The proteome of the blood-brain barrier in rat and mouse: highly specific identification of proteins on the luminal surface of brain microvessels by in vivo glycocapture.

BACKGROUND: The active transport of molecules into the brain from blood is regulated by receptors, transporters, and other cell surface proteins that are present on the luminal surface of endothelial cells at the blood-brain barrier (BBB). However, proteomic profiling of proteins present on the luminal endothelial cell surface of the BBB has proven challenging due to difficulty in labelling these proteins in a way that allows efficient purification of these relatively low abundance cell surface proteins. METHODS: Here we describe a novel perfusion-based labelling workflow: in vivo glycocapture. This workflow relies on the oxidation of glycans present on the luminal vessel surface via perfusion of a mild oxidizing agent, followed by subsequent isolation of glycoproteins by covalent linkage of their oxidized glycans to hydrazide beads. Mass spectrometry-based identification of the isolated proteins enables high-confidence identification of endothelial cell surface proteins in rats and mice. RESULTS: Using the developed workflow, 347 proteins were identified from the BBB in rat and 224 proteins in mouse, for a total of 395 proteins in both species combined. These proteins included many proteins with transporter activity (73 proteins), cell adhesion proteins (47 proteins), and transmembrane signal receptors (31 proteins). To identify proteins that are enriched in vessels relative to the entire brain, we established a vessel-enrichment score and showed that proteins with a high vessel-enrichment score are involved in vascular development functions, binding to integrins, and cell adhesion. Using publicly-available single-cell RNAseq data, we show that the proteins identified by in vivo glycocapture were more likely to be detected by scRNAseq in endothelial cells than in any other cell type. Furthermore, nearly 50% of the genes encoding cell-surface proteins that were detected by scRNAseq in endothelial cells were also identified by in vivo glycocapture. CONCLUSIONS: The proteins identified by in vivo glycocapture in this work represent the most complete and specific profiling of proteins on the luminal BBB surface to date. The identified proteins reflect possible targets for the development of antibodies to improve the crossing of therapeutic proteins into the brain and will contribute to our further understanding of BBB transport mechanisms.

Targeting insulin-like growth factor-1 receptor (IGF1R) for brain delivery of biologics.

The blood-brain barrier (BBB) prevents the majority of drugs from crossing into the brain and reaching neurons. To overcome this challenge, safe and non-invasive technologies targeting receptor-mediated pathways have been developed. In this study, three single-domain antibodies (sdAbs; IGF1R3, IGF1R4, and IGF1R5) targeting the extracellular domain of the human insulin-like growth factor-1 receptor (IGF1R), generated by llama immunization, showed enhanced transmigration across the rat BBB model (SV-ARBEC) in vitro. The rate of brain uptake of these sdAbs fused to mouse Fc (sdAb-mFc) in vivo was estimated using the fluorescent in situ brain perfusion (ISBP) technique followed by optical brain imaging and distribution volume evaluation. Compared to the brains perfused with the negative control A20.1-mFc, the brains perfused with anti-IGF1R sdAbs showed a significant increase of the total fluorescence intensity (~2-fold, p < .01) and the distribution volume (~4-fold, p < .01). The concentration curve for IGF1R4-mFc demonstrated a linear accumulation plateauing at approximately 400 µg (~1 µM), suggesting a saturable mechanism of transport. Capillary depletion and mass spectrometry analyses of brain parenchyma post-ISBP confirmed the IGF1R4-mFc brain uptake with ~25% of the total amount being accumulated in the parenchymal fraction in contrast to undetectable levels of A20.1-mFc after a 5-min perfusion protocol. Systemic administration of IGF1R4-mFc fused with the non-BBB crossing analgesic peptide galanin (2 and 5 mg/kg) induced a dose-dependent suppression of thermal hyperalgesia in the Hargreaves pain model. In conclusion, novel anti-IGF1R sdAbs showed receptor-mediated brain uptake with pharmacologically effective parenchymal delivery of non-permeable neuroactive peptides.

Label-free LC-HRMS-based enzymatic activity assay for the detection of DDC, MAO and COMT inhibitors.

Pharmacological treatment of Parkinson's disease consists of a combined chemotherapy that mostly relies on levodopa (L-DOPA) administration together with inhibitors of dopa-decarboxylase (DDC), monoamine oxidase (MAO) and catechol-methyltransferase (COMT). Identification of inhibitors specifically targeting these enzymes is still a significative part of the development of new alternative antiparkinsonian drugs. Most of the available methods use measurement of enzymatic reactions through radioactive labeling, antibody-recognized products or coupled enzymatic assays. Mass spectrometry (MS) represents an interesting alternative approach as it allows direct and specific detection and quantification of enzymatic reactions. We describe the development of a simple, reliable, label-free assay based on high-resolution mass spectrometry (HRMS) for the detection and relative quantification of three different enzymatic reactions using non-isolated enzymes. The assay was applied both to reference drugs and plant crude extracts. This method can be used to detect hits in extracts libraries as well as determine relative IC50 of inhibitors.

Transferrin Receptor-Mediated Uptake at the Blood-Brain Barrier Is Not Impaired by Alzheimer's Disease Neuropathology.

The transferrin receptor (TfR) is highly expressed by brain capillary endothelial cells (BCECs) forming the blood-brain barrier (BBB) and is therefore considered as a potential target for brain drug delivery. Monoclonal antibodies binding to the TfR, such as clone Ri7, have been shown to internalize into BCECs in vivo. However, since Alzheimer's disease (AD) is accompanied by a BBB dysfunction, it raises concerns about whether TfR-mediated transport becomes inefficient during the progression of the disease. Measurements of TfR levels using Western blot analysis in whole homogenates from human post-mortem parietal cortex and hippocampus did not reveal any significant difference between individuals with or without a neuropathological diagnosis of AD (respectively, n = 19 and 22 for the parietal cortex and n = 12 and 14 for hippocampus). Similarly, TfR concentrations in isolated human brain microvessels from parietal cortex were similar between controls and AD cases. TfR levels in isolated murine brain microvessels were not significantly different between groups of 12- and 18-month-old NonTg and 3xTg-AD mice, the latter modeling Aß and τ neuropathologies. In situ brain perfusion assays were then conducted to measure the brain uptake and internalization of fluorolabeled Ri7 in BCECs upon binding. Consistently, TfR-mediated uptake in BCECs was similar between 3xTg-AD mice and nontransgenic controls (∼0.3 µL·g-1·s-1) at 12, 18, and 22 months of age. Fluorescence microscopy analysis following intravenous administration of fluorolabeled Ri7 highlighted that the signal from the antibody was widely distributed throughout the cerebral vasculature but not in neurons or astrocytes. Overall, our data suggest that both TfR protein levels and TfR-dependent internalization mechanisms are preserved in the presence of Aß and τ neuropathologies, supporting the potential of TfR as a vector target for drug delivery into BCECs in AD.

Identification of steroidal derivatives inhibiting the transformations of allopregnanolone and estradiol by 17ß-hydroxysteroid dehydrogenase type 10.

17ß-Hydroxysteroid dehydrogenase type 10 (17ß-HSD10) is a mitochondrial enzyme known for its potential role in Alzheimer's Disease (AD). 17ß-HSD10, by its oxidative activity, could decrease the concentration of two important neurosteroids, allopregnanolone (ALLOP) and 17ß-estradiol (E2), respectively preventing their neurogenesis and neuroprotective effects. Since the inhibition of 17ß-HSD10 could lead to a new treatment for AD, we developed two biological assays using labeled ALLOP or E2 as substrates to measure the inhibitory activity of compounds against pure 17ß-HSD10 protein. After the optimization of different parameters (time, concentration of enzyme, substrate and cofactor), analogs of the first reported steroidal inhibitor of 17ß-HSD10 in intact cells were screened to determine their inhibitory potency for the ALLOP or the E2 oxidation. One compound, androstane derivative 5, possesses the best dual inhibition against both transformations (ALLOP, IC50 = 235 µM and E2, IC50 = 610 µM). Some compounds are dual inhibitors to a lesser extent, and others seem selective for one of the transformations in particular. By developing two reliable assays and by identifying a first generation of steroidal inhibitors of pure 17ß-HSD10, this preliminary study opens the door to new and more potent inhibitors.

Age-Dependent Regulation of the Blood-Brain Barrier Influx/Efflux Equilibrium of Amyloid-ß Peptide in a Mouse Model of Alzheimer's Disease (3xTg-AD).

The involvement of transporters located at the blood-brain barrier (BBB) has been suggested in the control of cerebral Aß levels, and thereby in Alzheimer's disease (AD). However, little is known about the regulation of these transporters at the BBB in animal models of AD. In this study, we investigated the BBB expression of Aß influx (Rage) and efflux (Abcb1-Abcg2-Abcg4-Lrp-1) transporters and cholesterol transporter (Abca1) in 3-18-month-old 3xTg-AD and control mice. The age-dependent effect of BBB transporters regulation on the brain uptake clearance (Clup) of [3H]cholesterol and [3H]Aß1 - 40 was then evaluated in these mice, using the in situ brain perfusion technique. Our data suggest that transgenes expression led to the BBB increase in Aß influx receptor (Rage) and decrease in efflux receptor (Lrp-1). Our data also indicate that mice have mechanisms counteracting this increased net influx. Indeed, Abcg4 and Abca1 are up regulated in 3- and 3/6-month-old 3xTg-AD mice, respectively. Our data show that the balance between the BBB influx and efflux of Aß is maintained in 3 and 6-month-old 3xTg-AD mice, suggesting that Abcg4 and Abca1 control the efflux of Aß through the BBB by a direct (Abcg4) or indirect (Abca1) mechanism. At 18 months, the BBB Aß efflux is significantly increased in 3xTg-AD mice compared to controls. This could result from the significant up-regulation of both Abcg2 and Abcb1 in 3xTg-AD mice compared to control mice. Thus, age-dependent regulation of several Aß and cholesterol transporters at the BBB could ultimately limit the brain accumulation of Aß.

Cerebrovascular and blood-brain barrier impairments in Huntington's disease: Potential implications for its pathophysiology.

OBJECTIVE: Although the underlying cause of Huntington's disease (HD) is well established, the actual pathophysiological processes involved remain to be fully elucidated. In other proteinopathies such as Alzheimer's and Parkinson's diseases, there is evidence for impairments of the cerebral vasculature as well as the blood-brain barrier (BBB), which have been suggested to contribute to their pathophysiology. We investigated whether similar changes are also present in HD. METHODS: We used 3- and 7-Tesla magnetic resonance imaging as well as postmortem tissue analyses to assess blood vessel impairments in HD patients. Our findings were further investigated in the R6/2 mouse model using in situ cerebral perfusion, histological analysis, Western blotting, as well as transmission and scanning electron microscopy. RESULTS: We found mutant huntingtin protein (mHtt) aggregates to be present in all major components of the neurovascular unit of both R6/2 mice and HD patients. This was accompanied by an increase in blood vessel density, a reduction in blood vessel diameter, as well as BBB leakage in the striatum of R6/2 mice, which correlated with a reduced expression of tight junction-associated proteins and increased numbers of transcytotic vesicles, which occasionally contained mHtt aggregates. We confirmed the existence of similar vascular and BBB changes in HD patients. INTERPRETATION: Taken together, our results provide evidence for alterations in the cerebral vasculature in HD leading to BBB leakage, both in the R6/2 mouse model and in HD patients, a phenomenon that may, in turn, have important pathophysiological implications.

Human apolipoprotein E ɛ4 expression impairs cerebral vascularization and blood-brain barrier function in mice.

Human apolipoprotein E (APOE) exists in three isoforms ɛ2, ɛ3, and ɛ4, of which APOE4 is the main genetic risk factor of Alzheimer's disease (AD). As cerebrovascular defects are associated with AD, we tested whether APOE genotype has an impact on the integrity and function of the blood-brain barrier (BBB) in human APOE-targeted replacement mice. Using the quantitative in situ brain perfusion technique, we first found lower (13.0% and 17.0%) brain transport coefficient (Clup) of [(3)H]-diazepam in APOE4 mice at 4 and 12 months, compared with APOE2 and APOE3 mice, reflecting a decrease in cerebral vascularization. Accordingly, results from immunohistofluorescence experiments revealed a structurally reduced cerebral vascularization (26% and 38%) and thinner basement membranes (30% and 35%) in 12-month-old APOE4 mice compared with APOE2 and APOE3 mice, suggesting vascular atrophy. In addition, APOE4 mice displayed a 29% reduction in [(3)H]-d-glucose transport through the BBB compared with APOE2 mice without significant changes in the expression of its transporter GLUT1 in brain capillaries. However, an increase of 41.3% of receptor for advanced glycation end products (RAGE) was found in brain capillaries of 12-month-old APOE4 mice. In conclusion, profound divergences were observed between APOE genotypes at the cerebrovascular interface, suggesting that APOE4-induced BBB anomalies may contribute to AD development.

Apolipoprotein E isoforms disrupt long-chain fatty acid distribution in the plasma, the liver and the adipose tissue of mice.

Evidences suggest that omega-3 fatty acid (n-3 PUFA) metabolism is imbalanced in apolipoprotein E epsilon 4 isoform carriers (APOE4). This study aimed to investigate APOE genotype-dependant modulation of FA profiles, protein and enzyme important to fatty acid (FA) metabolism in the adipose tissue, the liver and the plasma using human APOE-targeted replacement mouse-model (N=37). FA transport (FATP) and binding (FABP) protein levels in tissues and concentrations of liver carnitine palmitoyltransferase 1 (CPT1) were performed. N-3 PUFA concentration was >45% lower in the adipose tissue and liver of APOE4 mice compared to APOE3 mice. In APOE4 mice, there were higher levels of FATP and FABP in the liver and higher FATP in the adipose tissue compared to APOE2 mice. There was a trend towards higher CPT1 concentrations in APOE4 mice compared to APOE3 mice. Therefore, since APOE-isoform differences were not always in line with the unbalanced n-3 PUFA profiles in organs, other proteins may be involved in maintaining n-3 PUFA homeostasis in mice with different APOE-isoforms.

Altered cerebral vascular volumes and solute transport at the blood-brain barriers of two transgenic mouse models of Alzheimer's disease.

We evaluated the integrity and function of the blood-brain barrier in 3xTg-AD mice aged 3-18 months and in APP/PS1 mice aged 8-months to determine the impacts of changes in amyloid and tau proteins on the brain vascular changes. The vascular volume (Vvasc) was sub-normal in 3xTg-AD mice aged from 6 to 18 months, but not in the APP/PS1 mice. The uptakes of [(3)H]-diazepam by the brains of 3xTg-AD, APP/PS1 and their age-matched control mice were similar at all the times studied, suggesting that the simple diffusion of small solutes is unchanged in transgenic animals. The uptake of d-glucose by the brains of 18-month old 3xTg-AD mice, but not by those of 8-month old APP/PS1 mice, was reduced compared to their age-matched controls. Accordingly, the amount of Glut-1 protein was 1.4 times lower in the brain capillaries of 18 month-old 3xTg-AD mice than in those of age-matched control mice. We conclude that the brain vascular volume is reduced early in 3xTg-AD mice, 6 months before the appearance of pathological lesions, and that this reduction persists until they are at least 18 months old. The absence of alterations in the BBB of APP/PS1 mice suggests that hyperphosphorylated tau proteins contribute to the vascular changes that occur in AD.

Brain uptake of a fluorescent vector targeting the transferrin receptor: a novel application of in situ brain perfusion.

Monoclonal antibodies (mAbs) targeting blood-brain barrier (BBB) transporters are being developed for brain drug targeting. However, brain uptake quantification remains a challenge, particularly for large compounds, and often requires the use of radioactivity. In this work, we adapted an in situ brain perfusion technique for a fluorescent mAb raised against the mouse transferrin receptor (TfR) (clone Ri7). We first confirmed in vitro that the internalization of fluorolabeled Ri7 mAbs is saturable and dependent on the TfR in N2A and bEnd5 cells. We next showed that the brain uptake coefficient (Clup) of 100 µg (∼220 nM) of Ri7 mAbs fluorolabeled with Alexa Fluor 750 (AF750) was 0.27 ± 0.05 µL g(-1) s(-1) after subtraction of values obtained with a control IgG. A linear relationship was observed between the distribution volume VD (µL g(-1)) and the perfusion time (s) over 30-120 s (r(2) = 0.997), confirming the metabolic stability of the AF750-Ri7 mAbs during perfusion. Co-perfusion of increasing quantities of unlabeled Ri7 decreased the AF750-Ri7 Clup down to control IgG levels over 500 nM, consistent with a saturable mechanism. Fluorescence microscopy analysis showed a vascular distribution of perfused AF750-Ri7 in the brain and colocalization with a marker of basal lamina. To our knowledge, this is the first reported use of the in situ brain perfusion technique combined with quantification of compounds labeled with near-infrared fluorophores. Furthermore, this study confirms the accumulation of the antitransferrin receptor Ri7 mAb in the brain of mice through a saturable uptake mechanism.

Reduction in DHA transport to the brain of mice expressing human APOE4 compared to APOE2.

Benefits on cognition from docosahexaenoic acid (DHA, 22 : 6 n-3) intake are absent in humans carrying apolipoprotein E ε4 allele (APOE4), the most important genetic risk factor for Alzheimer's disease (AD). To test the hypothesis that carrying APOE4 impairs DHA distribution, we evaluated plasma and brain fatty acid profiles and uptake of [(14) C]-DHA using in situ cerebral perfusion through the blood-brain barrier in 4- and 13-month-old male and female APOE-targeted replacement mice (APOE2, APOE3, and APOE4), fed with a DHA-depleted diet. Cortical and plasma DHA were 9% lower and 34% higher in APOE4 compared to APOE2 mice, respectively. Brain uptake of [(14) C]-DHA was 24% lower in APOE4 versus APOE2 mice. A significant relationship was established between DHA and apoE concentrations in the cortex of mice (r(2) = 0.21) and AD patients (r(2) = 0.32). Altogether, our results suggest that lower brain uptake of DHA in APOE4 than in APOE2 mice may limit the accumulation of DHA in cerebral tissues. These data provide a mechanistic explanation for the lack of benefit of DHA in APOE4 carriers on cognitive function and the risk of AD. Using human APOE2, 3, and 4 isoform-specific transgenic mice, we found a lower brain uptake of docosahexaenoic acid (DHA) in APOE4 than in APOE2 mice that may limit the biodistribution of DHA in cerebral tissues. These data provide a mechanistic explanation for the lack of benefit of DHA in APOE4 carriers on cognitive function and the risk of Alzheimer's disease (AD).

Brain bioavailability of human intravenous immunoglobulin and its transport through the murine blood-brain barrier.

Intravenous immunoglobulin (IVIg) is currently evaluated in clinical trials for the treatment of various disorders of the central nervous system. To assess its capacity to reach central therapeutic targets, the brain bioavailability of IVIg must be determined. We thus quantified the passage of IVIg through the blood-brain barrier (BBB) of C57Bl/6 mice using complementary quantitative and qualitative methodologies. As determined by enzyme-linked immunosorbent assay, a small proportion of systemically injected IVIg was detected in the brain of mice (0.009±0.001% of injected dose in the cortex) whereas immunostaining revealed localization mainly within microvessels and less frequently in neurons. Pharmacokinetic analyses evidenced a low elimination rate constant (0.0053 per hour) in the cortex, consistent with accumulation within cerebral tissue. In situ cerebral perfusion experiments revealed that a fraction of IVIg crossed the BBB without causing leakage. A dose-dependent decrease of brain uptake was consistent with a saturable blood-to-brain transport mechanism. Finally, brain uptake of IVIg after a subchronic treatment was similar in the 3xTg-AD mouse model of Alzheimer disease compared with nontransgenic controls. In summary, our results provide evidence of BBB passage and bioavailability of IVIg into the brain in the absence of BBB leakage and in sufficient concentration to interact with the therapeutic targets.