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

Repetitive head trauma and apoE4 induce chronic cerebrovascular alterations that impair tau elimination from the brain

Repetitive mild traumatic brain injuries (r-mTBI) sustained in the military or contact sports have been associated with the accumulation of extracellular tau in the brain, which may contribute to the pathogenesis of neurodegenerative tauopathies. The expression of the apolipoprotein E4 (apoE4) isoform has been associated with higher levels of tau in the brain, and worse clinical outcomes after r-mTBI, though the influence of apoE genotype on extracellular tau dynamics in the brain is poorly understood. We recently demonstrated that extracellular tau can be eliminated across blood-brain barrier (BBB), which is progressively impaired following r-mTBI. The current studies investigated the influence of repetitive mild TBI (r-mTBI) and apoE genotype on the elimination of extracellular solutes from the brain. Following intracortical injection of biotin-labeled tau into humanized apoE-Tr mice, the levels of exogenous tau residing in the brain of apoE4 mice were elevated compared to other isoforms, indicating reduced tau elimination. Additionally, we found exposure to r-mTBI increased tau residence in apoE2 mice, similar to our observations in E2FAD animals. Each of these findings may be the result of diminished tau efflux via LRP1 at the BBB, as LRP1 inhibition significantly reduced tau uptake in endothelial cells and decreased tau transit across an in vitro model of the BBB (basolateral-to-apical). Notably, we showed that injury and apoE status, (particularly apoE4) resulted in chronic alterations in BBB integrity, pericyte coverage, and AQP4 polarization. These aberrations coincided with an atypical reactive astrocytic gene signature indicative of diminished CSF-ISF exchange. Our work found that CSF movement was reduced in the chronic phase following r-mTBI (>18 months post injury) across all apoE genotypes. In summary, we show that apoE genotype strongly influences cerebrovascular homeostasis, which can lead to age-dependent deficiencies in the elimination of toxic proteins from the brain, like tau, particularly in the aftermath of head trauma.

High levels of endothelial ICAM-1 prohibit natalizumab mediated abrogation of CD4+ T cell arrest on the inflamed BBB under flow in vitro

INTRODUCTION

The humanized anti-α4 integrin blocking antibody natalizumab (NTZ) is an effective treatment for relapsing-remitting multiple sclerosis (RRMS) that is associated with the risk of progressive multifocal leukoencephalopathy (PML). While extended interval dosing (EID) of NTZ reduces the risk for PML, the minimal dose of NTZ required to maintain its therapeutic efficacy remains unknown.

OBJECTIVE

Here we aimed to identify the minimal NTZ concentration required to inhibit the arrest of human effector/memory CD4⁺ T cell subsets or of PBMCs to the blood-brain barrier (BBB) under physiological flow in vitro.

RESULTS

Making use of three different human in vitro BBB models and in vitro live-cell imaging we observed that NTZ mediated inhibition of α4-integrins failed to abrogate T cell arrest to the inflamed BBB under physiological flow. Complete inhibition of shear resistant T cell arrest required additional inhibition of β2-integrins, which correlated with a strong upregulation of endothelial intercellular adhesion molecule (ICAM)-1 on the respective BBB models investigated. Indeed, NTZ mediated inhibition of shear resistant T cell arrest to combinations of immobilized recombinant vascular cell adhesion molecule (VCAM)-1 and ICAM-1 was abrogated in the presence of tenfold higher molar concentrations of ICAM-1 over VCAM-1. Also, monovalent NTZ was less potent than bivalent NTZ in inhibiting T cell arrest to VCAM-1 under physiological flow. In accordance with our previous observations ICAM-1 but not VCAM-1 mediated T cell crawling against the direction of flow.

CONCLUSION

Taken together, our in vitro observations show that high levels of endothelial ICAM-1 abrogate NTZ mediated inhibition of T cell interaction with the BBB. EID of NTZ in MS patients may thus require consideration of the inflammatory status of the BBB as high levels of ICAM-1 may provide an alternative molecular cue allowing for pathogenic T cell entry into the CNS in the presence of NTZ.

Cell models for Down syndrome-Alzheimer’s disease research

Down syndrome (DS) is the most common chromosomal abnormality and leads to intellectual disability, increased risk of cardiac defects, and an altered immune response. Individuals with DS have an extra full or partial copy of chromosome 21 (trisomy 21) and are more likely to develop early-onset Alzheimer’s disease (AD) than the general population. Changes in expression of human chromosome 21 (Hsa21)-encoded genes, such as amyloid precursor protein (APP), play an important role in the pathogenesis of AD in DS (DS-AD). However, the mechanisms of DS-AD remain poorly understood. To date, several mouse models with an extra copy of genes syntenic to Hsa21 have been developed to characterise DS-AD-related phenotypes. Nonetheless, due to genetic and physiological differences between mouse and human, mouse models cannot faithfully recapitulate all features of DS-AD. Cells differentiated from human-induced pluripotent stem cells (iPSCs), isolated from individuals with genetic diseases, can be used to model disease-related cellular and molecular pathologies, including DS. In this review, we will discuss the limitations of mouse models of DS and how these can be addressed using recent advancements in modelling DS using human iPSCs and iPSC-mouse chimeras, and potential applications of iPSCs in preclinical studies for DS-AD.

Influence of traumatic brain injury on extracellular tau elimination at the blood-brain barrier

Repetitive head trauma has been associated with the accumulation of tau species in the brain. Our prior work showed brain vascular mural cells contribute to tau processing in the brain, and that these cells progressively degenerate following repetitive mild traumatic brain injury (r-mTBI). The current studies investigated the role of the cerebrovasculature in the elimination of extracellular tau from the brain, and the influence of r-mTBI on these processes. Following intracranial injection of biotin-labeled tau, the levels of exogenous labeled tau residing in the brain were elevated in a mouse model of r-mTBI at 12 months post-injury compared to r-sham mice, indicating reduced tau elimination from the brain following head trauma. This may be the result of decreased caveolin-1 mediated tau efflux at the blood–brain barrier (BBB), as the caveolin inhibitor, methyl-β-cyclodextrin, significantly reduced tau uptake in isolated cerebrovessels and significantly decreased the basolateral-to-apical transit of tau across an in vitro model of the BBB. Moreover, we found that the upstream regulator of endothelial caveolin-1, Mfsd2a, was elevated in r-mTBI cerebrovessels compared to r-sham, which coincided with a decreased expression of cerebrovascular caveolin-1 in the chronic phase following r-mTBI (> 3 months post-injury). Lastly, angiopoietin-1, a mural cell-derived protein governing endothelial Mfsd2a expression, was secreted from r-mTBI cerebrovessels to a greater extent than r-sham animals. Altogether, in the chronic phase post-injury, release of angiopoietin-1 from degenerating mural cells downregulates caveolin-1 expression in brain endothelia, resulting in decreased tau elimination across the BBB, which may describe the accumulation of tau species in the brain following head trauma.

Hypoxia-Induced Neuroinflammation in Alzheimer's Disease: Potential Neuroprotective Effects of Centella asiatica

Alzheimer's disease (AD) is a neurodegenerative disorder that is characterised by the presence of extracellular beta-amyloid fibrillary plaques and intraneuronal neurofibrillary tau tangles in the brain. Recurring failures of drug candidates targeting these pathways have prompted research in AD multifactorial pathogenesis, including the role of neuroinflammation. Triggered by various factors, such as hypoxia, neuroinflammation is strongly linked to AD susceptibility and/or progression to dementia. Chronic hypoxia induces neuroinflammation by activating microglia, the resident immune cells in the brain, along with an increased in reactive oxygen species and pro-inflammatory cytokines, features that are common to many degenerative central nervous system (CNS) disorders. Hence, interests are emerging on therapeutic agents and plant derivatives for AD that target the hypoxia-neuroinflammation pathway. Centella asiatica is one of the natural products reported to show neuroprotective effects in various models of CNS diseases. Here, we review the complex hypoxia-induced neuroinflammation in the pathogenesis of AD and the potential application of Centella asiatica as a therapeutic agent in AD or dementia.

Atorvastatin disrupts primary human brain microvascular endothelial cell functions via prenylation-dependent mitochondrial inhibition and oxidative stress

Primary human brain microvascular endothelial cell (HBMEC) is the major component of the blood-brain barrier (BBB). Atorvastatin, a HMG-CoA reductase inhibitor, is a cholesterol-lowering drug commonly used to reduce the risk for cardiovascular disease. Numerous studies have reported the pleiotropic effects of atorvastatin on endothelial cells, but the findings are controversial and inconclusive. In addition, little is known about the biological effects of atorvastatin on HBMEC. In this work, we demonstrate that atorvastatin at micromolar but not nanomolar concentrations induces dysfunctions of a number of HBMEC events, including differentiation into capillary network, migration and growth but not cell adhesion. We further show that the inhibitory effects of atorvastatin on HBMEC are independent of angiogenesis stimulators. Atorvastatin induces HBMEC apoptosis even in the presence of vascular endothelial growth factor (VEGF) and serum. Mechanism studies indicate that atorvastatin at micromolar concentration leads to protein prenylation inhibition, mitochondrial dysfunction and thereby subsequent oxidative stress and damage in HBMEC. Rescue experiments confirm that atorvastatin inhibits HBMEC functions via prenylation-dependent mitochondrial inhibition. Our work reveals the inhibitory effects of atorvastatin on HBMEC and suggests the possible negative influence of atorvastatin in blood-brain barrier.

Influence of Matrix Metallopeptidase 9 on Beta-Amyloid Elimination Across the Blood-Brain Barrier

Lipoprotein receptor transport across the blood-brain barrier (BBB) mediates beta-amyloid (Aβ) accumulation in the brain and may be a contributing factor in Alzheimer's disease (AD) pathogenesis. Lipoprotein receptors are susceptible to proteolytic shedding at the cell surface, which precludes the endocytic transport of ligands. A ligand that closely interacts with the lipoprotein receptors is apolipoprotein E (apoE), which exists as three isoforms (apoE2, apoE3, apoE4). Our prior work showed an inverse relationship between lipoprotein receptor shedding and Aβ transport across the BBB, which was apoE-isoform dependent. To interrogate this further, the current studies investigated an enzyme implicated in lipoprotein receptor shedding, matrix metalloproteinase 9 (MMP9). Treatment with MMP9 dose-dependently elevated lipoprotein receptor shedding in brain endothelial cells and freshly isolated mouse cerebrovessels. Furthermore, treatment with a MMP9 inhibitor (SB-3CT) mitigated Aβ-induced lipoprotein receptor shedding in brain endothelial cells and the brains of apoE4 animals. In terms of BBB transit, SB-3CT treatment increased the transport of Aβ across an in vitro model of the BBB. In vivo, administration of SB-3CT to apoE4 animals significantly enhanced Aβ clearance from the brain to the periphery following intracranial administration of Aβ. The current studies show that MMP9 impacts lipoprotein receptor shedding and Aβ transit across the BBB, in an apoE  isoform-specific manner. In total, MMP9 inhibition can facilitate Aβ clearance across the BBB, which could be an effective approach to lowering Aβ levels in the brain and mitigating the AD phenotype, particularly in subjects carrying the apoE4 allele.

Engulfment of platelets delays endothelial cell aging via girdin and its phosphorylation

Endothelial cells are critical in angiogenesis and maintain the homeostasis of the blood‑brain barrier (BBB). Platelets (PLTs) are essential in vascular biology, including angiogenesis. The present study aimed to investigate the effect of PLTs on the aging of endothelial cells. Human brain microvascular endothelial cells (HBMECs) and human astrocytes were co‑cultured to mimic the BBB. Transmission electron microscopy was used to observe the engulfment of PLTs. Confocal microscopy was used to observe the co‑localization of PLTs, girders of actin filament (girdin) and phosphorylated (p‑)girdin. Senescence‑associated β‑galactosidase (β‑gal) staining, 3‑(4,5‑dimethylthiazol‑2‑yl)‑2,5‑diphenyltetrazolium bromide and flow cytometry were performed to examine the cell senescence, viability and apoptosis, respectively. Transwell assays were performed to examine cell invasion and migration. Western blot analysis was performed to detect the expression of girdin, AKT and p‑AKT. PLTs delayed senescence, and promoted the viability and resistance to apoptosis of the HBMECs. Cell invasion and migration were enhanced by PLTs. In addition, girdin and p‑girdin were essential to the engulfment of HBMECs to PLTs. Mechanically, the inhibition of AKT signals reversed the effect of PLTs on HBMECs by increasing the activity of β‑gal, decreasing the cell viability, and inhibiting the invasion and migration of the HBMECs. The engulfment of PLTs assisted in delaying the aging of endothelial cells via girdin and p‑girdin, in which the AKT signal was involved. The present study indicated a potential strategy for delaying endothelial cell aging in the treatment of central nervous system diseases.

Inhibition of mTOR protects the blood-brain barrier in models of Alzheimer's disease and vascular cognitive impairment

An intact blood-brain barrier (BBB) limits entry of proinflammatory and neurotoxic blood-derived factors into the brain parenchyma. The BBB is damaged in Alzheimer's disease (AD), which contributes significantly to the progression of AD pathologies and cognitive decline. However, the mechanisms underlying BBB breakdown in AD remain elusive, and no interventions are available for treatment or prevention. We and others recently established that inhibition of the mammalian/mechanistic target of rapamycin (mTOR) pathway with rapamycin yields significant neuroprotective effects, improving cerebrovascular and cognitive function in mouse models of AD. To test whether mTOR inhibition protects the BBB in neurological diseases of aging, we treated hAPP(J20) mice modeling AD and low-density lipoprotein receptor-null (LDLR-/-) mice modeling vascular cognitive impairment with rapamycin. We found that inhibition of mTOR abrogates BBB breakdown in hAPP(J20) and LDLR-/- mice. Experiments using an in vitro BBB model indicated that mTOR attenuation preserves BBB integrity through upregulation of specific tight junction proteins and downregulation of matrix metalloproteinase-9 activity. Together, our data establish mTOR activity as a critical mediator of BBB breakdown in AD and, potentially, vascular cognitive impairment and suggest that rapamycin and/or rapalogs could be used for the restoration of BBB integrity. NEW & NOTEWORTHY This report establishes mammalian/mechanistic target of rapamycin as a critical mediator of blood-brain barrier breakdown in models of Alzheimer's disease and vascular cognitive impairment and suggests that drugs targeting the target of rapamycin pathway could be used for the restoration of blood-brain barrier integrity in disease states.

Asiaticoside Attenuates Cell Growth Inhibition and Apoptosis Induced by Aβ1-42 via Inhibiting the TLR4/NF-κB Signaling Pathway in Human Brain Microvascular Endothelial Cells

Alzheimer's disease (AD) is a very common progressive neurodegenerative disorder with the highest incidence in the world. Dysfunction of the blood-brain barrier (BBB) may be responsible for the pathogenesis and pathology of AD for abnormally transporting amyloid-β (Aβ, the main component of the senile plaques) from the sera into the central nervous system. Aβ peptides induce apoptosis in human brain microvascular endothelial cells (hBMECs), the main component of BBB. Apoptosis in neuronal cells plays a critical role in the pathogenesis of AD. Asiaticoside, a natural glycoside extracted from Centella asiatica (L.) Urban, has an anti-apoptotic effect on hBMECs but the molecule mechanism remains unclear. Therefore, we investigate the protective effect of asiaticoside on Aβ_1-42-induced cytotoxicity and apoptosis as well as associated mechanism in hBMECs with commonly used in vitro methods for clinical development of asiaticoside as a novel anti-AD agent. In the present study, we investigated the effects of asiaticoside on cytotoxicity by Cell Counting Kit-8 assay, mitochondrial membrane potential by JC-1 fluorescence analysis, anti-apoptosis by Hoechst 33258 staining and Annexin V-FITC (fluorescein isothiocyanate) and propidium iodide (PI) analyses, the expressions of TNF-α and IL-6 by enzyme-linked immunosorbent assay (ELISA) and TLR4, MyD88, TRAF6, p-NF-κB p65, and total NF-κB p65 by Western blotting, and nuclear translocation of NF-κB p65 by immunofluorescence analysis in hBMECs. The results showed that pretreatment of asiaticoside (25, 50, and 100 μM) for 12 h significantly attenuated cell growth inhibition and apoptosis, and restored declined mitochondrial membrane potential induced by Aβ_1-42 (50 μM) in hBMECs. Asiaticoside also significantly downregulated the elevated expressions of TNF-α, IL-6, TLR4, MyD88, TRAF6, and p-NF-κB p65, as well as inhibited NF-κB p65 translocation from cytoplasm to nucleus induced by Aβ_1-42 in hBMECs in a concentration-dependent manner. The possible underlying molecular mechanism of asiaticoside may be through inhibiting the TLR4/NF-κB signaling pathway. Therefore, asiaticoside may be developed as a novel agent for the prevention and/or treatment of AD clinically.

Modulation of Amyloid-β1-40 Transport by ApoA1 and ApoJ Across an in vitro Model of the Blood-Brain Barrier

Amyloid-β (Aβ) accumulation in Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA) is likely caused by the impairment of its brain clearance that partly occurs through the blood-brain barrier (BBB). In this context, an in vitro BBB model is a valuable tool for studying the molecular mechanisms that regulate this process. This study assessed brain Aβ elimination across the BBB and its modulation by the natural chaperones Apolipoprotein A1 (ApoA1) and Apolipoprotein J/Clusterin (ApoJ). The model was based on primary cerebral endothelial cells that were cultured on Matrigel-coated Transwells and treated with fluorescently labeled-Aβ1-40 to track its efflux across the BBB, which corresponds to trafficking from the basolateral (brain) to apical (blood) compartments. We observed that the transport of basolateral Aβ1-40 was enhanced when it was complexed to rApoJ, whereas the complex formed with rApoA1 did not influence Aβ1-40 efflux. However, the presence of rApoA1 in the apical compartment was able to mobilize Aβ1-40 from the basolateral side. We also observed that both rApoA1 and rApoJ moderately crossed the monolayer (from blood to brain) through a mechanism involving the LDL receptor-related protein family. In contrast to the increased rApoJ efflux when complexed to Aβ1-40, rApoA1 trafficking was restricted when it was bound to the Aβ peptide. In summary, the present study highlights the role of ApoJ and ApoA1 in the in vitro modulation of Aβ elimination across the BBB.

Immobilization of Neurotrophin Peptides on Gold Nanoparticles by Direct and Lipid-Mediated Interaction: A New Multipotential Therapeutic Nanoplatform for CNS Disorders

Neurotrophins are essential proteins for the development and maintenance of neural functions as well as promising drugs in neurodegenerative disorders. Current limits in their effective clinical applications can be overwhelmed by the combined use of peptidomimetic and nanomedicine approaches. Indeed, neurotrophin-mimicking peptides may allow minimizing the adverse side effects of the whole protein drug. Moreover, the immobilization of such peptides on nanomaterials may offer additional advantages, including protection against degradation, enhanced permeability of barrier membranes, and intrinsic therapeutic properties of the nanoparticles (e.g., antiangiogenic and plasmonic features of gold nanoparticles (AuNPs)). In the present article, we scrutinize the functionalization of spherical AuNPs of diameter 12 nm by peptides because of the N-terminal domains of the nerve growth factor (NGF) and the brain-derived neurotrophic factor (BDNF), NGF1-14 and BDNF1-12, respectively. The hybrid gold-peptide nanobiointerface was investigated, both in the direct physisorption and in the lipid-bilayer-mediated adsorption processes, by a multitechnique study that included UV-vis and X-ray photoelectron spectroscopies, dynamic light scattering, zeta-potential analyses, and atomic force microscopy. Both peptide- and lipid-dependent features were identified, to have a modulation in the peptide coverage of nanoparticles as well as in the cellular uptake of NGF and BDNF peptides, as investigated by confocal microscopy. The promising potentials of the neurotrophins to cross the blood-brain barrier were demonstrated.

Inhibition of ADAM10 promotes the clearance of Aβ across the BBB by reducing LRP1 ectodomain shedding

Background

Transport across the blood–brain barrier (BBB) is an important mediator of beta-amyloid (Aβ) accumulation in the brain and a contributing factor in the pathogenesis of Alzheimer’s disease (AD). One of the receptors responsible for the transport of Aβ in the BBB is the low density lipoprotein receptor-related protein 1 (LRP1). LRP1 is susceptible to proteolytic shedding at the cell surface, which prevents endocytic transport of ligands. Previously, we reported a strong inverse correlation between LRP1 shedding in the brain and Aβ transit across the BBB. Several proteases contribute to the ectodomain shedding of LRP1 including the α-secretase, a desintegrin and metalloproteinase domain containing protein 10 (ADAM10).

Methods

The role of ADAM10 in the shedding of LRP1 and Aβ BBB clearance was assessed through pharmacological inhibition of ADAM10 in an in vitro model of the BBB and through the use of ADAM10 endothelial specific knock-out mice. In addition, an acute treatment paradigm with an ADAM10 inhibitor was also tested in an AD mouse model to assess the effect of ADAM10 inhibition on LRP1 shedding and Aβbrain accumulation.

Results

In the current studies, inhibition of ADAM10 reduced LRP1 shedding in brain endothelial cultures and increased Aβ42 transit across an in vitro model of the BBB. Similarly, transgenic ADAM10 endothelial knockout mice displayed lower LRP1 shedding in the brain and significantly enhanced Aβ clearance across the BBB compared to wild-type animals. Acute treatment with the ADAM10-selective inhibitor GI254023X in an AD mouse model substantially reduced brain LRP1 shedding and increased Aβ40 levels in the plasma, indicating enhanced Aβ transit from the brain to the periphery. Furthermore, both soluble and insoluble Aβ40 and Aβ42 brain levels were decreased following GI254023X treatment, but these effects lacked statistical significance.

Conclusions

These studies demonstrate a role for ADAM10 in the ectodomain shedding of LRP1 in the brain and the clearance of Aβ across the BBB, which may provide a novel strategy for attenuating Aβ accumulation in the AD brain.

The hunt for brain Aβ oligomers by peripherally circulating multi-functional nanoparticles: Potential therapeutic approach for Alzheimer disease

We previously showed the ability of liposomes bi-functionalized with phosphatidic acid and an ApoE-derived peptide (mApoE-PA-LIP) to reduce brain Aβ in transgenic Alzheimer mice. Herein we investigated the efficacy of mApoE-PA-LIP to withdraw Aβ peptide in different aggregation forms from the brain, using a transwell cellular model of the blood-brain barrier and APP/PS1 mice. The spontaneous efflux of Aβ oligomers (Aβo), but not of Aβ fibrils, from the 'brain' side of the transwell was strongly enhanced (5-fold) in presence of mApoE-PA-LIP in the 'blood' compartment. This effect is due to a withdrawal of Aβo exerted by peripheral mApoE-PA-LIP by sink effect, because, when present in the brain side, they did not act as Aβo carrier and limit the oligomer efflux. In vivo peripheral administration of mApoE-PA-LIP significantly increased the plasma Aβ level, suggesting that Aβ-binding particles exploiting the sink effect can be used as a therapeutic strategy for Alzheimer disease. From the Clinical Editor: Alzheimer disease (AD) at present is an incurable disease, which is thought to be caused by an accumulation of amyloid-β (Aβ) peptides in the brain. Many strategies in combating this disease have been focused on either the prevention or dissolving these peptides. In this article, the authors showed the ability of liposomes bi-functionalized with phosphatidic acid and with an ApoE- derived peptide to withdraw amyloid peptides from the brain. The data would help the future design of more novel treatment for Alzheimer disease.

Possible roles of amyloids in malaria pathophysiology

The main therapeutic and prophylactic tools against malaria have been locked for more than a century in the classical approaches of using drugs targeting metabolic processes of the causing agent, the protist Plasmodium spp., and of designing vaccines against chosen antigens found on the parasite's surface. Given the extraordinary resources exhibited by Plasmodium to escape these traditional strategies, which have not been able to free humankind from the scourge of malaria despite much effort invested in them, new concepts have to be explored in order to advance toward eradication of the disease. In this context, amyloid-forming proteins and peptides found in the proteome of the pathogen should perhaps cease being regarded as mere anomalous molecules. Their likely functionality in the pathophysiology of Plasmodium calls for attention being paid to them as a possible Achilles' heel of malaria. Here we will give an overview of Plasmodium-encoded amyloid-forming polypeptides as potential therapeutic targets and toxic elements, particularly in relation to cerebral malaria and the blood-brain barrier function. We will also discuss the recent finding that the genome of the parasite contains an astonishingly high proportion of prionogenic domains.

Apolipoprotein E isoform-specific effects on lipoprotein receptor processing

Recent findings indicate an isoform-specific role for apolipoprotein E (apoE) in the elimination of beta-amyloid (Aβ) from the brain. ApoE is closely associated with various lipoprotein receptors, which contribute to Aβ brain removal via metabolic clearance or transit across the blood–brain barrier (BBB). These receptors are subject to ectodomain shedding at the cell surface, which alters endocytic transport and mitigates Aβ elimination. To further understand the manner in which apoE influences Aβ brain clearance, these studies investigated the effect of apoE on lipoprotein receptor shedding. Consistent with prior reports, we observed an increased shedding of the low-density lipoprotein receptor (LDLR) and the LDLR-related protein 1 (LRP1) following Aβ exposure in human brain endothelial cells. When Aβ was co-treated with each apoE isoform, there was a reduction in Aβ-induced shedding with apoE2 and apoE3, while lipoprotein receptor shedding in the presence of apoE4 remained increased. Likewise, intracranial administration of Aβ to apoE-targeted replacement mice (expressing the human apoE isoforms) resulted in an isoform-dependent effect on lipoprotein receptor shedding in the brain (apoE4 > apoE3 > apoE2). Moreover, these results show a strong inverse correlation with our prior work in apoE transgenic mice in which apoE4 animals showed reduced Aβ clearance across the BBB compared to apoE3 animals. Based on these results, apoE4 appears less efficient than other apoE isoforms in regulating lipoprotein receptor shedding, which may explain the differential effects of these isoforms in removing Aβ from the brain.

The spleen tyrosine kinase (Syk) regulates Alzheimer amyloid-β production and Tau hyperphosphorylation

We have previously shown that the L-type calcium channel (LCC) antagonist nilvadipine reduces brain amyloid-β (Aβ) accumulation by affecting both Aβ production and Aβ clearance across the blood-brain barrier (BBB). Nilvadipine consists of a mixture of two enantiomers, (+)-nilvadipine and (-)-nilvadipine, in equal proportion. (+)-Nilvadipine is the active enantiomer responsible for the inhibition of LCC, whereas (-)-nilvadipine is considered inactive. Both nilvadipine enantiomers inhibit Aβ production and improve the clearance of Aβ across the BBB showing that these effects are not related to LCC inhibition. In addition, treatment of P301S mutant human Tau transgenic mice (transgenic Tau P301S) with (-)-nilvadipine reduces Tau hyperphosphorylation at several Alzheimer disease (AD) pertinent epitopes. A search for the mechanism of action of (-)-nilvadipine revealed that this compound inhibits the spleen tyrosine kinase (Syk). We further validated Syk as a target-regulating Aβ by showing that pharmacological inhibition of Syk or down-regulation of Syk expression reduces Aβ production and increases the clearance of Aβ across the BBB mimicking (-)-nilvadipine effects. Moreover, treatment of transgenic mice overexpressing Aβ and transgenic Tau P301S mice with a selective Syk inhibitor respectively decreased brain Aβ accumulation and Tau hyperphosphorylation at multiple AD relevant epitopes. We show that Syk inhibition induces an increased phosphorylation of the inhibitory Ser-9 residue of glycogen synthase kinase-3β, a primary Tau kinase involved in Tau phosphorylation, by activating protein kinase A, providing a mechanism explaining the reduction of Tau phosphorylation at GSK3β-dependent epitopes following Syk inhibition. Altogether our data highlight Syk as a promising target for preventing both Aβ accumulation and Tau hyperphosphorylation in AD.

Role of the cannabinoid system in the transit of beta-amyloid across the blood-brain barrier

Emerging evidence suggests beta-amyloid (Aβ) deposition in the Alzheimer's disease (AD) brain is the result of impaired clearance, due in part to diminished Aβ transport across the blood-brain barrier (BBB). Recently, modulation of the cannabinoid system was shown to reduce Aβ brain levels and improve cognitive behavior in AD animal models. The purpose of the current studies was to investigate the role of the cannabinoid system in the clearance of Aβ across the BBB. Using in vitro and in vivo models of BBB clearance, Aβ transit across the BBB was examined in the presence of cannabinoid receptor agonists and inhibitors. In addition, expression levels of the Aβ transport protein, lipoprotein receptor-related protein1 (LRP1), were determined in the brain and plasma of mice following cannabinoid treatment. Cannabinoid receptor agonism or inhibition of endocannabinoid-degrading enzymes significantly enhanced Aβ clearance across the BBB (2-fold). Moreover, cannabinoid receptor inhibition negated the stimulatory influence of cannabinoid treatment on Aβ BBB clearance. Additionally, LRP1 levels in the brain and plasma were elevated following cannabinoid treatment (1.5-fold), providing rationale for the observed increase in Aβ transit from the brain to the periphery. The current studies demonstrate, for the first time, a role for the cannabinoid system in the transit of Aβ across the BBB. These findings provide insight into the mechanism by which cannabinoid treatment reduces Aβ burden in the AD brain and offer additional evidence on the utility of this pathway as a treatment for AD.

Immortalized human cerebral microvascular endothelial cells maintain the properties of primary cells in an in vitro model of immune migration across the blood brain barrier

The immortalized human cerebral microvascular endothelial cell line HCMEC/D3 presents a less expensive and more logistically feasible alternative to primary human brain microvascular endothelial cells (HBMEC's) for use in constructing in vitro models of the blood brain barrier (BBB). However, the fidelity of the HCMEC/D3 cell line to primary HBMEC's in studies of immune transmigration has yet to be established. Flow cytometric analysis of primary human leukocyte migration across in vitro BBB's generated with either HCMEC/D3 or primary HBMEC's revealed that HCMEC/D3 maintains the immune barrier properties of primary HBMEC's. Leukocyte migration responses and inflammatory cytokine production were statistically indistinguishable between both endothelial cell types, and both cell types responded similarly to astrocyte coculture, stimulation of leukocytes with phorbol myristate acetate (PMA) and ionomycin, and inflammatory cytokine treatment. This report is the first to validate the HCMEC/D3 cell line in a neuroimmunological experimental system via direct comparison to primary HBMEC's, demonstrating remarkable fidelity in terms of barrier resistance, immune migration profiles, and responsiveness to inflammatory cytokines. Moreover, we report novel findings demonstrating that interaction effects between immune cells and resident CNS cells are preserved in HCMEC/D3, suggesting that important characteristics of neuroimmune interactions during CNS inflammation are preserved in systems utilizing this cell line. Together, these findings demonstrate that HCMEC/D3 is a valid and powerful tool for less expensive and higher throughput in vitro investigations of immune migration at the BBB.

A multifaceted role for apoE in the clearance of beta-amyloid across the blood-brain barrier

BACKGROUND

While apolipoprotein E4 (apoE4) is highly correlated with the development of Alzheimer's disease (AD), its role in AD pathology and, in particular, beta-amyloid (Aβ) removal from the brain, is not clearly defined.

OBJECTIVE

To elucidate the influence of apoE on the clearance of Aβ across the blood-brain barrier (BBB).

METHODS

Aβ(1-42) was intracerebrally administered to transgenic mice expressing human apoE isoforms and examined in the periphery.

RESULTS

apoE3 and apoE4 mice had 5 times and 2 times, respectively, more Aβ(1-42) appearing in the plasma than wild-type or apoE knockout mice, indicating an enhanced clearance of Aβ from the brain to the periphery. In vitro, unbound basolateral apoE3 (i.e., not bound to Aβ), and to a lesser extent unbound apoE4, at concentrations ≤10 nM facilitated basolateral-to-apical fluorescein-Aβ(1-42) transcytosis across a BBB model, while apoE isoforms bound to Aβ significantly disrupted Aβ transcytosis. Additionally, following apical exposure to the BBB model, we found that apoE4 bound to Aβ is able to penetrate the BBB more readily than apoE3 bound to Aβ and does so via the RAGE (receptor for advanced glycation end products) transporter.

CONCLUSION

These studies indicate a multifaceted, isoform-dependent role for apoE in the exchange of Aβ across the BBB and may partially explain the association of apoE4 and Aβ brain accumulation in AD.

Selective dihydropyiridine compounds facilitate the clearance of β-amyloid across the blood-brain barrier

Increasing evidence suggests that the soluble form of the β-amyloid peptide (Aβ) plays a critical role in the pathogenesis of Alzheimer's disease. Previously, we reported that treatment with certain antihypertensive dihydropyridine (DHP) compounds can mitigate Aβ production in whole cells and reduce brain Aβ burden in a mouse model of Alzheimer's disease. As Aβ clearance across the blood-brain barrier (BBB) is a key regulatory step in the deposition of Aβ in the brain, we examined the effect of DHP treatment on Aβ brain clearance. Treatment with certain DHP compounds significantly increased Aβ(1-42) transcytosis across the BBB in an in vitro model. The rank order of these compounds was nitrendipine>nicardipine=cilnidipine=lercanidipine>nimodipine>azelnidipine=nilvadipine. Conversely, amlodipine, felodipine, isradipine, and nifedipine had no effect on Aβ(1-42) BBB transcytosis. In an in vivo paradigm of Aβ clearance across the BBB, peripheral administration of nitrendipine, cilnidipine, and nilvadipine to wild-type animals facilitated the brain clearance of centrally administered exogenous Aβ(1-42), whereas with amlodipine, there was no effect. We also observed improved cognitive function in mice treated with nilvadipine following central Aβ(1-42) insult. Thus, in addition to the effect of certain DHP compounds on Aβ production, we demonstrate that certain DHP compounds also facilitate the clearance of Aβ across the BBB. This dual mechanism of action may be particularly effective in attenuating Aβ brain burden in Alzheimer's disease and could open the door to a new class of therapies for the treatment of this disease.

Induction of drug efflux protein expression by venlafaxine but not desvenlafaxine

Venlafaxine and its metabolite desvenlafaxine are serotonin-norepinephrine reuptake inhibitors currently prescribed for the treatment of depression. Previously, it was reported that venlafaxine is an inducer of MDR1, the gene responsible for P-glycoprotein (P-gp). The present study expanded upon these findings by examining the effect of venlafaxine and desvenlafaxine on the expression of both P-gp and the breast cancer resistance protein (BCRP) in human brain endothelial cells (HBMEC), an in vitro model of the blood-brain barrier (BBB). The HBMEC were treated for 1 h with various concentrations (500 nM to 50 µM) of venlafaxine and desvenlafaxine. Western blot analysis revealed treatment with venlafaxine significantly induced the expression of P-gp (2-fold) and BCRP (1.75-fold) in a dose-dependent manner, while treatment with desvenlafaxine had no effect on drug efflux transporter expression. To determine the functional significance of this effect, the permeability of a known drug efflux probe, rhodamine 123, across the BBB model and Caco-2 cells, a model of intestinal absorption, were examined. Treatment with venlafaxine (1-50 µM) for 1 h significantly reduced the apical-to-basolateral permeability of R123 across the BBB model (30%) and Caco-2 cell monolayers (25%), indicative of increased drug efflux transporter expression at the apical membrane. Conversely, desvenlafaxine had no effect on R123 permeability in either cellular model. These studies indicate that venlafaxine, but not desvenlafaxine is an inducer of drug efflux transporter expression, which consequently increases the potential for clinical drug-drug interactions. Therefore, based on these preliminary results, caution should be taken when prescribing venlafaxine with other P-gp substrates.

Selective antihypertensive dihydropyridines lower Aβ accumulation by targeting both the production and the clearance of Aβ across the blood-brain barrier

Several large population-based or clinical trial studies have suggested that certain dihydropyridine (DHP) L-type calcium channel blockers (CCBs) used for the treatment of hypertension may confer protection against the development of Alzheimer disease (AD). However, other studies with drugs of the same class have shown no beneficial clinical effects. To determine whether certain DHPs are able to impact underlying disease processes in AD (specifically the accumulation of the Alzheimer Aβ peptide), we investigated the effect of several antihypertensive DHPs and non-DHP CCBs on Aβ production. Among the antihypertensive DHPs tested, a few, including nilvadipine, nitrendipine and amlodipine inhibited Aβ production in vitro, whereas others had no effect or raised Aβ levels. In vivo, nilvadipine and nitrendipine acutely reduced brain Aβ levels in a transgenic mouse model of AD (Tg PS1/APPsw) and improved Aβ clearance across the blood-brain barrier (BBB), whereas amlodipine and nifedipine were ineffective showing that the Aβ-lowering activity of the DHPs is independent of their antihypertensive activity. Chronic oral treatment with nilvadipine decreased Aβ burden in the brains of Tg APPsw (Tg2576) and Tg PS1/APPsw mice, and also improved learning abilities and spatial memory. Our data suggest that the clinical benefit conferred by certain antihypertensive DHPs against AD is unrelated to their antihypertensive activity, but rely on their ability to lower brain Aβ accumulation by affecting both Aβ production and Aβ clearance across the BBB.