Impact of Pesticide Residues on the Gut-Microbiota-Blood-Brain Barrier Axis: A Narrative Review.

Accumulating evidence indicates that chronic exposure to a low level of pesticides found in diet affects the human gut-microbiota-blood-brain barrier (BBB) axis. This axis describes the physiological and bidirectional connection between the microbiota, the intestinal barrier (IB), and the BBB. Preclinical observations reported a gut microbial alteration induced by pesticides, also known as dysbiosis, a condition associated not only with gastrointestinal disorders but also with diseases affecting other distal organs, such as the BBB. However, the interplay between pesticides, microbiota, the IB, and the BBB is still not fully explored. In this review, we first consider the similarities/differences between these two physiological barriers and the different pathways that link the gut microbiota and the BBB to better understand the dialogue between bacteria and the brain. We then discuss the effects of chronic oral pesticide exposure on the gut-microbiota-BBB axis and raise awareness of the danger of chronic exposure, especially during the perinatal period (pregnant women and offspring).

TNF-α and IL-1ß Modulate Blood-Brain Barrier Permeability and Decrease Amyloid-ß Peptide Efflux in a Human Blood-Brain Barrier Model.

The blood-brain barrier (BBB) is a selective barrier and a functional gatekeeper for the central nervous system (CNS), essential for maintaining brain homeostasis. The BBB is composed of specialized brain endothelial cells (BECs) lining the brain capillaries. The tight junctions formed by BECs regulate paracellular transport, whereas transcellular transport is regulated by specialized transporters, pumps and receptors. Cytokine-induced neuroinflammation, such as the tumor necrosis factor-α (TNF-α) and interleukin-1ß (IL-1ß), appear to play a role in BBB dysfunction and contribute to the progression of Alzheimer's disease (AD) by contributing to amyloid-ß (Aß) peptide accumulation. Here, we investigated whether TNF-α and IL-1ß modulate the permeability of the BBB and alter Aß peptide transport across BECs. We used a human BBB in vitro model based on the use of brain-like endothelial cells (BLECs) obtained from endothelial cells derived from CD34+ stem cells cocultivated with brain pericytes. We demonstrated that TNF-α and IL-1ß differentially induced changes in BLECs' permeability by inducing alterations in the organization of junctional complexes as well as in transcelluar trafficking. Further, TNF-α and IL-1ß act directly on BLECs by decreasing LRP1 and BCRP protein expression as well as the specific efflux of Aß peptide. These results provide mechanisms by which CNS inflammation might modulate BBB permeability and promote Aß peptide accumulation. A future therapeutic intervention targeting vascular inflammation at the BBB may have the therapeutic potential to slow down the progression of AD.

Ketone Bodies Promote Amyloid-ß1-40 Clearance in a Human in Vitro Blood-Brain Barrier Model.

Alzheimer's disease (AD) is characterized by the abnormal accumulation of amyloid-ß (Aß) peptides in the brain. The pathological process has not yet been clarified, although dysfunctional transport of Aß across the blood-brain barrier (BBB) appears to be integral to disease development. At present, no effective therapeutic treatment against AD exists, and the adoption of a ketogenic diet (KD) or ketone body (KB) supplements have been investigated as potential new therapeutic approaches. Despite experimental evidence supporting the hypothesis that KBs reduce the Aß load in the AD brain, little information is available about the effect of KBs on BBB and their effect on Aß transport. Therefore, we used a human in vitro BBB model, brain-like endothelial cells (BLECs), to investigate the effect of KBs on the BBB and on Aß transport. Our results show that KBs do not modify BBB integrity and do not cause toxicity to BLECs. Furthermore, the presence of KBs in the culture media was combined with higher MCT1 and GLUT1 protein levels in BLECs. In addition, KBs significantly enhanced the protein levels of LRP1, P-gp, and PICALM, described to be involved in Aß clearance. Finally, the combined use of KBs promotes Aß efflux across the BBB. Inhibition experiments demonstrated the involvement of LRP1 and P-gp in the efflux. This work provides evidence that KBs promote Aß clearance from the brain to blood in addition to exciting perspectives for studying the use of KBs in therapeutic approaches.

ABCA7 Downregulation Modifies Cellular Cholesterol Homeostasis and Decreases Amyloid-ß Peptide Efflux in an in vitro Model of the Blood-Brain Barrier.

The role of ABCA7 in brain homeostasis and Alzheimer's disease (AD) is currently under intense scrutiny, since it has been reported that polymorphisms in the Abca7 gene and a loss of function of the protein are closely linked to excessive accumulation of amyloid peptides and disturbed cholesterol homeostasis. The blood-brain barrier (BBB), which isolates the brain from the blood compartment, is involved in both of these processes. We therefore hypothesized that ABCA7 downregulation might affect cholesterol and amyloid exchanges at the BBB. Using siRNA and primary cultures of mouse endothelial cells purified from brain microvessels and seeded on Transwell ® inserts, we investigated the role of ABCA7 in cholesterol and amyloid exchanges across the BBB. Our results showed that a decrease in ABCA7 expression at the BBB provokes in vitro a reduction in ABCA1 expression and a decrease in APOE secretion. In vitro, these decreases reduce cholesterol exchange across the BBB, particularly for high-density lipoproteins and ApoA-I particles. When ABCA7 was absent, we observed a reduction in Aß peptide basolateral-to-apical transport in the presence of ApoA-I, with non-significant changes in the expression levels of Rage, Lrp1, Abcb1, Abcc1, and Abcg2. Our study in murine BBB model highlighted a putative new role for ABCA7 in AD via the protein's involvement in cholesterol metabolism and amyloid clearance at the BBB.

Bexarotene Promotes Cholesterol Efflux and Restricts Apical-to-Basolateral Transport of Amyloid-ß Peptides in an In Vitro Model of the Human Blood-Brain Barrier.

One of the prime features of Alzheimer's disease (AD) is the excessive accumulation of amyloid-ß (Aß) peptides in the brain. Several recent studies suggest that this phenomenon results from the dysregulation of cholesterol homeostasis in the brain and impaired bidirectional Aß exchange between blood and brain. These mechanisms appear to be closely related and are controlled by the blood-brain barrier (BBB) at the brain microvessel level. In animal models of AD, the anticancer drug bexarotene (a retinoid X receptor agonist) has been found to restore cognitive functions and decrease the brain amyloid burden by regulating cholesterol homeostasis. However, the drug's therapeutic effect is subject to debate and the exact mechanism of action has not been characterized. Therefore, the objective of this present study was to determine bexarotene's effects on the BBB. Using an in vitro model of the human BBB, we investigated the drug's effects on cholesterol exchange between abluminal and luminal compartments and the apical-to-basolateral transport of Aß peptides across the BBB. Our results demonstrated that bexarotene induces the expression of ABCA1 but not ApoE. This upregulation correlates with an increase in ApoE2-, ApoE4-, ApoA-I-, and HDL-mediated cholesterol efflux. Regarding the transport of Aß peptides, bexarotene increases the expression of ABCB1, which in turn decreases Aß apical-to-basolateral transport. Our results showed that bexarotene not only promotes the cholesterol exchange between the brain and the blood but also decreases the influx of Aß peptides across BBB, suggesting that bexarotene is a promising drug candidate for the treatment of AD.

In vitro discrimination of the role of LRP1 at the BBB cellular level: focus on brain capillary endothelial cells and brain pericytes.

Several studies have demonstrated that the blood-brain barrier (BBB) (dynamic cellular complex composed by brain capillary endothelial cells (BCECs) and surrounded by astrocytic end feet and pericytes) regulates the exchanges of amyloid ß (Aß) peptide between the blood and the brain. Deregulation of these exchanges seems to be a key trigger for the brain accumulation of Aß peptide observed in Alzheimer's disease (AD). Whereas the involvement of receptor for advanced glycation end-products in Aß peptide transcytosis has been demonstrated in our laboratory, low-density lipoprotein receptor's role at the cellular level needs to be clarified. For this, we used an in vitro BBB model that consists of a co-culture of bovine BCECs and rat glial cells. This model has already been used to characterize low-density lipoprotein receptor-related peptide (LRP)'s involvement in the transcytosis of molecules such as tPA and angiopep-2. Our results suggest that Aß peptide efflux across the BCEC monolayer involves a transcellular transport. However, the experiments with RAP discard an involvement of LRP family members at BCECs level. In contrast, our results show a strong transcriptional expression of LRP1 in pericytes and suggest its implication in Aß endocytosis. Moreover, the observations of pericytes contraction and local downregulation of LRP1 in response to Aß treatment opens up perspectives for studying this cell type with respect to Aß peptide metabolism and AD.

Amyloid-ß peptides, Alzheimer's disease and the blood-brain barrier.

Ever since amyloid-ß (Aß) peptides were first identified in cerebral plaques in patients with Alzheimer's disease (AD), much research work has focused on the complex mechanisms through which these peptides are synthesized, transported and degraded. Although new information emerges on a regular basis, we consider that the importance of the blood-brain barrier (BBB) in the pathogenesis of AD has been underestimated. In fact, there are a number of obstacles that make it difficult to convince specialists in AD that the BBB indeed plays a key role in this disease: these include the complex physiology of the BBB and the technical difficulty of studying the barrier in vivo and reproducing its main properties in vitro. With these considerations in mind, the present review sets out summarize our current knowledge about the physiology of the BBB and describe recent research findings on the barrier's role in Aß peptide proteostasis and thus in the mechanism of AD.

Oxysterols decrease apical-to-basolateral transport of Aß peptides via an ABCB1-mediated process in an in vitro Blood-brain barrier model constituted of bovine brain capillary endothelial cells.

It is known that activation of the liver X receptors (LXRs) by natural or synthetic agonists decreases the amyloid burden and enhances cognitive function in transgenic murine models of Alzheimer's disease (AD). Recent evidence suggests that LXR activation may affect the transport of amyloid ß (Aß) peptides across the blood-brain barrier (the BBB, which isolates the brain from the peripheral circulation). By using a well-characterized in vitro BBB model, we demonstrated that LXR agonists (24S-hydroxycholesterol, 27-hydroxycholesterol and T0901317) modulated the expression of target genes involved in cholesterol homeostasis (such as ATP-binding cassette sub-family A member 1 (ABCA1)) and promoted cellular cholesterol efflux to apolipoprotein A-I and high density lipoproteins. Interestingly, we also observed a decrease in Aß peptide influx across brain capillary endothelial cells, although ABCA1 did not appear to be directly involved in this process. By focusing on others receptors and transporters that are thought to have major roles in Aß peptide entry into the brain, we then demonstrated that LXR stimulation provoked an increase in expression of the ABCB1 transporter (also named P-glycoprotein (P-gp)). Further investigations confirmed ABCB1's involvement in the restriction of Aß peptide influx. Taken as a whole, our results not only reinforce the BBB's key role in cerebral cholesterol homeostasis but also demonstrate the importance of the LXR/ABCB1 axis in Aß peptide influx-highlighting an attractive new therapeutic approach whereby the brain could be protected from peripheral Aß peptide entry.

Brain pericytes ABCA1 expression mediates cholesterol efflux but not cellular amyloid-ß peptide accumulation.

In brain, excess cholesterol is metabolized into 24S-hydroxycholesterol (24S-OH-chol) and eliminated into the circulation across the blood-brain barrier. 24S-OH-chol is a natural agonist of the nuclear liver X receptors (LXRs) involved in peripheral cholesterol homeostasis. The effects of this oxysterol on the pericytes embedded in the basal lamina of this barrier (close to the brain compartment) have not been previously studied. We used primary cultures of brain pericytes to demonstrate that the latter express LXR nuclear receptors and their target gene ATP-binding cassette, sub-family A, member 1 (ABCA1), known to be one of the major transporters involved in peripheral lipid homeostasis. Treatment with 24S-OH-chol caused an increase in ABCA1 expression that correlated with a reverse cholesterol transfer to apolipoprotein E, apolipoprotein A-I, and high density lipoprotein particles. Inhibition of ABCA1 decreased this efflux. As pericytes are able to internalize the amyloid-ß peptides which accumulate in brain of Alzheimer's disease patients, we then investigated the effects of 24S-OH-chol on this process. We found that the cellular accumulation process was not modified by 24S-OH-chol treatment. Overall, our results highlight the importance of the LXR/ABCA1 system in brain pericytes and suggest a new role for these cells in brain cholesterol homeostasis.

[Role of the blood-brain barrier in Alzheimer's disease]. / La barrière hémato-encéphalique - Une nouvelle cible thérapeutique dans la maladie d'Alzheimer ?

The blood-brain barrier (BBB), which isolates the brain from the whole body, restricts exchanges between the brain and the peripheral compartments. Several studies highlight the importance of this barrier in neurodegenerative diseases such as Alzheimer's disease (AD). This pathology is characterized by an abnormal accumulation and aggregation of Aß peptides which contribute to the neurodegenerative processes and the BBB seems to play a key role for the brain Aß peptides metabolism. This review focuses on recent data demonstrating the important role of the BBB in AD, suggesting that this barrier is a possible new therapeutic target in this disease.

Apical-to-basolateral transport of amyloid-ß peptides through blood-brain barrier cells is mediated by the receptor for advanced glycation end-products and is restricted by P-glycoprotein.

Several studies have highlighted the close relationship between Alzheimer's disease (AD) and alterations in the bidirectional transport of amyloid-ß (Aß) peptides across the blood-brain barrier (BBB). The brain capillary endothelial cells (BCECs) that compose the BBB express the receptors and transporters that enable this transport process. There is significant in vivo evidence to suggest that P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) restrict Aß peptides entry into the brain, whereas the receptor for advanced glycation end-products (RAGE) seems to mediate apical-to-basolateral passage across the BBB. However, deciphering the molecular mechanisms underlying these in vivo processes requires further in vitro characterization. Using an in vitro BBB model and specific competition experiments against RAGE, we have observed a significant decrease in apical-to-basolateral (but not basolateral-to-apical) transport of Aß1-40 and Aß1-42 peptides through BCECs. This transport is a caveolae-dependent process and fits with the apical location of RAGE observed in confocal microscopy experiments. Inhibition of P-gp and BCRP using different inhibitors increases transport of Aß peptides suggesting that these efflux pumps are involved in Aß peptide transport at the BCECs level. Taken as a whole, these results demonstrate the involvement of the caveolae-dependent transcytosis of Aß peptides through the BBB in a RAGE-mediated transport process, reinforcing the hypothesis whereby this receptor is a potential drug target in AD.

Transcriptional profiles of receptors and transporters involved in brain cholesterol homeostasis at the blood-brain barrier: use of an in vitro model.

Brain is the most cholesterol rich organ of the whole body and recent studies suggest a role for the blood-brain barrier (BBB) in cerebral cholesterol homeostasis. Low density lipoprotein receptor (LDLR)-related receptors and ATP-binding Cassette (ABC) transporters play an important role in peripheral sterol homeostasis. The purpose of this study was to determine the mRNA expression profiles of ABC transporters (ABCA1, 2, 3, 7 and ABCG1) and low density lipoprotein receptor (LDLR)-related receptors (LDLR, vLDLR, LRP1, LRP2 and LRP8) in BBB endothelium using an in vitro co-culture model of bovine brain capillary endothelial cells (BCECs) and rat glial cells. All transcripts tested are expressed by BCECs and in capillary extract, except vLDLR. Glial cells influence ABCG1, A1, 2, 7 and LRP1 transcription, suggesting a role in cerebral lipid supply/elimination through the modulation of the expression of these transporters and receptors by these cells. Altogether, these results highlight the importance of glial input in the BBB transport phenotype for sterol homeostasis in the central nervous system, and confirm the importance of the BBB in this process.

Physiological pathway for low-density lipoproteins across the blood-brain barrier: transcytosis through brain capillary endothelial cells in vitro.

Although an immense knowledge has accumulated concerning regulation of cholesterol homeostasis in the body, this does not include the brain, where details are just emerging. Using an in vitro blood-brain barrier model, the authors have demonstrated that low-density lipoprotein (LDL) underwent transcytosis through the endothelial cells (ECs) by a receptor-mediated process, bypassing the lysosomal compartment. Moreover, caveolae might be involved in these blood-borne molecule transports from the blood to the brain. Although several ligands are known to be internalized through cell surface caveolae, the subsequent intracellular pathways have remained elusive. By cell fractionation experiment and Western blot, the authors have demonstrated that the LDL receptor is located in the caveolae membrane fraction. Then, LDLs internalized were detected by electron microscopy in multivesicular bodies. The authors identified in brain capillary ECs a novel endosomal compartment, mildly acidic, positive for marker Lamp-1 but devoid of any degradative capability. From the point of view of pH, cellular location, and caveolae-derived formation, the multivesicular organelles described here can be related to the caveosome structure. These results could provide clues to physiological functions of caveolae-caveosome transcellular pathway in brain capillary ECs and may help in the rational design of more effective therapeutic drugs to the brain.