Blood-brain Barrier Maturation Research Articles

Wnt/\u03b2-catenin signaling regulates VE-cadherin-mediated anastomosis of brain capillaries by counteracting S1pr1 signaling

Canonical Wnt signaling is crucial for vascularization of the central nervous system and blood-brain barrier (BBB) formation. BBB formation and modulation are not only important for development, but also relevant for vascular and neurodegenerative diseases. However, there is little understanding of how Wnt signaling contributes to brain angiogenesis and BBB formation. Here we show, using high resolution in vivo imaging and temporal and spatial manipulation of Wnt signaling, different requirements for Wnt signaling during brain angiogenesis and BBB formation. In the absence of Wnt signaling, premature Sphingosine-1-phosphate receptor (S1pr) signaling reduces VE-cadherin and Esama at cell-cell junctions. We suggest that Wnt signaling suppresses S1pr signaling during angiogenesis to enable the dynamic junction formation during anastomosis, whereas later S1pr signaling regulates BBB maturation and VE-cadherin stabilization. Our data provides a link between brain angiogenesis and BBB formation and identifies Wnt signaling as coordinator of the timing and as regulator of anastomosis.

3D self-organized microvascular model of the human blood-brain barrier with endothelial cells, pericytes and astrocytes

The blood-brain barrier (BBB) regulates molecular trafficking, protects against pathogens, and prevents efficient drug delivery to the brain. Models to date failed to reproduce the human anatomical complexity of brain barriers, contributing to misleading results in clinical trials. To overcome these limitations, a novel 3-dimensional BBB microvascular network model was developed via vasculogenesis to accurately replicate the in vivo neurovascular organization. This microfluidic system includes human induced pluripotent stem cell-derived endothelial cells, brain pericytes, and astrocytes as self-assembled vascular networks in fibrin gel. Gene expression of membrane transporters, tight junction and extracellular matrix proteins, was consistent with computational analysis of geometrical structures and quantitative immunocytochemistry, indicating BBB maturation and microenvironment remodelling. Confocal microscopy validated microvessel-pericyte/astrocyte dynamic contact-interactions. The BBB model exhibited perfusable and selective microvasculature, with permeability lower than conventional in vitro models, and similar to in vivo measurements in rat brain. This robust and physiologically relevant BBB microvascular model offers an innovative and valuable platform for drug discovery to predict neuro-therapeutic transport efficacy in pre-clinical applications as well as recapitulate patient-specific and pathological neurovascular functions in neurodegenerative disease.

Brain Barrier Disruption and Region-Specific Neuronal Degeneration during Necrotizing Enterocolitis in Preterm Pigs

Necrotizing enterocolitis (NEC) increases the risk of brain injury and impaired neurodevelopment. Rapid brain maturation prior to birth may explain why preterm brains are particularly vulnerable to serious infections. Using pigs as models, we hypothesized that preterm birth was associated with altered blood-cerebrospinal fluid (CSF) barrier (BCSFB) function and cerebral structural deficits, and that NEC was associated with systemic inflammation, BCSFB disruption, and neuroinflammation. First, cesarean-delivered preterm and term pigs (n = 43–44) were euthanized at birth to investigate BCSFB function and markers of brain structural maturation, or on day 5 to measure markers of blood-brain barrier maturation in the hippocampus and striatum (experiment 1). Next, preterm pigs (n = 162) were fed increasing volumes of infant formula to assess NEC lesions, systemic inflammation, BCSFB permeability, cerebral histopathology, hippocampal micro­glial density, and cytokine levels on day 5 (experiments 2 and 3). In experiment 1, preterm newborns had increased CSF-plasma ratios of albumin and raffinose, reduced CSF glucose levels, as well as increased cerebral hydration and reduced white matter myelination compared with term animals. We observed lower hippocampal (but not striatal) perivascular astrocyte coverage for the first 5 days after preterm birth, accompanied by altered cell junction protein levels. In experiments 2 and– 3, piglets with severe NEC lesions showed reduced blood thrombocytes and increased plasma C-reactive protein and interleukin-6 levels. NEC was associated with increased CSF-plasma albumin and raffinose ratios, reduced CSF leukocyte numbers, and increased cerebral hydration. In the hippocampus, NEC was associated with pyramidal neuron loss and increased interleukin-6 levels. In the short term, NEC did not affect cerebral myelination or microglia density. In conclusion, altered BCSFB properties and brain structural deficits were observed in pigs after preterm birth. Acute gastrointestinal NEC lesions were associated with systemic inflammation, increased BCSFB permeability and region-specific neuronal damage. The results demonstrate the importance of early interventions against NEC to prevent brain injury in preterm infants.

CD146 coordinates brain endothelial cell–pericyte communication for blood–brain barrier development

The blood-brain barrier (BBB) establishes a protective interface between the central neuronal system and peripheral blood circulation and is crucial for homeostasis of the CNS. BBB formation starts when the endothelial cells (ECs) invade the CNS and pericytes are recruited to the nascent vessels during embryogenesis. Despite the essential function of pericyte-EC interaction during BBB development, the molecular mechanisms coordinating the pericyte-EC behavior and communication remain incompletely understood. Here, we report a single cell receptor, CD146, that presents dynamic expression patterns in the cerebrovasculature at the stages of BBB induction and maturation, coordinates the interplay of ECs and pericytes, and orchestrates BBB development spatiotemporally. In mouse brain, CD146 is first expressed in the cerebrovascular ECs of immature capillaries without pericyte coverage; with increased coverage of pericytes, CD146 could only be detected in pericytes, but not in cerebrovascular ECs. Specific deletion of Cd146 in mice ECs resulted in reduced brain endothelial claudin-5 expression and BBB breakdown. By analyzing mice with specific deletion of Cd146 in pericytes, which have defects in pericyte coverage and BBB integrity, we demonstrate that CD146 functions as a coreceptor of PDGF receptor-β to mediate pericyte recruitment to cerebrovascular ECs. Moreover, we found that the attached pericytes in turn down-regulate endothelial CD146 by secreting TGF-β1 to promote further BBB maturation. These results reveal that the dynamic expression of CD146 controls the behavior of ECs and pericytes, thereby coordinating the formation of a mature and stable BBB.

Melanocortins, Melanocortin Receptors and Multiple Sclerosis.

The melanocortins and their receptors have been extensively investigated for their roles in the hypothalamo-pituitary-adrenal axis, but to a lesser extent in immune cells and in the nervous system outside the hypothalamic axis. This review discusses corticosteroid dependent and independent effects of melanocortins on the peripheral immune system, central nervous system (CNS) effects mediated through neuronal regulation of immune system function, and direct effects on endogenous cells in the CNS. We have focused on the expression and function of melanocortin receptors in oligodendroglia (OL), the myelin producing cells of the CNS, with the goal of identifying new therapeutic approaches to decrease CNS damage in multiple sclerosis as well as to promote repair. It is clear that melanocortin signaling through their receptors in the CNS has potential for neuroprotection and repair in diseases like MS. Effects of melanocortins on the immune system by direct effects on the circulating cells (lymphocytes and monocytes) and by signaling through CNS cells in regions lacking a mature blood brain barrier are clear. However, additional studies are needed to develop highly effective MCR targeted therapies that directly affect endogenous cells of the CNS, particularly OL, their progenitors and neurons.

Abcb1a (P-glycoprotein) limits brain exposure of the anticancer drug candidate seliciclib in vivo in adult mice

Seliciclib displayed limited brain exposure in vivo in adult rats with mature blood-brain barrier (BBB). Selicilib was shown to be a specific substrate of human ABCB1 in vitro.To demonstrate that ABCB1/Abcb1 can limit brain exposure in vivo in mice we are showing that seliciclib is a substrate of mouse Abcb1a, the murine ABCB1 ortholog expressed in the BBB as LLC-PK-Abcb1a cells displayed an efflux ratio (ER) of 15.31±3.54 versus an ER of 1.44±0.10 in LLC-PK1-mock cells. Additionally, in the presence of LY335979, an ABCB1/Abcb1a specific inhibitor, the observed ER for seliciclib in the LLC-PK1-mMdr1a cells decreased to 1.05±0.25. To demonstrate in vivo relevance of seliciclib transport by Abcb1a mouse brain microdialysis experiments were carried out that showed that the AUCbrain/AUCblood ratio of 0.143 in anesthetized mice increased about two-fold to 0.279 in the presence of PSC833 another ABCB1/Abcb1a specific inhibitor. PSC833 also increased the brain exposure (AUCbrain) of seliciclib close to 2-fold (136 vs 242) in awake mice.In sum, Abcb1a significantly decreases seliciclib permeability in vitro and is partly responsible for limited brain exposure of seliciclib in vivo in mice.

Brain uptake of deltamethrin in rats as a function of plasma protein binding and blood–brain barrier maturation

Pyrethroids, including permethrin and deltamethrin (DLM), are very widely used of insecticides. It was hypothesized that lower plasma binding and increased blood–brain barrier (BBB) penetration of DLM in immature rats contribute to the higher brain concentrations of DLM and more pronounced neurotoxicity reported in this age group. The left brain of anesthetized adult rats was perfused for 2min via a carotid artery with 1μM 14C-DLM in: 2–5% human serum albumin (HSA); plasma from adult and 15- and 21-d-old rats; and plasma from human donors of: birth–1 week, 1–4 weeks, 4 weeks–1 year, 1–3 years and adults. The fraction of DLM bound and brain uptake of DLM did not vary significantly with the HSA concentration nor with the age of rat or human plasma donors. One, 10 and 50μM 14C-DLM were perfused into the left-brain of anesthetized adult, 15- and 21-d-old rats. DLM deposition in the brain was linear over this range of concentrations and inversely related to age. The results of this investigation indicate that increased BBB permeability in the youngest rats enhances brain deposition of the insecticide. Plasma protein binding of DLM in immature rats and humans is not sufficiently diminished to impact its brain uptake.

CNS angiogenesis and barriergenesis occur simultaneously

The blood-brain barrier (BBB) plays a vital role in the central nervous system (CNS). A comprehensive understanding of BBB development has been hampered by difficulties in observing the differentiation of brain endothelial cells (BECs) in real-time. Here, we generated two transgenic zebrafish line, Tg(glut1b:mCherry) and Tg(plvap:EGFP), to serve as in vivo reporters of BBB development. We showed that barriergenesis (i.e. the induction of BEC differentiation) occurs immediately as endothelial tips cells migrate into the brain parenchyma. Using the Tg(glut1b:mCherry) transgenic line, we performed a genetic screen and identified a zebrafish mutant with a nonsense mutation in gpr124, a gene known to play a role in CNS angiogenesis and BBB development. We also showed that our transgenic plvap:EGFP line, a reporter of immature brain endothelium, is initially expressed in newly formed brain endothelial cells, but subsides during BBB maturation. Our results demonstrate the ability to visualize the in vivo differentiation of brain endothelial cells into the BBB phenotype and establish that CNS angiogenesis and barriergenesis occur simultaneously.

363. Improvement of Sandhoff Phenotype Following Intravenous Injection of Adeno-Associated Viral Vector Expressing a Hexosaminidase Isoenzyme in Adult Sandhoff Mice: Preclinical Safety and Efficacy Study

GM2 gangliosidosis disorders stem from a Hexosaminidase A (HexA) isoenzyme deficiency. In humans, HexA is the sole enzyme able to catabolize GM2 ganglioside (GM2). The inability to effectively catabolize GM2 leads to neurodegeneration of the central nervous system. HexA is comprised of 2 subunits (α, β) and works with the GM2 activator protein (GM2AP). In the recent work by Tropak et al. (Mol Ther Met Clin Dev, in press), a hybrid subunit, named µ-subunit, was created (patent pending) by combining the stabilization and GM2AP binding sites of the β-subunit while conserving the catalytic properties of the α-subunit. The ‘µ’-subunit, coded by HEXM, can homodimerize and form a stable, functional enzyme, named HexM, which can interact with GM2AP to hydrolyse GM2. Previous work for successful correction of Sandhoff mice using AAV was only shown in neonatal mice (with immature blood-brain barrier (BBB)) and may not directly help in designing a human clinical trial. In the current study, we examined the efficacy/safety of IV injections of the scAAV9/HEXM vector at two doses in adult SD mice (with mature BBB). In addition, we also tested if an adjunct IV injection of mannitol provides any enhancement in efficacy. At 6 weeks old, the vector was injected via tail vein in cohorts of n=17 and n=15 SD mice at 2.5E+12 or 1.0E+13 vg/mouse, respectively. Another cohort of 16 mice received IV mannitol (3g/kg) prior to an IV injection of 2.5E+12 vg scAAV9/HEXM. Some mice from low dose group were euthanized at 16 weeks for direct analysis with untreated SD control mice, while the remainder were left until for terminal survival. Analysis of survival benefit, locomotor behaviour, biochemical and molecular parameters were performed. While untreated SD mice had a 16 week humane endpoint, 4 of 7 mice in higher dose group are now surviving past 56 weeks, 1 of 12 mice in the low dose cohort, and 4 of 9 mice in the mannitol cohort are surviving past 52 weeks. These increases in survival are all highly significant compared to the ~16 week humane endpoint of untreated SD mice. Behaviourally, there are no major significant differences in locomotion between the groups until after 15 weeks, when the adjunct mannitol group significantly outperforms the PBS group. Survival and behaviour monitoring, and the biochemical analyses for this study are ongoing. The preliminary results from this study show delayed onset of the SD phenotype with a single AAV9/HEXM injection and a significant benefit of a pre-injection of IV mannitol. This study is the first to show that an IV gene transfer using a scAAV/HEXM vector can provide survival and behavioural benefit in adult SD mice especially with adjunct use of mannitol. We propose that these results can advise the design of a human gene therapy trial for SD and the related Tay-Sachs disease.

Investigation of iron's neurotoxicity during cerebral maturation in the neonatal rat model of haemolysis.

Haemolytic disease of newborns due to rhesus and AB0 incompatibility is encountered frequently in neonatal clinics and may lead to severe haemolysis. In this study, it is suggested that important amounts of iron released with haemolysis may have a toxic effect on the brain parenchymal tissue, and the severity of the toxic effect can be correlated with the maturation of the brain barrier systems. To demonstrate the accumulation and the neuro-toxic effects of free iron (Fe) in the brain an experimental haemolysis model with various maturation phases was performed. The study was composed of 48 Wistar rats with the following ages: five days old (Group A), 10 days old (Group B), and 19 days old (Group C). Each group was divided into three experimental subgroups and three control groups. Experimental groups were treated with intraperitoneal 75 mg/kg/day phenyl hydrazine hydrochloride for haemolysis. We demonstrated that the blood brain barrier (BBB) is permeable in five-day-old newborn rats and is mature in 10- and 19-day-old rats. Iron staining and neuronal damage were detected in group A and group B rats. No damage was detected in the brain tissue of group C animals. The presence of iron staining and neuronal damage in group B with mature BBB may suggest the existence of other incomplete barrier systems different from BBB that lead to iron accumulation in the brain. Blood brain barrier has a partial role in Fe transport, and the alternative barrier systems may also be involved. It could be supposed that after maturation of all barrier systems, excessive Fe penetration to the brain cannot occur. Our findings showed that the toxic amounts of iron may penetrate into the brain parenchyma of newborns despite the BBB preservation and cause neuronal damage in newborns, but the mature brain is not affected by the same magnitude blood levels.

Shiga toxin-2 enhances heat-shock-induced apoptotic cell death in cultured and primary glial cells.

The blood-brain barrier (BBB) selectively controls the homeostasis of the central nervous system (CNS) environment using specific structural and biochemical features of the endothelial cells, pericytes, and glial limitans. Glial cells, which represent the cellular components of the mature BBB, are the most numerous cells in the brain and are indispensable for neuronal functioning. We investigated the effects of Shiga toxin on glial cells in vitro. Shiga toxin failed to inhibit cell proliferation but attenuated expression of heat shock protein 70, which is one of the chaperone proteins, in cultured and primary glial cells. Furthermore, the combination of Shiga toxin and a heat shock procedure induced cell apoptosis and decreased cell proliferation in both cells. Thus, we speculate that glial cell death in response to the combination of Shiga toxin and heat shock might weaken the BBB and induce central nervous system complications.

The heparan sulfate proteoglycan agrin contributes to barrier properties of mouse brain endothelial cells by stabilizing adherens junctions.

Barrier characteristics of brain endothelial cells forming the blood–brain barrier (BBB) are tightly regulated by cellular and acellular components of the neurovascular unit. During embryogenesis, the accumulation of the heparan sulfate proteoglycan agrin in the basement membranes ensheathing brain vessels correlates with BBB maturation. In contrast, loss of agrin deposition in the vasculature of brain tumors is accompanied by the loss of endothelial junctional proteins. We therefore wondered whether agrin had a direct effect on the barrier characteristics of brain endothelial cells. Agrin increased junctional localization of vascular endothelial (VE)-cadherin, β-catenin, and zonula occludens-1 (ZO-1) but not of claudin-5 and occludin in the brain endothelioma cell line bEnd5 without affecting the expression levels of these proteins. This was accompanied by an agrin-induced reduction of the paracellular permeability of bEnd5 monolayers. In vivo, the lack of agrin also led to reduced junctional localization of VE-cadherin in brain microvascular endothelial cells. Taken together, our data support the notion that agrin contributes to barrier characteristics of brain endothelium by stabilizing the adherens junction proteins VE-cadherin and β-catenin and the junctional protein ZO-1 to brain endothelial junctions.Electronic supplementary materialThe online version of this article (doi:10.1007/s00441-014-1969-7) contains supplementary material, which is available to authorized users.

Oligodendrocyte precursor cells support blood-brain barrier integrity via TGF-β signaling.

Trophic coupling between cerebral endothelium and their neighboring cells is required for the development and maintenance of blood-brain barrier (BBB) function. Here we report that oligodendrocyte precursor cells (OPCs) secrete soluble factor TGF-β1 to support BBB integrity. Firstly, we prepared conditioned media from OPC cultures and added them to cerebral endothelial cultures. Our pharmacological experiments showed that OPC-conditioned media increased expressions of tight-junction proteins and decreased in vitro BBB permeability by activating TGB-β-receptor-MEK/ERK signaling pathway. Secondly, our immuno-electron microscopic observation revealed that in neonatal mouse brains, OPCs attach to cerebral endothelial cells via basal lamina. And finally, we developed a novel transgenic mouse line that TGF-β1 is knocked down specifically in OPCs. Neonates of these OPC-specific TGF-β1 deficient mice (OPC-specific TGF-β1 partial KO mice: PdgfraCre/Tgfb1flox/wt mice or OPC-specific TGF-β1 total KO mice: PdgfraCre/Tgfb1flox/flox mice) exhibited cerebral hemorrhage and loss of BBB function. Taken together, our current study demonstrates that OPCs increase BBB tightness by upregulating tight junction proteins via TGF-β signaling. Although astrocytes and pericytes are well-known regulators of BBB maturation and maintenance, these findings indicate that OPCs also play a pivotal role in promoting BBB integrity.

Novel insights into the development and maintenance of the blood-brain barrier.

The blood–brain barrier (BBB) is essential for maintaining homeostasis within the central nervous system (CNS) and is a prerequisite for proper neuronal function. The BBB is localized to microvascular endothelial cells that strictly control the passage of metabolites into and out of the CNS. Complex and continuous tight junctions and lack of fenestrae combined with low pinocytotic activity make the BBB endothelium a tight barrier for water soluble moleucles. In combination with its expression of specific enzymes and transport molecules, the BBB endothelium is unique and distinguishable from all other endothelial cells in the body. During embryonic development, the CNS is vascularized by angiogenic sprouting from vascular networks originating outside of the CNS in a precise spatio-temporal manner. The particular barrier characteristics of BBB endothelial cells are induced during CNS angiogenesis by cross-talk with cellular and acellular elements within the developing CNS. In this review, we summarize the currently known cellular and molecular mechanisms mediating brain angiogenesis and introduce more recently discovered CNS-specific pathways (Wnt/β−catenin, Norrin/Frizzled4 and hedgehog) and molecules (GPR124) that are crucial in BBB differentiation and maturation. Finally, based on observations that BBB dysfunction is associated with many human diseases such as multiple sclerosis, stroke and brain tumors, we discuss recent insights into the molecular mechanisms involved in maintaining barrier characteristics in the mature BBB endothelium.

Abstract T MP111: Gpr124 Regulates Post-Stroke Cerebrovascular Integrity

Introduction: GPR124/TEM5 is an orphan G-protein coupled receptor (GPCR) with a large extracellular domain. We and others have previously demonstrated that GPR124 exerts CNS-specific angiogenesis regulation with knockout mice exhibiting embryonic lethality from hemorrhagic glomeruloid vascular malformations in forebrain and neural tube (c.f. Kuhnert et al., Science , Nov 12;330(6006):985-9 . (2010)). Hypothesis: GPR124 regulates adult angiogenesis and blood-brain barrier (BBB) integrity during homeostasis or after stroke. Methods: To bypass GPR124 embryonic lethality, we generated GPR124 conditional knockout (cko) mice allowing temporally-regulated deletion. Tamoxifen treatment of GPR124 flox/- ; ROSA-CreER mice versus GPR124 flox/+; ROSA-CreER littermate controls allowed GPR124 cko versus heterozygosity, respectively, in adult mice. GPR124 deletion was followed by analyses of microvascular structure and patterning and blood-brain barrier (BBB) integrity). GPR124 cko mice versus controls were also subjected to 60 minute middle cerebral artery occlusion (MCAO) and effects on stroke volume, survival and microvascular structure assessed. Results: GPR124 deletion in neonatal or adult mice was well-tolerated without impairment of postnatal vascular patterning, BBB maturation or BBB integrity. However, GPR124 cko mice subjected to the middle cerebral artery occlusion (MCAO) stroke model exhibited impaired survival and a profound microvascular hemorrhagic transformation that was confined to the infarct region relative to wild-type controls. GPR124 cko stroke vasculature also exhibited numerous cellular and architectural defects relative to controls that will be discussed. Conclusions: GPR124 deletion is well tolerated in adult mice but results in marked hemorrhagic transformation in the MCAO stroke model. GPR124 represents a novel receptor whose function is essential for cerebrovascular integrity in the post-stroke setting, with attendant therapeutic implications.

Vascular endothelial growth factor increases during blood-brain barrier-enhanced permeability caused by Phoneutria nigriventer spider venom.

Phoneutria nigriventer spider accidental envenomation provokes neurotoxic manifestations, which when critical, results in epileptic-like episodes. In rats, P. nigriventer venom (PNV) causes blood-brain barrier breakdown (BBBb). The PNV-induced excitotoxicity results from disturbances on Na+, K+ and Ca2+ channels and glutamate handling. The vascular endothelial growth factor (VEGF), beyond its angiogenic effect, also, interferes on synaptic physiology by affecting the same ion channels and protects neurons from excitotoxicity. However, it is unknown whether VEGF expression is altered following PNV envenomation. We found that adult and neonates rats injected with PNV showed immediate neurotoxic manifestations which paralleled with endothelial occludin, β-catenin, and laminin downregulation indicative of BBBb. In neonate rats, VEGF, VEGF mRNA, and Flt-1 receptors, glutamate decarboxylase, and calbindin-D28k increased in Purkinje neurons, while, in adult rats, the BBBb paralleled with VEGF mRNA, Flk-1, and calbindin-D28k increases and Flt-1 decreases. Statistically, the variable age had a role in such differences, which might be due to age-related unequal maturation of blood-brain barrier (BBB) and thus differential cross-signaling among components of the glial neurovascular unit. The concurrent increases in the VEGF/Flt-1/Flk-1 system in the cerebellar neuron cells and the BBBb following PNV exposure might imply a cytokine modulation of neuronal excitability consequent to homeostatic perturbations induced by ion channels-acting PNV neuropeptides. Whether such modulation represents neuroprotection needs further investigation.

Partial recovery of the damaged rat blood–brain barrier is mediated by adherens junction complexes, extracellular matrix remodeling and macrophage infiltration following focal astrocyte loss

Blood–brain barrier (BBB) dysfunction is a feature of many neurodegenerative disorders. The mechanisms and interactions between astrocytes, extracellular matrix and vascular endothelial cells in regulating the mature BBB are poorly understood. We have previously shown that transitory glial fibrillary acidic protein (GFAP)-astrocyte loss, induced by the systemic administration of 3-chloropropanediol, leads to reversible disruption of tight junction complexes and BBB integrity to a range of markers. However, early restoration of BBB integrity to dextran (10–70kDa) and fibrinogen was seen in the absence of paracellular tight junction proteins claudin-5 and occludin. In the present study we show that in the GFAP-astrocyte-lesioned rat inferior colliculus, paracellular expression of adherens junction proteins (vascular endothelial (VE)-cadherin and β-catenin) was maintained in vascular endothelial cells that lacked paracellular claudin-5 expression and which showed reversible post-translational occludin modification. Claudin-1 expression paralleled the loss and recovery of claudin-5, while claudin-3 or -12 immunoreactivity was not detected. In addition, the extracellular matrix, as visualized by laminin and fibronectin, underwent extensive reversible remodeling and perivascular CD169 macrophages become abundant throughout the lesioned inferior colliculus. At a time that GFAP-astrocytes repopulated the lesion area and tight junction proteins were returned to paracellular domains, the extracellular matrix and leukocyte profiles normalized and resembled profiles seen in control tissue. This study supports the hypothesis that a combination of paracellular adherens junctional proteins, remodeled basement membrane and the presence of perivascular leukocytes provide a temporary barrier to limit the extravasation of macromolecules and potentially neurotoxic substances into the brain parenchyma until tight junction proteins are restored to paracellular domains.

Foxc1 is required by pericytes during fetal brain angiogenesis

SummaryBrain pericytes play a critical role in blood vessel stability and blood–brain barrier maturation. Despite this, how brain pericytes function in these different capacities is only beginning to be understood. Here we show that the forkhead transcription factor Foxc1 is expressed by brain pericytes during development and is critical for pericyte regulation of vascular development in the fetal brain. Conditional deletion of Foxc1 from pericytes and vascular smooth muscle cells leads to late-gestation cerebral micro-hemorrhages as well as pericyte and endothelial cell hyperplasia due to increased proliferation of both cell types. Conditional Foxc1 mutants do not have widespread defects in BBB maturation, though focal breakdown of BBB integrity is observed in large, dysplastic vessels. qPCR profiling of brain microvessels isolated from conditional mutants showed alterations in pericyte-expressed proteoglycans while other genes previously implicated in pericyte–endothelial cell interactions were unchanged. Collectively these data point towards an important role for Foxc1 in certain brain pericyte functions (e.g. vessel morphogenesis) but not others (e.g. barriergenesis).

Pericytes and Astrocytes Crosstalk: Understanding Perivascular Synergism at the BBB

The Blood brain barrier (BBB) plays a critical role in maintaining brain homeostasis. Astrocytes (AC) lying adjacent to BBB endothelial cells (EC) have been reported to play a major role in ECs polarization and differentiation into a BBB phenotype. Pericytes (PC) have been reported to synthesize most elements of the basement membrane and provide structural support and vasodynamic capabilities to the brain microvasculature thus suggesting a more primary role in BBB differentiation than previously anticipated. Whether PC and AC modulate each other functions and activities (thus affecting the BBB) are still poorly understood. For this purpose we analyzed the conditioned media from human AC, PC and AC‐PC cultures and assessed the morphological changes in correspondingly treated human BBB EC. Analysis of culture media have shown that Ang‐1 (important for BBB TJ formation) was primarily released by PC rather than AC (release of Ang‐2) thus suggesting that PC play an important role in BBB maturation. Furthermore, PC induced AC redistribution (cytosol to membrane) of glut‐1, important for glucose transport across the BBB was seen. This suggests that PC can modulate AC functions thus affecting the BBB. In addition, EC culture in presence of PC and/or AC‐PC conditioned media (but not AC alone) reassembled in a capillary like structure formation. This suggests that PC also play a role during angiogenesis of brain microcapillary.

P-Glycoprotein, Breast Cancer Resistance Protein, Organic Anion Transporter 3, and Transporting Peptide 1a4 during Blood–Brain Barrier Maturation: Involvement of Wnt/β-Catenin and Endothelin-1 Signaling

Our current knowledge about drug transporters in the maturational brain is very limited. In this study, we provide a comprehensive overview of the expression and activity profile of P-glycoprotein (P-gp), Breast Cancer Resistance Protein (bcrp), Organic Anion Transporter 3 (oat3), and Transporting Peptide 1a4 (oatp1a4) transporters during blood-brain barrier (BBB) maturation. Gene and protein expressions of the analyzed transporters increase as the brain matures, with no variation in their activity for P-gp and bcrp, while the transport activity of oat3 and oatp1a4 increases during brain maturation from preterm up to adulthood. For the first time, we illustrate a downregulation of nuclear β-catenin expression in brain capillaries when bcrp, P-gp, oat3, and oatp1a4 transporters are at their highest expression levels. In vivo activation of β-catenin in rat brains, by intracerebroventricular (ICV) injection of a GSK-3 inhibitor, enhances the activity of P-gp, bcrp, oat3, and oatp1a4. Interestingly, in an in vitro BBB model consisting of a coculture of primary endothelial brain cells with astrocytes or in vivo, activation of β-catenin enhances the mRNA expression of ET-1. Interestingly, blocking the ETA receptor for endothelin-1 in vivo by ICV injection of a ETA antagonist decreases transporter activity mediated by the activation of β-catenin. These findings shed light on the role of an interaction between β-catenin and endothelin-1 signaling in the regulation of these transporters at the BBB.