurotoxic substances by the BBB. We will also discuss the interaction of the BBB with components of the neurovascular unit in relation to AD and the potential contribution of AD risk factors to aspects of BBB dysfunction. From the results discussed herein, we conclude that BBB dysfunction contributes to AD through a number of mechanisms that could be initiated in the presence or absence of Aβ pathology.

Altered brain uptake of therapeutics in a triple transgenic mouse model of Alzheimer's disease

PURPOSE

The purpose of this study was to systematically assess the impact of Alzheimer's disease (AD)-associated blood-brain barrier (BBB) alterations on the uptake of therapeutics into the brain.

METHODS

The brain uptake of probe compounds was measured in 18-20 month old wild type (WT) and triple transgenic (3×TG) AD mice using an in situ transcardiac perfusion technique. These results were mechanistically correlated with immunohistochemical and molecular studies.

RESULTS

The brain uptake of the paracellular marker, [(14)C] sucrose, did not differ between WT and 3×TG mice. The brain uptake of passively diffusing markers, [(3)H] diazepam and [(3)H] propranolol, decreased 54-60% in 3×TG mice, consistent with a 33.5% increase in the thickness of the cerebrovascular basement membrane in 3×TG mice. Despite a 42.4% reduction in P-gp expression in isolated brain microvessels from a sub-population of 3×TG mice (relative to WT mice), the brain uptake of P-gp substrates ([(3)H] digoxin, [(3)H] loperamide and [(3)H] verapamil) was not different between genotypes, likely due to a compensatory thickening in the cerebrovascular basement membrane counteracting any reduced efflux of these lipophilic substrates.

CONCLUSION

These studies systematically assessed the impact of AD on BBB drug transport in a relevant animal model, and have demonstrated a reduction in the brain uptake of passively-absorbed molecules in this mouse model of AD.

Effects of Amyloid Beta Peptide on Neurovascular Cells

Alzheimer’s disease (AD) is a chronic neurodegenerative disorder, which is characterized by the accumulation of amyloid plaques and neurofibrillary tangles in specific regions of the brain, accompanied by impairment of the neurons, and progressive deterioration of cognition and memory of affected individuals. Although the cause and progression of AD are still not well understood, the amyloid hypothesis is dominant and widely accepted. According to this hypothesis, an increased deposition of amyloid-β peptide (Aβ) in the brain is the main cause of the AD’s onset and progression. There is increasing body of evidence that blood-brain barrier (BBB) dysfunction plays an important role in the development of AD, and may even precede neuron degeneration in AD brain. In the early stage of AD, microvasculature deficiencies, inflammatory reactions, surrounding the cerebral vasculature and endothelial dysfunctions are commonly observed. Continuous neurovascular degeneration and accumulation of Aβ on blood vessels resulting in cerebral amyloid angiopathy is associated with further progression of the disease and cognitive decline. However, little is known about molecular mechanisms that underlie Aβ induced damage of neurovascular cells. In this regards, this review is aimed to address how Aβ impacts the cerebral endothelium. Understanding the cellular pathways triggered by Aβ leading to alterations in cerebral endothelial cells structure and functions would provide insights into the mechanism of BBB dysfunction and inflammatory processes in Alzheimer’s, and may offer new approaches for prevention and treatment strategies for AD.

Membrane configuration optimization for a murine in vitro blood-brain barrier model

A powerful experimental tool used to study the dynamic functions of the blood-brain barrier (BBB) is an in vitro cellular based system utilizing cell culture inserts in multi-well plates. Currently, usage of divergent model configurations without explanation of selected variable set points renders data comparisons difficult and limits widespread understanding. This work presents for the first time in literature a comprehensive screening study to optimize membrane configuration, with aims to unveil influential membrane effects on the ability of cerebral endothelial cells to form a tight monolayer. First, primary murine brain endothelial cells and astrocytes were co-cultured in contact and non-contact orientations on membranes of pore diameter sizes ranging from 0.4 μm to 8.0 μm, and the non-contact orientation and smallest pore diameter size were shown to support a significantly tighter monolayer formation. Then, membranes made from polyethylene terephthalate (PET) and polycarbonate (PC) purchased from three different commercial sources were compared, and PET membranes purchased from two manufacturers facilitated a significantly tighter monolayer formation. Models were characterized by transendothelial electrical resistance (TEER), sodium fluorescein permeability, and immunocytochemical labeling of tight junction proteins. Finally, a murine brain endothelial cell line, bEnd.3, was grown on the different membranes, and similar results were obtained with respect to optimal membrane configuration selection. The results and methodology presented here on high throughput 24-well plate inserts can be translated to other BBB systems to advance model understanding.

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.

Structural transitions and oligomerization along polyalanine fibril formation pathways from computer simulations

The results of a computer simulation study of the aggregation kinetics of a large system of model peptides with particular focus on the formation of intermediates are presented. Discontinuous molecular dynamic simulations were used in combination with our intermediate-resolution protein model, PRIME, to simulate the aggregation of a system of 192 polyalanine (KA(14) K) peptides at a concentration of 5 mM and a reduced temperature of T* = 0.13 starting from a random configuration and ending in the assembly of a fibrillar structure. The population of various structures, including free monomers, beta sheets, amorphous aggregates, hybrid aggregates, and fibrils, and the transitions between the structures were tracked over the course of 30 independent simulations and averaged together. The aggregation pathway for this system starts with the association of free monomers into small amorphous aggregates that then grow to moderate size by incorporating other free monomers or merging with other small amorphous aggregates. These then rearrange into either small beta sheets or hybrid aggregates formed by association between unstructured chains and beta sheets, both of which grow in size by adding free monomer chains or other small aggregates, one at a time. Fibrillar structures are formed initially either by the stacking of beta sheets, rearrangement of hybrid aggregates or association between beta sheets and hybrid aggregates. They grow by the addition of beta sheets, hybrid aggregates, and other small fibrillar structures. The rearrangement of amorphous aggregates into beta sheets is a critical and necessary step in the fibril formation pathway.

Unraveling the early events of amyloid-β protein (Aβ) aggregation: techniques for the determination of Aβ aggregate size

The aggregation of proteins into insoluble amyloid fibrils coincides with the onset of numerous diseases. An array of techniques is available to study the different stages of the amyloid aggregation process. Recently, emphasis has been placed upon the analysis of oligomeric amyloid species, which have been hypothesized to play a key role in disease progression. This paper reviews techniques utilized to study aggregation of the amyloid-β protein (Aβ) associated with Alzheimer's disease. In particular, the review focuses on techniques that provide information about the size or quantity of oligomeric Aβ species formed during the early stages of aggregation, including native-PAGE, SDS-PAGE, Western blotting, capillary electrophoresis, mass spectrometry, fluorescence correlation spectroscopy, light scattering, size exclusion chromatography, centrifugation, enzyme-linked immunosorbent assay, and dot blotting.

Amyloid-β contributes to blood-brain barrier leakage in transgenic human amyloid precursor protein mice and in humans with cerebral amyloid angiopathy

BACKGROUND AND PURPOSE

Cerebral amyloid angiopathy (CAA) is a degenerative disorder characterized by amyloid-β (Aβ) deposition in the blood-brain barrier (BBB). CAA contributes to injuries of the neurovasculature including lobar hemorrhages, cortical microbleeds, ischemia, and superficial hemosiderosis. We postulate that CAA pathology is partially due to Aβ compromising the BBB.

METHODS

We characterized 19 patients with acute stroke with "probable CAA" for neurovascular pathology based on MRI and clinical findings. Also, we studied the effect of Aβ on the expression of tight junction proteins and matrix metalloproteases (MMPs) in isolated rat brain microvessels.

RESULTS

Two of 19 patients with CAA had asymptomatic BBB leakage and posterior reversible encephalopathic syndrome indicating increased BBB permeability. In addition to white matter changes, diffusion abnormality suggesting lacunar ischemia was found in 4 of 19 patients with CAA; superficial hemosiderosis was observed in 7 of 9 patients. Aβ(40) decreased expression of the tight junction proteins claudin-1 and claudin-5 and increased expression of MMP-2 and MMP-9. Analysis of brain microvessels from transgenic mice overexpressing human amyloid precursor protein revealed the same expression pattern for tight junction and MMP proteins. Consistent with reduced tight junction and increased MMP expression and activity, permeability was increased in brain microvessels from human amyloid precursor protein mice compared with microvessels from wild-type controls.

CONCLUSIONS

Our findings indicate that Aβ contributes to changes in brain microvessel tight junction and MMP expression, which compromises BBB integrity. We conclude that Aβ causes BBB leakage and that assessing BBB permeability could potentially help characterize CAA progression and be a surrogate marker for treatment response.

Beta amyloid effects on expression of multidrug efflux transporters in brain endothelial cells

ABC (ATP Binding Cassette) efflux transporters at the blood-brain barrier, P-glycoprotein (ABCB1), multidrug resistance associated protein 4 (ABCC4) and breast cancer resistance protein (ABCG2), are important for protecting the brain from circulating xenobiotics. Their expression is regulated by signals from surrounding brain tissue that may alter in CNS pathologies. Differences have been reported in transporter expression on brain vasculature of Alzheimer's subjects where raised levels of β-amyloid (Aβ) occur. The present study examines in vitro the effects of Aβ using immortalised brain endothelial cells (hCMEC/D3). Significantly lower expression of ABCB1 but not ABCC4 or ABCG2 was found following exposure to Aβ(1-42) peptide but not its scrambled equivalent. This was evident at both protein and transcript level and was reflected in lower transcriptional activity of the ABCB1 promoter as judged from the luciferase reporter gene assay and in decreases in ABCB1-mediated efflux of rhodamine 123. Aβ exposure also affected Wnt/β-catenin signalling, decreasing levels of β-catenin protein, reducing activation of TOPFLASH and increasing transcript levels of endogenous inhibitor, Dkk-1. Application of Wnt3a reversed the Aβ-induced changes to ABCB1 protein. These results suggest that Aβ may impair Wnt/β-catenin signalling at the blood-brain barrier but that activation of this pathway may restore ABCB1.

Alterations in the blood brain barrier in ageing cerebral cortex in relationship to Alzheimer-type pathology: a study in the MRC-CFAS population neuropathology cohort

Impairment of the blood brain barrier (BBB) in human brain ageing and its relationship to Alzheimer-type pathology remains poorly defined. We have investigated the BBB in temporal cortex of brain donations from a population-representative sample of 92 participants from the Medical Research Council Cognitive Function and Ageing Study (MRC CFAS), a longitudinal study with a programme of brain donation. BBB alteration was investigated by immunohistochemistry to albumin and fibrinogen and to the tight junction proteins claudin-5, zonula occludens-1 (ZO-1) and occludin. BBB leakage showed wide population-variation and increased with progression of Alzheimer-type pathology, though with considerable overlap between different levels of Alzheimer-type pathology. This was accompanied by increased mean vascular density, but not by down-regulation of tight junction proteins. ZO-1 and occludin were also expressed in glia. Mechanisms leading to BBB leakage in brain ageing remain to be defined, but the population-variation in BBB changes and its early increase in relationship to Alzheimer-type pathology progression suggest that BBB dysfunction contributes to brain ageing.

Amyloid Beta induces oxidative stress-mediated blood-brain barrier changes in capillary amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is frequently observed in Alzheimer's disease (AD) and is characterized by deposition of amyloid beta (Aβ) in leptomeningeal and cortical brain vasculature. In 40% of AD cases, Aβ mainly accumulates in cortical capillaries, a phenomenon referred to as capillary CAA (capCAA). The aim of this study was to investigate blood-brain barrier (BBB) alterations in CAA-affected capillaries with the emphasis on tight junction (TJ) changes. First, capCAA brain tissue was analyzed for the distribution of TJs. Here, we show for the first time a dramatic loss of occludin, claudin-5, and ZO-1 in Aβ-laden capillaries surrounded by NADPH oxidase-2 (NOX-2)-positive activated microglia. Importantly, we observed abundant vascular expression of the Aβ transporter receptor for advanced glycation endproducts (RAGE). To unravel the underlying mechanism, a human brain endothelial cell line was stimulated with Aβ1-42 to analyze the effects of Aβ. We observed a dose-dependent cytotoxicity and increased ROS generation, which interestingly was reversed by administration of exogenous antioxidants, NOX-2 inhibitors, and by blocking RAGE. Taken together, our data evidently show that Aβ is toxic to brain endothelial cells via binding to RAGE and induction of ROS production, which ultimately leads to disruption of TJs and loss of BBB integrity.

Amyloid triggers extensive cerebral angiogenesis causing blood brain barrier permeability and hypervascularity in Alzheimer's disease

Evidence of reduced blood-brain barrier (BBB) integrity preceding other Alzheimer's disease (AD) pathology provides a strong link between cerebrovascular angiopathy and AD. However, the "Vascular hypothesis", holds that BBB leakiness in AD is likely due to hypoxia and neuroinflammation leading to vascular deterioration and apoptosis. We propose an alternative hypothesis: amyloidogenesis promotes extensive neoangiogenesis leading to increased vascular permeability and subsequent hypervascularization in AD. Cerebrovascular integrity was characterized in Tg2576 AD model mice that overexpress the human amyloid precursor protein (APP) containing the double missense mutations, APPsw, found in a Swedish family, that causes early-onset AD. The expression of tight junction (TJ) proteins, occludin and ZO-1, were examined in conjunction with markers of apoptosis and angiogenesis. In aged Tg2576 AD mice, a significant increase in the incidence of disrupted TJs, compared to age matched wild-type littermates and young mice of both genotypes, was directly linked to an increased microvascular density but not apoptosis, which strongly supports amyloidogenic triggered hypervascularity as the basis for BBB disruption. Hypervascularity in human patients was corroborated in a comparison of postmortem brain tissues from AD and controls. Our results demonstrate that amylodogenesis mediates BBB disruption and leakiness through promoting neoangiogenesis and hypervascularity, resulting in the redistribution of TJs that maintain the barrier and thus, provides a new paradigm for integrating vascular remodeling with the pathophysiology observed in AD. Thus the extensive angiogenesis identified in AD brain, exhibits parallels to the neovascularity evident in the pathophysiology of other diseases such as age-related macular degeneration.

Spatial frequency domain imaging of intrinsic optical property contrast in a mouse model of Alzheimer's disease

Extensive changes in neural tissue structure and function accompanying Alzheimer's disease (AD) suggest that intrinsic signal optical imaging can provide new contrast mechanisms and insight for assessing AD appearance and progression. In this work, we report the development of a wide-field spatial frequency domain imaging (SFDI) method for non-contact, quantitative in vivo optical imaging of brain tissue composition and function in a triple transgenic mouse AD model (3xTg). SFDI was used to generate optical absorption and scattering maps at up to 17 wavelengths from 650 to 970 nm in 20-month-old 3xTg mice (n = 4) and age-matched controls (n = 6). Wavelength-dependent optical properties were used to form images of tissue hemoglobin (oxy-, deoxy-, and total), oxygen saturation, and water. Significant baseline contrast was observed with 13-26% higher average scattering values and elevated water content (52 ± 2% vs. 31 ± 1%); reduced total tissue hemoglobin content (127 ± 9 μM vs. 174 ± 6 μM); and lower tissue oxygen saturation (57 ± 2% vs. 69 ± 3%) in AD vs. control mice. Oxygen inhalation challenges (100% oxygen) resulted in increased levels of tissue oxy-hemoglobin (ctO(2)Hb) and commensurate reductions in deoxy-hemoglobin (ctHHb), with ~60-70% slower response times and ~7 μM vs. ~14 μM overall changes for 3xTg vs. controls, respectively. Our results show that SFDI is capable of revealing quantitative functional contrast in an AD model and may be a useful method for studying dynamic alterations in AD neural tissue composition and physiology.

Characterization and use of human brain microvascular endothelial cells to examine β-amyloid exchange in the blood-brain barrier

Alzheimer's disease (AD) is characterized by excessive cerebrovascular deposition of the β-amyloid peptide (Aβ). The investigation of Aβ transport across the blood-brain barrier (BBB) has been hindered by inherent limitations in the cellular systems currently used to model the BBB, such as insufficient barrier properties and poor reproducibility. In addition, many of the existing models are not of human or brain origin and are often arduous to establish and maintain. Thus, we characterized an in vitro model of the BBB employing human brain microvascular endothelial cells (HBMEC) and evaluated its utility to investigate Aβ exchange at the blood-brain interface. Our HBMEC model offers an ease of culture compared with primary isolated or coculture BBB models and is more representative of the human brain endothelium than many of the cell lines currently used to study the BBB. In our studies, the HBMEC model exhibited barrier properties comparable to existing BBB models as evidenced by the restricted permeability of a known paracellular marker. In addition, using a simple and rapid fluormetric assay, we showed that antagonism of key Aβ transport proteins significantly altered the bi-directional transcytosis of fluorescein-Aβ (1-42) across the HBMEC model. Moreover, the magnitude of these effects was consistent with reports in the literature using the same ligands in existing in vitro models of the BBB. These studies establish the HBMEC as a representative in vitro model of the BBB and offer a rapid fluorometric method of assessing Aβ exchange between the periphery and the brain.

Transport of drugs across the blood–brain barrier in Alzheimer’s disease

Alzheimer’s disease, a neurodegenerative disorder, is associated with various pathological alterations to the blood–brain barrier, including disruption to the inter-endothelial tight junction proteins, altered expression of transport proteins involved in drug efflux, a reduction in cerebral blood flow and a thickening of the brain capillary basement membrane. There are many conflicting reports on whether such changes alter the ability of endogenous proteins to extravasate into the brain parenchyma, and there are even fewer reports focusing on the potential impact of these changes on drug transport into the CNS. The purpose of this review is to critically evaluate how the reported changes to the blood–brain barrier in Alzheimer’s disease have (or have not) resulted in altered CNS drug delivery, and to highlight the requirement for more rigorous and systematic studies in this field for the benefit of drug discovery and delivery scientists.

Gelsolin restores A beta-induced alterations in choroid plexus epithelium

Histologically, Alzheimer's disease (AD) is characterized by senile plaques and cerebrovascular amyloid deposits. In previous studies we demonstrated that in AD patients, amyloid-β (Aβ) peptide also accumulates in choroid plexus, and that this process is associated with mitochondrial dysfunction and epithelial cell death. However, the molecular mechanisms underlying Aβ accumulation at the choroid plexus epithelium remain unclear. Aβ clearance, from the brain to the blood, involves Aβ carrier proteins that bind to megalin, including gelsolin, a protein produced specifically by the choroid plexus epithelial cells. In this study, we show that treatment with gelsolin reduces Aβ-induced cytoskeletal disruption of blood-cerebrospinal fluid (CSF) barrier at the choroid plexus. Additionally, our results demonstrate that gelsolin plays an important role in decreasing Aβ-induced cytotoxicity by inhibiting nitric oxide production and apoptotic mitochondrial changes. Taken together, these findings make gelsolin an appealing tool for the prophylactic treatment of AD.

Soluble aggregates of the amyloid-beta peptide are trapped by serum albumin to enhance amyloid-beta activation of endothelial cells

Background

Self-assembly of the amyloid-β peptide (Aβ) has been implicated in the pathogenesis of Alzheimer's disease (AD). As a result, synthetic molecules capable of inhibiting Aβ self-assembly could serve as therapeutic agents and endogenous molecules that modulate Aβ self-assembly may influence disease progression. However, increasing evidence implicating a principal pathogenic role for small soluble Aβ aggregates warns that inhibition at intermediate stages of Aβ self-assembly may prove detrimental. Here, we explore the inhibition of Aβ_1–40 self-assembly by serum albumin, the most abundant plasma protein, and the influence of this inhibition on Aβ_1–40 activation of endothelial cells for monocyte adhesion.

Results

It is demonstrated that serum albumin is capable of inhibiting in a dose-dependent manner both the formation of Aβ_1–40 aggregates from monomeric peptide and the ongoing growth of Aβ_1–40 fibrils. Inhibition of fibrillar Aβ_1–40 aggregate growth is observed at substoichiometric concentrations, suggesting that serum albumin recognizes aggregated forms of the peptide to prevent monomer addition. Inhibition of Aβ_1–40 monomer aggregation is observed down to stoichiometric ratios with partial inhibition leading to an increase in the population of small soluble aggregates. Such partial inhibition of Aβ_1–40 aggregation leads to an increase in the ability of resulting aggregates to activate endothelial cells for adhesion of monocytes. In contrast, Aβ_1–40 activation of endothelial cells for monocyte adhesion is reduced when more complete inhibition is observed.

Conclusion

These results demonstrate that inhibitors of Aβ self-assembly have the potential to trap small soluble aggregates resulting in an elevation rather than a reduction of cellular responses. These findings provide further support that small soluble aggregates possess high levels of physiological activity and underscore the importance of resolving the effect of Aβ aggregation inhibitors on aggregate size.