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Background: Age is the most common risk factor for Alzheimer’s disease (AD), a neurodegenerative disorder characterized by the hallmarks of toxic amyloid-β (Aβ) plaques and hyperphosphorylated tau tangles. Moreover, sub-physiological brain insulin levels have emerged as a pathological manifestation of AD. Objective: Identify age-related changes in the plasma disposition and blood-brain barrier (BBB) trafficking of Aβ peptides and insulin in mice. Methods: Upon systemic injection of 125I-Aβ40, 125I-Aβ42, or 125I-insulin, the plasma pharmacokinetics and brain influx were assessed in wild-type (WT) or AD transgenic (APP/PS1) mice at various ages. Additionally, publicly available single-cell RNA-Seq data [GSE129788] was employed to investigate pathways regulating BBB transport in WT mice at different ages. Results: The brain influx of 125I-Aβ40, estimated as the permeability-surface area product, decreased with age, accompanied by an increase in plasma AUC. In contrast, the brain influx of 125I-Aβ42 increased with age, accompanied by a decrease in plasma AUC. The age-dependent changes observed in WT mice were accelerated in APP/PS1 mice. As seen with 125I-Aβ40, the brain influx of 125I-insulin decreased with age in WT mice, accompanied by an increase in plasma AUC. This finding was further supported by dynamic single-photon emission computed tomography (SPECT/CT) imaging studies. RAGE and PI3K/AKT signaling pathways at the BBB, which are implicated in Aβ and insulin transcytosis, respectively, were upregulated with age in WT mice, indicating BBB insulin resistance. Conclusion: Aging differentially affects the plasma pharmacokinetics and brain influx of Aβ isoforms and insulin in a manner that could potentially augment AD risk.

Disruptions in glucose uptake and metabolism in the brain are implicated in metabolic disorders and Alzheimer’s disease (AD). Toxic soluble amyloid-beta (sAß) peptides accumulating in the brain and plasma of AD patients were suggested to promote blood-brain barrier (BBB) dysfunction, brain hypometabolism, and cognitive decline. Exposure to sAß peptides is reported to interfere with glucose metabolism in the brain parenchyma, although their effects on the BBB have not been fully characterized. Our data showed that the brain uptake of glucose surrogate, [18F]-fluorodeoxyglucose (18FDG), was reduced significantly in APP/PS1 transgenic mice (overproduce Aß) compared to wild-type (WT) mice. In addition, the influx rate of18FDG was also decreased in both Aß40 and Aß42 pre-infused mice compared to control mice. Glucose is primarily delivered from blood into the brain via glucose transporter 1 (GLUT1). The confocal microscopy experiment showed that Aß40 and Aß42 peptides significantly decreased GLUT1 expression in polarized human cerebral microvascular endothelial cell (hCMEC/D3) monolayers. Insulin-AKT pathway has been observed to induce glucose uptake via regulating the expression of TXNIP, the only α-arrestin protein known to bind to thioredoxin. We found that Aß40 and Aß42 peptides decreased p-AKT and increased TXNIP expression in the hCMEC/D3 cell monolayers. MK2206, a kinase inhibitor of AKT, was used to confirm that inhibition of insulin/AKT pathway reduced GLUT1 expression in an insulin-independent manner in the hCMEC/D3 cell monolayers. These results suggest that inhibitory effects of sAß on GLUT1 expression are mediated by inhibition of the insulin/AKT pathway. The role of TXNIP on endothelial GLUT1 expression was investigated using resveratrol, which has been reported to downregulate TXNIP overexpression. Consistently, resveratrol treatment led to a significant increase in GLUT1 expression in the hCMEC/D3 cell monolayers. Furthermore, by co-incubation of resveratrol and sAß peptides in hCMEC/D3 cell monolayers, we found that resveratrol rectified the aberrant TXNIP expression caused by sAß peptides. Together, these findings provide novel evidence that toxic sAß peptide exposure inhibits glucose transport at the BBB by decreasing GLUT1 expression via the insulin/Akt/TXNIP axis.

Blood-brain barrier (BBB) is a critical portal regulating the bidirectional transport of amyloid beta (Aβ) proteins between blood and brain. Disrupted trafficking at the BBB may not only promote the build-up of Aβ plaques in the brain parenchyma, but also facilitate Aβ accumulation within the BBB endothelium, which aggravates BBB dysfunction. Soluble Aβ42:Aβ40 ratios in plasma and cerebrospinal fluid have been reported to decrease during Alzheimer’s disease (AD) progression. Our previous publications demonstrated that trafficking of Aβ42 and Aβ40 at the BBB is distinct and is disrupted under various pathophysiological conditions. However, the intracellular mechanisms that allow BBB endothelium to differentially handle Aβ40 and Aβ42 have not been clearly elucidated. In this study, we identified mechanisms of fluorescently labeled Aβ (F-Aβ) endocytosis in polarized human cerebral microvascular endothelial (hCMEC/D3) cell monolayers using pharmacological inhibition and siRNA knock-down approaches. Further, intracellular transit of F-Aβ following endocytosis was tracked using live cell imaging. Our studies demonstrated that both F-Aβ peptides were internalized by BBB endothelial cells via energy, dynamin and actin dependent endocytosis. Interestingly, endocytosis of F-Aβ40 is found to be clathrin-mediated, whereas F-Aβ42 endocytosis is caveolae-mediated. Following endocytosis, both isoforms were sorted by the endo-lysosomal system. While Aβ42 was shown to accumulate more in the lysosome which could lead to its higher degradation and/or aggregation at lower lysosomal pH, Aβ40 demonstrated robust accumulation in recycling endosomes which may facilitate its transcytosis across the BBB. These results provide a mechanistic insight into the selective ability of BBB endothelium to transport Aβ40 versus Aβ42. This knowledge contributes to the understanding of molecular pathways underlying Aβ accumulation in the BBB endothelium and associated cerebrovascular dysfunction as well as amyloid deposition in the brain parenchyma which are implicated in AD pathogenesis.

Plasma pharmacokinetic (PK) data is required as an input function for graphical analysis (e.g., Patlak plot) of single positron emission computed tomography/computed tomography (SPECT/CT) and positron emission tomography/CT (PET/CT) data to evaluate tissue influx rate of radiotracers. Dynamic heart imaging data is often used as a surrogate of plasma PK. However, accumulation of radiolabel (representing both intact and degraded tracer) in the heart tissue may interfere with accurate prediction of plasma PK from the heart data. Therefore, we developed a compartmental model, which involves forcing functions to describe intact and degraded radiolabeled proteins in plasma and their accumulation in heart tissue, to deconvolve plasma PK of125I-amyloid beta 40 (125I-Aβ40) and125I-insulin from their dynamic heart imaging data. The three-compartment model was shown to adequately describe the plasma concentration-time profile of intact/degraded proteins and the heart radioactivity time data obtained from SPECT/CT imaging for both tracers. The model was successfully applied to deconvolve the plasma PK of both tracers from their naïve datasets of dynamic heart imaging. In agreement with our previous observations made by conventional serial plasma sampling, the deconvolved plasma PK of125I-Aβ40and125I-insulin in young mice exhibited lower area under the curve (AUC) than the aged mice. Further, Patlak plot parameters (Ki) extracted using deconvolved plasma PK as input function successfully recapitulated age-dependent blood-to-brain influx kinetics changes for both125I-Aβ40and125I-insulin. Therefore, the compartment model developed in this study provides a novel approach to deconvolve plasma PK of radiotracers from their noninvasive dynamic heart imaging. This method facilitates the application of preclinical SPECT or PET imaging data to characterize distribution kinetics of tracers where simultaneous plasma sampling is not feasible.

The compositionally distinct lipid rafts present in the plasma membrane regulate the restrictive trafficking and signal transduction in the blood-brain barrier (BBB) endothelium. Several metabolic and neurodegenerative diseases are associated with lipid homeostasis disruption within the BBB endothelium. Here, we hypothesized that the delivery of lipid triglyceride based nanoemulsions containing unsaturated fatty acids (UFAs) provides a novel non-pharmacological approach to modulate lipid raft integrity and rectify the aberrant trafficking and signal transduction. The current study has shown that soybean oil nanoemulsions (SNEs) altered the morphology of lipid rafts that are stained by Alex Fluor 647 labelled cholera toxin (AF647-CTX) in polarized human cerebral microvascular endothelial (hCMEC/D3) cell monolayers. Moreover, western blot and flow cytometry analysis showed that SNEs containing polyunsaturated fatty acids (PUFAs) increased phospo-AKT (p-AKT) expression, a marker for the stimulation of metabolic arm of insulin signaling, and insulin uptake in hCMEC/D3 monolayers. However, olive oil nanoemulsions (ONEs) containing monounsaturated fatty acids (MUFAs) had no detectable impact on lipid raft integrity, AKT phosphorylation, or insulin uptake. These findings provided direct evidence that SNEs containing PUFAs can upregulate insulin-pAKT pathway, facilitate insulin trafficking at the BBB, and potentially address cerebrovascular dysfunction in metabolic and neurodegenerative diseases.