Expression of GFRalpha-1, receptor for GDNF, in rat brain capillary during postnatal development of the BBB

Effects of early-life stress and sex on blood-brain barrier permeability and integrity in juvenile and adult rats

Early-life stress (ELS) is considered a relevant etiological factor for neurodegenerative and mental disorders. In the present study, we hypothesized that ELS may persistently and sex dependently influence blood-brain barrier (BBB) integrity and function during critical periods of brain development and consequently determine susceptibility to and sex-related prevalence of chronic diseases in adult life. We used the maternal separation (MS) procedure in rats to model ELS and evaluated BBB permeability and gene expression of selected tight junction (TJ) proteins, glucose transporter type 1 (Slc2a1) and aquaporin 4 (Aqp4) in the medial prefrontal cortex (mPFC), dorsal striatum (dSTR) and hippocampus of juvenile and adult rats. Serum concentrations of a peripheral marker of BBB function (S100β) and proinflammatory cytokines were also assessed. We observed developmental sealing of the BBB and sex differences in the permeability of the BBB and the mRNA expression of TJ proteins and Slc2a1. Adult females showed lower BBB permeability and higher levels of Cldn3, Cldn5, Ocln, and Slc2a1 in the mPFC and dSTR than males. MS temporarily increased BBB permeability in the dSTR of juvenile males and affected mRNA expression of the majority of studied proteins related to BBB function in age-, region- and sex-dependent manners. Additionally, MS sex dependently decreased serum S100β levels and did not affect proinflammatory cytokine concentrations. In general, our study did not reveal a clear or strong negative effect of MS on BBB integrity. However, the results suggest that ELS may induce adaptive/maladaptive changes or compensatory mechanisms within the BBB of unknown yet consequences.

Blood-Brain Barrier and Neurodegenerative Diseases-Modeling with iPSC-Derived Brain Cells

The blood-brain barrier (BBB) regulates the delivery of oxygen and important nutrients to the brain through active and passive transport and prevents neurotoxins from entering the brain. It also has a clearance function and removes carbon dioxide and toxic metabolites from the central nervous system (CNS). Several drugs are unable to cross the BBB and enter the CNS, adding complexity to drug screens targeting brain disorders. A well-functioning BBB is essential for maintaining healthy brain tissue, and a malfunction of the BBB, linked to its permeability, results in toxins and immune cells entering the CNS. This impairment is associated with a variety of neurological diseases, including Alzheimer's disease and Parkinson's disease. Here, we summarize current knowledge about the BBB in neurodegenerative diseases. Furthermore, we focus on recent progress of using human-induced pluripotent stem cell (iPSC)-derived models to study the BBB. We review the potential of novel stem cell-based platforms in modeling the BBB and address advances and key challenges of using stem cell technology in modeling the human BBB. Finally, we highlight future directions in this area.

Serum levels of glial cell line-derived neurotrophic factor as a biomarker for mood disorders and lithium response

Glial cell line-derived neurotrophic factor (GDNF) plays an important role in the pathophysiology of neuropsychiatric disorders. We examined serum GDNF levels in bipolar disorder (BD) patients and major depressive disorder (MDD) patients and their association with response to lithium therapy. We used a multicenter (six sites), exploratory, cross-sectional case-control design and recruited 448 subjects: 143 BD patients, 116 MDD patients, and 158 healthy controls (HCs). We evaluated the patients' clinical severity using the Clinical Global Impression (CGI), and responses to lithium therapy using the Alda scale. The serum GDNF levels were significantly decreased in the BD and MDD groups compared to the HCs, with no significant difference between the BD and MDD groups. After adjustment, the serum GDNF levels in the BD and MDD patients in remission or depressive states were decreased compared to the HC values. Lower serum GDNF levels in BD patients were associated with higher CGI and Alda scores (i.e., severe illness and good response to lithium therapy, respectively). Our findings suggest that the serum GDNF level may be a biomarker for both BD and MDD in remission or depressive states. The serum GDNF level may be associated with the lithium response of BD patients.

Dual Roles of Astrocyte-Derived Factors in Regulation of Blood-Brain Barrier Function after Brain Damage

The blood-brain barrier (BBB) is a major functional barrier in the central nervous system (CNS), and inhibits the extravasation of intravascular contents and transports various essential nutrients between the blood and the brain. After brain damage by traumatic brain injury, cerebral ischemia and several other CNS disorders, the functions of the BBB are disrupted, resulting in severe secondary damage including brain edema and inflammatory injury. Therefore, BBB protection and recovery are considered novel therapeutic strategies for reducing brain damage. Emerging evidence suggests key roles of astrocyte-derived factors in BBB disruption and recovery after brain damage. The astrocyte-derived vascular permeability factors include vascular endothelial growth factors, matrix metalloproteinases, nitric oxide, glutamate and endothelin-1, which enhance BBB permeability leading to BBB disruption. By contrast, the astrocyte-derived protective factors include angiopoietin-1, sonic hedgehog, glial-derived neurotrophic factor, retinoic acid and insulin-like growth factor-1 and apolipoprotein E which attenuate BBB permeability resulting in recovery of BBB function. In this review, the roles of these astrocyte-derived factors in BBB function are summarized, and their significance as therapeutic targets for BBB protection and recovery after brain damage are discussed.

Role of glial cell line-derived neurotrophic factor in the pathogenesis and treatment of mood disorders

Glial cell line-derived neurotrophic factor (GDNF) is widely recognized as a survival factor for dopaminergic neurons, but GDNF has also been shown to promote development, differentiation, and protection of other central nervous system neurons and was thought to play an important role in various neuropsychiatric disorders. Severe mood disorders, such as primarily major depressive disorder and bipolar affective disorder, attract particular attention. These psychopathologies are characterized by structural alterations accompanied by the dysregulation of neuroprotective and neurotrophic signaling mechanisms required for the maturation, growth, and survival of neurons and glia. The main objective of this review is to summarize the recent findings and evaluate the potential role of GDNF in the pathogenesis and treatment of mood disorders. Specifically, it describes (1) the implication of GDNF in the mechanism of depression and in the effect of antidepressant drugs and mood stabilizers and (2) the interrelation between GDNF and brain neurotransmitters, playing a key role in the pathogenesis of depression. This review provides converging lines of evidence that (1) brain GDNF contributes to the mechanism underlying depressive disorders and the effect of antidepressants and mood stabilizers and (2) there is a cross-talk between GDNF and neurotransmitters representing a feedback system: GDNF-neurotransmitters and neurotransmitters-GDNF.

Different segments of the cerebral vasculature reveal specific endothelial specifications, while tight junction proteins appear equally distributed

The identification of the "paucity of transportation vesicles" and "belt-like" tight junctions (TJs) of endothelial cells as the "morphological correlate of a blood-brain barrier" (BBB) by Reese and Karnovsky (J Cell Biol 34:207-217, 1967) has become textbook knowledge, and countless studies have helped to further define the elements, functions, and dynamics of the BBB. Most work, however, has focused on parenchymal capillaries or less clearly defined "microvessels", while a systematic study on similarities and differences between BBB architecture along the vascular tree within the brain and the meninges has been lacking. Since astrocytes induce endothelial cells to display BBB-typical characteristics by sonic hedgehog and Wnt/β-catenin signaling, we hypothesized that BBB-typical features should be most pronounced in parenchymal capillaries, where endothelium and astrocytes are separated by a basement membrane only. In contrast, this intimate contact is absent in leptomeningeal vessels, thereby potentially affecting BBB architecture. However, here, we show that claudin-3, claudin-5, zonula occludens-1, and occludin as typical constitutes of BBB TJs are comparably distributed in all segments of the parenchymal and the meningeal vascular tree of C57Bl6 mice. While electron microscopy revealed equally occluded interendothelial clefts, arterial vessels of the brain parenchyma but not within the meninges exhibited significantly longer TJ overlaps compared to capillaries. The highest density of endothelial vesicles was found in arterial vessels. Thus, endothelial expression of BBB-typical TJ proteins is not reflected by the distance to surrounding astrocytes, but electron microscopy reveals significant differences of endothelial specification along different segments of the CNS vasculature.

Growth factor treatment to overcome Alzheimer's dysfunctional signaling

The number of people suffering from Alzheimer's disease (AD) will increase as the world population ages, creating a huge socio-economic burden. The three pathophysiological hallmarks of AD are the cholinergic system dysfunction, the β-amyloid peptide deposition and the Tau protein hyperphosphorylation. Current treatments have only transient effects and each tends to concentrate on a single pathophysiological aspect of AD. This review first provides an overall view of AD in terms of its pathophysiological symptoms and signaling dysfunction. We then examine the therapeutic potential of growth factors (GFs) by showing how they can overcome the dysfunctional cell signaling that occurs in AD. Finally, we discuss new alternatives to GFs that help overcome the problem of brain uptake, such as small peptides, with evidence from some of our unpublished data on human neuronal cell line.

Development of the blood-brain barrier within the paraventricular nucleus of the hypothalamus: influence of fetal glucocorticoid excess

The blood–brain barrier (BBB) is a critical contributor to brain function. To understand its development and potential function in different brain regions, the postnatal (P) BBB was investigated in the mouse cortex (CTX), lateral hypothalamus, and paraventricular nucleus of the hypothalamus (PVN). Brains were examined on postnatal days (P)12, P22 and P52 for BBB competency and for pericytes as key cellular components of the BBB demarcated by immunoreactive desmin. Glucocorticoid influences (excess dexamethasone; dex) during prenatal development were also assessed for their impact on the blood vessels within these regions postnatally. At P12, there was significantly more extravascular leakage of a low molecular weight dye (fluorescein isothiocyanate) in the CTX than within hypothalamic regions. For pericytes, there were low levels of desmin immunoreactivity at P12 that increased with age for all regions. There was more desmin immunoreactivity present in the PVN at each age examined. Fetal dex exposure resulted in decreased blood vessel density within the PVN at P20. In the CTX, dex exposure increased BBB competency, in contrast to the PVN where there was a decrease in BBB competency and increased pericyte presence. Overall, unique alterations in the functioning of the BBB within the PVN may provide a novel mechanism for fetal antecedent programming that may influence adult disorders.

New players in the neurovascular unit: insights from experimental and clinical epilepsy

The conventional notion that neurons are exclusively responsible for brain signaling is increasingly challenged by the idea that brain function in fact depends on a complex interplay between neurons, glial cells, vascular endothelium, and immune-related blood cells. Recent data demonstrates that neuronal activity is profoundly affected by an entire cellular and extracellular 'orchestra', the so-called neurovascular unit (NVU). Among the 'musical instruments' of this orchestra, there may be molecules long-known in biomedicine as important mediators of inflammatory and immune responses in the organism, as well as non-neuronal cells, e.g., leukocytes. We here review recent evidence on the structure and function of the NVU, both in the healthy brain and in pathological conditions, such as the abnormal NVU activation observed in epilepsy. We will argue that a better understanding of NVU function will require the addition of new players to the 'orchestra'.

Endothelial calcium dynamics, connexin channels and blood-brain barrier function

Situated between the circulation and the brain, the blood-brain barrier (BBB) protects the brain from circulating toxins while securing a specialized environment for neuro-glial signaling. BBB capillary endothelial cells exhibit low transcytotic activity and a tight, junctional network that, aided by the cytoskeleton, restricts paracellular permeability. The latter is subject of extensive research as it relates to neuropathology, edema and inflammation. A key determinant in regulating paracellular permeability is the endothelial cytoplasmic Ca(2+) concentration ([Ca(2+)]i) that affects junctional and cytoskeletal proteins. Ca(2+) signals are not one-time events restricted to a single cell but often appear as oscillatory [Ca(2+)]i changes that may propagate between cells as intercellular Ca(2+) waves. The effect of Ca(2+) oscillations/waves on BBB function is largely unknown and we here review current evidence on how [Ca(2+)]i dynamics influence BBB permeability.

VEGF-A165 potently induces human blood-nerve barrier endothelial cell proliferation, angiogenesis, and wound healing in vitro

Several mitogens such as vascular endothelial growth factor (VEGF) have been implicated in mammalian vascular proliferation and repair. However, the molecular mediators of human blood-nerve barrier (BNB) development and specialization are unknown. Primary human endoneurial endothelial cells (pHEndECs) were expanded in vitro and specific mitogen receptors detected by western blot. pHEndECs were cultured with basal medium containing different mitogen concentrations with or without heparin. Non-radioactive cell proliferation, Matrigel(™)-induced angiogenesis and sterile micropipette injury wound healing assays were performed. Proliferation rates, number and total length of induced microvessels, and rate of endothelial cell monolayer wound healing were determined and compared to basal conditions. VEGF-A165 in the presence of heparin, was the most potent inducer of pHEndEC proliferation, angiogenesis, and wound healing in vitro. 1.31 nM VEGF-A165 induced ~110 % increase in cell proliferation relative to basal conditions (∼51 % without heparin). 2.62 pM VEGF-A165 induced a three-fold increase in mean number of microvessels and 3.9-fold increase in total capillary length/field relative to basal conditions. In addition, 0.26 nM VEGF-A165 induced ∼1.3-fold increased average rate of endothelial wound healing 4-18 h after endothelial monolayer injury, mediated by increased cell migration. VEGF-A165 was the only mitogen capable of complete wound closure, occurring within 30 h following injury via increased cell proliferation. This study demonstrates that VEGF-A165, in the presence of heparin, is a potent inducer of pHEndEC proliferation, angiogenesis, and wound healing in vitro. VEGF-A165 may be an important mitogen necessary for human BNB development and recovery in response to peripheral nerve injury.

Tight junction-related human diseases

Tight junctions are intercellular junctions adjacent to the apical ends of paracellular spaces. They have two classical functions, the barrier function and the fence function. The former regulates the passage of ions, water and various molecules through paracellular spaces, and is thus related to edema, jaundice, diarrhea and blood‐borne metastasis. The latter function maintains cell polarity by forming a fence to prevent intermixing of molecules in the apical membrane with those in the lateral membrane. This function is deeply involved in cancer cell properties in terms of loss of cell polarity. Recently, two novel aspects of tight junctions have been reported. One is their involvement in signal transduction. The other is that fact that tight junctions are considered to be a crucial component of innate immunity. In addition, since some proteins comprising tight junctions work as receptors for viruses and extracellular stimuli, pathogenic bacteria and viruses target and affect the tight junction functions, leading to diseases. In this review, the relationship between tight junctions and human diseases will be described.

GDNF restores human blood-nerve barrier function via RET tyrosine kinase-mediated cytoskeletal reorganization

Endoneurial microvessels and the perineurium are responsible for maintaining homeostasis in peripheral nerves. Endoneurial endothelial cells form the blood-nerve barrier (BNB). The molecular pathways responsible for endoneurial microvascular barrier formation in humans are not fully understood. We tested the effect of different mitogens on the transendothelial electrical resistance (TEER) of confluent primary human endoneurial endothelial cell (pHEndEC) cultures following serum withdrawal (mimicking diffuse endothelial injury) in vitro. We show that glial-derived neurotrophic factor (GDNF, 1 ng/mL) sufficiently induced a maximal 114.2% recovery in TEER over basal conditions 48 h after serum withdrawal. Solute permeability to high molecular weight dextran was reduced by 52.4% following GDNF treatment. GDNF-mediated increase in TEER was dependent on RET tyrosine-kinase signaling pathways and mildly enhanced by cyclic adenosine monophosphate in combination with maximal concentrations of multiple redundant mitogens. There was no significant increase in adherens or tight junction proteins β-catenin, VE-Cadherin, zona occludens-1 and occludin following GDNF treatment. GDNF induced a small increase in total claudin-5 protein expression without significant increase in messenger RNA or modulation in tyrosine phosphorylation following serum withdrawal. Indirect immunocytochemistry revealed membrane relocation of longitudinal F-actin cytoskeletal filaments in pHEndECs following GDNF treatment, resulting in more continuous intercellular contacts that formed adherens and tight junctions. Together, these results demonstrate a sufficient role for GDNF in human BNB recovery following serum withdrawal in vitro, facilitated primarily by endothelial cell cytoskeletal reorganization. These observations provide insights into the regulation of human BNB function during recovery from peripheral nerve injury.

Pericyte-derived glial cell line-derived neurotrophic factor increase the expression of claudin-5 in the blood-brain barrier and the blood-nerve barrier

The destruction of blood-brain barrier (BBB) and blood-nerve barrier (BNB) has been considered to be a key step in the disease process of a number of neurological disorders including cerebral ischemia, Alzheimer's disease, multiple sclerosis, and diabetic neuropathy. Although glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) facilitate neuronal or axonal regeneration in the brain or peripheral nerves, their action in the BBB and BNB remains unclear. The purpose of the present study was to elucidate whether these neurotrophic factors secreted from the brain or peripheral nerve pericytes increase the barrier function of the BBB or BNB, using our newly established human brain microvascular endothelial cell (BMEC) line or peripheral nerve microvascular endothelial cell (PnMEC) line. GDNF increased the expression of claudin-5 and the transendothelial electrical resistance (TEER) of BMECs and PnMECs, whereas BDNF did not have this effect. Furthermore, we herein demonstrate that the GDNF secreted from the brain and peripheral nerve pericytes was one of the key molecules responsible for the up-regulation of claudin-5 expression and the TEER value in the BBB and BNB. These results indicate that the regulation of GDNF secreted from pericytes may therefore be a novel therapeutic strategy to modify the BBB or BNB functions and promote brain or peripheral nerve regeneration.

Neurovascular coupling in cognitive impairment associated with diabetes mellitus

Although it is feared that diabetes-induced cognitive decline will become a major clinical problem worldwide in the future, the detailed pathological mechanism is not well known. Because patients with diabetes have various complications of vascular disease, with not only macrovascular but also microvascular disorders, vascular disorders in the brain are considered to be one of the mechanisms in diabetes-induced cognitive impairment. Indeed, disruption of the blood-brain barrier (BBB) has been observed in some diabetic patients and experimental diabetes models. Moreover, white matter lesions, part of the evidence of BBB dysfunction, are reported to be observed more frequently in patients with diabetes. Animal studies demonstrate that diabetes enhances BBB permeability through a decrease in the level of tight junction proteins and an increase in matrix metalloproteinase activity. However, there are several reports indicating that BBB disruption does not occur with diabetes. Therefore, the association of BBB breakdown with diabetes-induced cognitive impairment is not conclusive. Recently, neuronal diseases involving dementia have been induced experimentally through dysfunction of neurovascular coupling, which involves blood vessels, astrocytes and neutrons. Diabetes-induced cognitive decline may be induced via disruption of neurovascular coupling, with not only vascular disorder but also impairment of astrocytic trafficking. Here, the relation between vascular disorder and cognitive impairment in diabetes is discussed.

Ontogeny and the effects of exogenous and endogenous glucocorticoids on tight junction protein expression in ovine cerebral cortices

Maternal glucocorticoid treatment reduces blood-brain permeability early, but not late in fetal development, and pretreatment with glucocorticoids does not affect barrier permeability in newborn lambs. In addition, endogenous increases in plasma cortisol levels are associated with decreases in blood-brain barrier permeability during normal fetal development. Therefore, we tested the hypotheses that development as well as endogenous and exogenous glucocorticoids alters the expression of tight junction proteins in the cerebral cortex of sheep. Cerebral cortices from fetuses at 60%, 70%, and 90% of gestation, newborn and adult sheep were snap frozen after four 6-mg dexamethasone or placebo injections were given over 48-h to the ewes and adult sheep. Lambs were treated similarly with 0.25 mg/kg-dexamethasone or placebo. Tight junction protein expression was measured by Western immunoblot. Claudin-1 was higher (P<0.05) in fetuses at 60% of gestation than in newborn and adult sheep. Claudin-5 was higher at 60% than 70% of gestation, and than in newborn and adult sheep. ZO-1 was higher in newborn than adult sheep. ZO-2 was higher at 90% gestation, in newborn and adult sheep than 60% gestation. Claudin-5 was higher in dexamethasone than placebo-treated lambs, and ZO-2 was higher in fetuses of dexamethasone than placebo-treated ewes at 90% gestation. ZO-2 expression demonstrated a direct correlation with increases in plasma cortisol during fetal development. We conclude that claudin-1, claudin-5, ZO-1, and ZO-2 expression exhibit differential developmental regulation, exogenous glucocorticoids regulate claudin-5 and ZO-2 in vivo at some, but not all ages, and increases in endogenous fetal glucocorticoids are associated with increases in ZO-2 expression, but not with occludin, claudin-1, claudin-5 or ZO-1 expression in ovine cerebral cortices.

Corticotropin-releasing factor modulates maternal separation-induced ultrasonic vocalization in rat pups via activation of CRF1 receptor

An infant animal isolated from its mother emits vocalizations spanning from the audible to the ultrasonic. These vocalizations are believed to represent distress signals from the pup. However, the neurobiological basis for vocalizations elicited by isolation has not been well characterized under different environmental conditions. The present study was designed to clarify the role of the corticotropin-releasing factor (CRF) system in vocalizations elicited by isolating a rat pup at ambient temperatures of 37 degrees C (temperature of the nest in which the mother and littermates are present) and 24 degrees C (room temperature). Sprague-Dawley rat pups at 7 days old were isolated from their dam, then the number of vocalizations was measured for 5 min. The number of vocalizations increased when ambient temperature was changed from 37 degrees C to 24 degrees C. Systemic administration of CRF (3 or 10 mg/kg) increased the number of vocalizations at 37 degrees C in a dose-dependent manner. CRF-induced increases in the number of vocalizations at 3 mg/kg were completely blocked by a selective CRF1 receptor antagonist, NBI27914 (3 mg/kg), but not by a selective CRF2 receptor antagonist, K41498 (3 mg/kg). NBI27914 (30 mg/kg), but not K41498 (3 mg/kg), suppressed the increased number of vocalizations at 24 degrees C. These results demonstrate involvement of the CRF-CRF1 receptor regulatory system on the modulation of ultrasonic vocalizations by rat pups separated from their dam.

The temporal dynamics of consolidation and reconsolidation decrease during postnatal development

The temporal dynamics of consolidation and reconsolidation of taste/odor aversion memory are evaluated during rat pup growth at postnatal days 3, 10, and 18. This is assessed through the temporal gradients of efficacy of a protein synthesis inhibitor (anisomycin) in inducing amnesia after either acquisition (consolidation) or reactivation (reconsolidation). The results show a progressive reduction with age of the delay during which the inhibitor is able to induce amnesia. Control experiments rule out a reduction of anisomycin efficacy due to blood brain barrier growth or decrease in protein synthesis inhibition. Thus, these results present the first evidence that the protein synthesis-dependent phase of memory stabilization requires less time with age. This decrease occurs in parallel for consolidation and reconsolidation. Such changes in the dynamics of memory processing could contribute to the cognitive improvement associated with development.

Altered ATP7A expression and other compensatory responses in a murine model of Menkes disease

Mutations in the copper-transporter ATP7A lead to severe neurodegeneration in the mottled brindled hemizygous male (MoBr/y) mouse and human patients with Menkes disease. Our earlier studies demonstrated cell-type- and -stage-specific changes in ATP7A protein expression during postnatal neurodevelopment. Here we examined copper and cuproenzyme levels in MoBr/y mice to search for compensatory responses. While all MoBr/y neocortical subcellular fractions had decreased copper levels, the greatest decrease (8-fold) was observed in cytosol. Immunostaining for ATP7A revealed decreased levels in MoBr/y hippocampal pyramidal and cerebellar Purkinje neurons. In contrast, an upregulation of ATP7A protein occurred in MoBr/y endothelial cells, perhaps to compensate for a lack of copper in the neuropil. MoBr/y astrocytes and microglia increased their physical association with the blood-brain barrier. No alterations in ATP7A levels were observed in ependymal cells, arguing for specificity in the alteration observed at the blood-brain barrier. The decreased expression of ATP7A protein in MoBr/y Purkinje cells was associated with impaired synaptogenesis and dramatic cytoskeletal dysfunction. Immunoblotting failed to reveal any compensatory increase in levels of ATP7B. While total levels of several cuproenzymes (peptide-amidating monooxygenase, SOD1, and SOD3) were unaltered in the MoBr/y brain, levels of amidated cholecystokinin (CCK8) and amidated pituitary adenylate cyclase-activating polypeptide (PACAP38) were reduced in a tissue-specific fashion. The compensatory changes observed in the neurovascular unit provide insight into the success of copper injections within a defined neurodevelopmental period.

Glial cell-derived cytokines attenuate the breakdown of vascular integrity in diabetic retinopathy

The blood-retinal barrier (BRB) is a biological unit comprised of specialized capillary endothelial cells firmly connected by intercellular tight junctions and endothelium-surrounding glial cells. The BRB is essential for maintaining the retinal microenvironment and low permeability and is compromised in an early phase during the progression of diabetic retinopathy. Here, we demonstrate that retinoic acid receptor (RAR)alpha stimulants preferentially act on glial cells rather than endothelial cells, resulting in the enhanced expression of glial cell line-derived neurotrophic factor (GDNF) through recruitment of the RARalpha-driven trans-acting coactivator to the 5'-flanking region of the gene promoter. Conversely, RARalpha decreases expression of vascular endothelial growth factor (VEGF)/vascular permeability factor. These gene expression alterations causally limit vascular permeability by modulating the tight junction function of capillary endothelium in a paracrine manner in vitro. The phenotypic transformation of glial cells mediated by RARalpha is sufficient for significant reductions of vascular leakage in the diabetic retina, suggesting that RARalpha antagonizes the loss of tight junction integrity induced by diabetes. These findings reveal that glial cell-derived cytokines such as GDNF and VEGF regulate BRB function, implying that the glial cell can be a possible therapeutic target in diabetic retinopathy.

Kim Y, Liu S, Kim KS. Human astrocytes/astrocyte-conditioned medium and shear stress enhance the barrier properties of human brain microvascular endothelial cells

The blood-brain barrier (BBB) is a structural and functional barrier that regulates the passage of molecules into and out of the brain to maintain the neural microenvironment. We have previously developed the in vitro BBB model with human brain microvascular endothelial cells (HBMEC). However, in vivo HBMEC are shown to interact with astrocytes and also exposed to shear stress through blood flow. In an attempt to develop the BBB model to mimic the in vivo condition we constructed the flow-based in vitro BBB model using HBMEC and human fetal astrocytes (HFA). We also examined the effect of astrocyte-conditioned medium (ACM) in lieu of HFA to study the role of secreted factor(s) on the BBB properties. The tightness of HBMEC monolayer was assessed by the permeability of dextran and propidium iodide as well as by measuring the transendothelial electrical resistance (TEER). We showed that the HBMEC permeability was reduced and TEER was increased by non-contact, co-cultivation with HFA and ACM. The exposure of HBMEC to shear stress also exhibited decreased permeability. Moreover, HFA/ACM and shear flow exhibited additive effect of decreasing the permeability of HBMEC monolayer. In addition, we showed that the HBMEC expression of ZO-1 (tight junction protein) was increased by co-cultivation with ACM and in response to shear stress. These findings suggest that the non-contact co-cultivation with HFA helps maintain the barrier properties of HBMEC by secreting factor(s) into the medium. Our in vitro flow model system with the cells of human origin should be useful for studying the interactions between endothelial cells, glial cells, and secreted factor(s) as well as the role of shear stress in the barrier property of HBMEC.

Possible involvement of gap junctions in the barrier function of tight junctions of brain and lung endothelial cells

Gap-junction plaques are often observed with tight-junction strands of vascular endothelial cells but the molecular interaction and functional relationships between these two junctions remain obscure. We herein show that gap-junction proteins connexin40 (Cx40) and Cx43 are colocalized and coprecipitated with tight-junction molecules occludin, claudin-5, and ZO-1 in porcine blood-brain barrier (BBB) endothelial cells. Gap junction blockers 18beta-glycyrrhetinic acid (18beta-GA) and oleamide (OA) did not influence expression of Cx40, Cx43, occludin, claudin-5, junctional adhesion molecule (JAM)-A, JAM-B, JAM-C, or ZO-1, or their subcellular localization in the porcine BBB endothelial cells. In contrast, these gap-junction blocking agents inhibited the barrier function of tight junctions in cells, determined by measurement of transendothelial electrical resistance and paracellular flux of mannitol and inulin. 18beta-GA also significantly reduced the barrier property in rat lung endothelial (RLE) cells expressing doxycycline-induced claudin-1, but did not change the interaction between Cx43 and either claudin-1 or ZO-1, nor their expression levels or subcellular distribution. These findings suggest that Cx40- and/or Cx43-based gap junctions might be required to maintain the endothelial barrier function without altering the expression and localization of the tight-junction components analyzed.

Blood-brain barrier interfaces and brain tumors

In the developing brain, capillaries are differentiated and matured into the blood-brain barrier (BBB), which is composed of cerebral endothelial cells, astrocyte end-feet, and pericytes. Since the BBB regulates the homeostasis of central nervous system (CNS), the maintenance of the BBB is important for CNS function. The disruption of the BBB may result in many brain disorders including brain tumors. However, the molecular mechanism of BBB formation and maintenance is poorly understood. Here, we summarize recent advances in the role of oxygen tension and growth factors on BBB development and maintenance, and in BBB dysfunction related with brain tumors.

Glyceraldehyde-derived advanced glycation end-products preferentially induce VEGF expression and reduce GDNF expression in human astrocytes

The blood-brain barrier (BBB) is a biological unit composed of capillary endothelial cells and astrocytes. Here we examined the effects of various types of advanced glycation end-products (AGEs) on astrocytes and BBB-forming endothelial cells. While no type of AGE we examined changed the permeability of endothelial sheets, glyceraldehyde-derived AGE induced VEGF expression most significantly in astrocytes. The expression of glial cell line-derived neurotrophic factor (GDNF), which reduces the vascular permeability, was decreased in the astrocytes by treatment with glyceraldehyde-derived AGE. These results indicate that glyceraldehyde-derived AGE is the biologically active substance for astrocytes by regulating the VEGF and GDNF expression, which is causally contributing to an increase in the permeability of the BBB.

Dynamics of CNS Barriers: Evolution, Differentiation, and Modulation

(1) Three main barrier layers at the interface between blood and tissue protect the central nervous system (CNS): the endothelium of brain capillaries, and the epithelia of the choroid plexus (CP) and the arachnoid. The classical work on these barriers in situ until the 1970s laid the foundations for modern understanding. Techniques for brain endothelial cell isolation and culture pioneered by Ferenc Joó in the 1970s opened up new fields of examination, enabling study of mechanisms at the cellular and molecular level. (2) Astrocytic glial cells are closely associated with the brain endothelial barrier. During evolution the barrier appears to have shifted from the glial to the endothelial layer, in parallel with the increasing importance of the microvasculature and its regulation. Vestiges of the barrier potential of glia remain in the modern mammalian CNS. (3) Evolutionary evidence suggests that the advantage derived from ionic homeostasis around central synapses was the major selective pressure leading to refinement of CNS barrier systems. This is one element of the modern 'multitasking' barrier function. (4) While epithelia are constitutively able to form barriers at appropriate interfaces, the 'default' condition for endothelia is more leaky; inductive influences from associated cells especially astrocytes are important in generating the full blood-brain barrier (BBB) phenotype in brain capillaries. The underlying mechanisms are being elucidated at the molecular and genomics level. (5) The barrier layers of the nervous system can be modulated by a number of receptor-mediated processes, involving several signal transduction pathways, both calcium dependent and independent. Some agents acting as 'inducers' in the long term can act as 'modulators' in the short-term, with some overlap of signaling pathways. Modulating agents may be derived both from the blood and from cells associated with cerebral vessels. Less is known about the modulation of the CP. (6) The challenge for the next era of CNS barrier studies will be to apply new knowledge from proteomics and genomics to understanding the in vivo condition in physiology and pathology.

The blood-brain barrier: an overview: structure, regulation, and clinical implications

The blood-brain barrier (BBB) is a diffusion barrier, which impedes influx of most compounds from blood to brain. Three cellular elements of the brain microvasculature compose the BBB-endothelial cells, astrocyte end-feet, and pericytes (PCs). Tight junctions (TJs), present between the cerebral endothelial cells, form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the TJ barrier, but astrocytes are not believed to have a barrier function in the mammalian brain. Dysfunction of the BBB, for example, impairment of the TJ seal, complicates a number of neurologic diseases including stroke and neuroinflammatory disorders. We review here the recent developments in our understanding of the BBB and the role of the BBB dysfunction in CNS disease. We have focused on intraventricular hemorrhage (IVH) in premature infants, which may involve dysfunction of the TJ seal as well as immaturity of the BBB in the germinal matrix (GM). A paucity of TJs or PCs, coupled with incomplete coverage of blood vessels by astrocyte end-feet, may account for the fragility of blood vessels in the GM of premature infants. Finally, this review describes the pathogenesis of increased BBB permeability in hypoxia-ischemia and inflammatory mechanisms involving the BBB in septic encephalopathy, HIV-induced dementia, multiple sclerosis, and Alzheimer disease.

The temporospatial expression of peripheral myelin protein 22 at the developing blood-nerve and blood-brain barriers

Peripheral myelin protein 22 (PMP22), also known as growth arrest-specific gene 3 (gas3), is a tetraspan membrane protein whose misexpression is associated with demyelinating peripheral neuropathies. Although the function of PMP22 in Schwann cells is unknown, the protein is found at intercellular junctions of various epithelia and endothelia. To begin to elucidate the role of PMP22 at cell junctions, we examined the temporal expression and protein localization during development and maturation of the rat blood-nerve barrier (BNB) and blood-brain barrier (BBB). Developing and adult rat sciatic nerves and brains were coimmunostained for PMP22 and known junctional proteins including zonula occludens-1 (ZO-1), occludin, and claudin-5. Prior to the maturation of the BNB and BBB and detection of the tight junction protein occludin, PMP22 is present at ZO-1 positive endothelial junctions of the sciatic nerve and brain cortex. The subcellular localization of PMP22 in cultured brain endothelia was confirmed by internalization with ZO-1 after EGTA-induced disruption of cell junctions. In choroid epithelia, PMP22 is detected along with occludin and ZO-1 as early as embryonic day 15 (E15). In agreement, PMP22 message is elevated in P1 rat brain microvasculature and choroid epithelia, compared with total cortex. Additionally, neuroepithelial cell junctions in the embryonic rat brain are immunoreactive for PMP22, ZO-1, and beta-catenin but not occludin. Together, these studies identify PMP22 as an early constituent of intercellular junctions in the developing and mature rat BNB and BBB.

Gonadotropin-releasing hormone agonist administration reduced vascular endothelial growth factor (VEGF), VEGF receptors, and vascular permeability of the ovaries of hyperstimulated rats

OBJECTIVE

Using hyperstimulated rats, to elucidate the mechanisms of gonadotropin-releasing hormone agonist (GnRH-a) treatment to prevent early ovarian hyperstimulation syndrome (OHSS).

DESIGN

Descriptive study of hyperstimulated rats as an early OHSS model with Western blot analysis, Northern blot hybridization, and vascular permeability assay.

SETTING

Experimental laboratory research.

ANIMAL(S)

Sprague-Dawley female rats were used for collecting ovarian samples.

INTERVENTION(S)

Hyperstimulated rats received consecutive GnRH-a treatment from the start of pregnant mare serum gonadotropin (PMSG) treatment through 2 days after hCG administration.

MAIN OUTCOME MEASURE(S)

Expressions of vascular endothelial growth factor (VEGF), VEGF receptor-1 (VEGFR-1: Flt-1), VEGF receptor-2 (VEGFR-2: KDR/Flk-1), and vascular permeability by Evans blue leakage.

RESULT(S)

GnRH-a treatment significantly reduced expressions of VEGF, VEGFR-1, and VEGFR-2 both in mRNA and protein levels in the ovaries of hyperstimulated rats. GnRH-a treatment also reduced vascular permeability in the ovaries of hyperstimulated rats.

CONCLUSION(S)

It is speculated that GnRH-a treatment may prevent early OHSS by reducing vascular permeability through the decrease in VEGF and its receptors.

Cyclic AMP induces phosphorylation of claudin-5 immunoprecipitates and expression of claudin-5 gene in blood-brain-barrier endothelial cells via protein kinase A-dependent and -independent pathways

Cyclic AMP (cAMP) promotes functions of tight junctions in endothelial cells, although its target remains unknown. We showed here that cAMP increased gene expression of claudin-5 and decreased that of claudin-1 in porcine blood-brain-barrier endothelial cells via protein kinase A (PKA)-independent and -dependent pathways, respectively. cAMP also enhanced immunoreactivity of claudin-5 along cell borders and in the cytoplasm, reorganized actin filaments, and altered signals of claudin-5, occludin, ZO-1, and ZO-2 along cell boundaries from zipperlike to linear patterns. In contrast, claudin-1 was detected only in the cytoplasm in a dotlike pattern, and its immunolabeling was reduced by cAMP. Interestingly, 31- and 62-kDa claudin-5 immunoprecipitates in the NP-40-soluble and -insoluble fractions, respectively, were highly phosphorylated on threonine residue(s) upon cAMP treatment. All these changes induced by cAMP, except for claudin-5 expression and its signals in the cytoplasm, were reversed by an inhibitor of PKA, H-89. We also demonstrated that cAMP elevated the barrier function of tight junctions in porcine blood-brain-barrier endothelial cells in PKA-dependent and -independent manners. These findings indicate that both PKA-induced phosphorylation of claudin-5 immunoprecipitates and cAMP-dependent but PKA-independent induction of claudin-5 expression could be involved in promotion of tight-junction function in endothelial cells.

Polyunsaturated fatty acids induce tight junctions to form in brain capillary endothelial cells

Tight junctions create a rate-limiting barrier to the diffusion of solutes between vertebrate epithelial cells and endothelial cells. They are also controlled within individual cells by a variety of physiologically relevant signals. We investigated the effects of polyunsaturated fatty acids on the formation of tight junctions in brain capillary endothelial cells, monitoring the transepithelial electrical resistance, and analyzed the expression of occludin messenger RNA. Brain-capillary endothelial cells were grown to confluence on filters and exposed to eicosapentaenoic acids, gamma linolenic acid and linoleic acid. Transepithelial electrical resistance was determined with voltage-measuring electrodes. The messenger RNA expression of occludin was quantitated by real-time quantitative reverse transcriptase-polymerase chain reaction. The basal resistance across monolayers of porcine brain capillary endothelial cells was 83+/-8.1 Omega cm(2). Cells cultured in eicosapentaenoic acids and gamma linolenic acid, but not linolenic acid, displayed a 2.7-fold increase in transepithelial electrical resistance at 10 microM in brain capillary endothelial cells. The expression level of occludin messenger RNA increased markedly immediately after the exposure to eicosapentaenoic acids or gamma linolenic acid. Following an 8 h exposure to exogenous eicosapentaenoic acids or gamma linolenic acid, occludin messenger RNA levels were significantly increased. In addition, the rise in transepithelial electrical resistance induced by eicosapentaenoic acids and gamma linolenic acid was markedly inhibited by the tyrosine kinase inhibitors genistein and PP2 and protein kinase C inhibitor, calphostin C. In contrast, the rise in transepithelial electrical resistance induced by eicosapentaenoic acids and gamma linolenic acid was not inhibited by the PI 3-kinase inhibitor, LY294002. We conclude that eicosapentaenoic acids and gamma linolenic acid increased the transepithelial electrical resistance and the expression of occludin messenger RNA in brain capillary endothelial cells. This gamma linolenic acid and eicosapentaenoic acid induced assembly of tight junction is likely to be regulated by protein kinase C and tyrosine kinase activity.

Glial cell line-derived neurotrophic factor does not enter normal mouse brain

Glial cell line-derived neurotrophic factor (GDNF) is produced both in the central nervous system (CNS) and the periphery. Effective in ameliorating neurodegeneration in several animal models of CNS disease, its promise as a therapeutic agent would be greatly enhanced if it readily crossed the blood-brain barrier (BBB) in unmodified form. Here, we used the sensitive techniques of multiple-time regression analysis and ex-vivo perfusion in blood-free buffer to examine the entry of (125)I-GDNF into mouse brain. The integrity of GDNF in blood and brain was examined by high performance liquid chromatography and the physicochemical properties determining permeability were measured by octanol/buffer partition coefficient and hydrogen bonding. The efflux of (125)I-GDNF was determined to test for the presence of a bidirectional transport system. The results show that (125)I-GDNF differs from other peptides and polypeptides in that it does not enter brain any faster than (99m)Tc-albumin, an effect that cannot be explained by degradation, rapid efflux, protein binding, or inadequate lipophilicity. Thus, GDNF shows a different type of interaction with the BBB. In normal mice, the BBB functions as a substantial physical barrier; in pathological or traumatic situations when the barrier is partially disrupted, the lack of restriction by a saturable transport system could make GDNF a suitable candidate for peripheral delivery in promoting neuroregeneration.

Astrocyte-endothelial interactions and blood-brain barrier permeability

The blood-brain barrier (BBB) is formed by brain endothelial cells lining the cerebral microvasculature, and is an important mechanism for protecting the brain from fluctuations in plasma composition, and from circulating agents such as neurotransmitters and xenobiotics capable of disturbing neural function. The barrier also plays an important role in the homeostatic regulation of the brain microenvironment necessary for the stable and co-ordinated activity of neurones. The BBB phenotype develops under the influence of associated brain cells, especially astrocytic glia, and consists of more complex tight junctions than in other capillary endothelia, and a number of specific transport and enzyme systems which regulate molecular traffic across the endothelial cells. Transporters characteristic of the BBB phenotype include both uptake mechanisms (e.g. GLUT-1 glucose carrier, L1 amino acid transporter) and efflux transporters (e.g. P-glycoprotein). In addition to a role in long-term barrier induction and maintenance, astrocytes and other cells can release chemical factors that modulate endothelial permeability over a time-scale of seconds to minutes. Cell culture models, both primary and cell lines, have been used to investigate aspects of barrier induction and modulation. Conditioned medium taken from growing glial cells can reproduce some of the inductive effects, evidence for involvement of diffusible factors. However, for some features of endothelial differentiation and induction, the extracellular matrix plays an important role. Several candidate molecules have been identified, capable of mimicking aspects of glial-mediated barrier induction of brain endothelium; these include TGFbeta, GDNF, bFGF, IL-6 and steroids. In addition, factors secreted by brain endothelial cells including leukaemia inhibitory factor (LIF) have been shown to induce astrocytic differentiation. Thus endothelium and astrocytes are involved in two-way induction. Short-term modulation of brain endothelial permeability has been shown for a number of small chemical mediators produced by astrocytes and other nearby cell types. It is clear that endothelial cells are involved in both long- and short-term chemical communication with neighbouring cells, with the perivascular end feet of astrocytes being of particular importance. The role of barrier induction and modulation in normal physiology and in pathology is discussed.

Tight junctions of the blood-brain barrier: development, composition and regulation

1. The blood-brain barrier is essential for the maintenance and regulation of the neural microenvironment. The main characteristic features of blood-brain barrier endothelial cells are an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. 2. Endothelial blood-brain barrier tight junctions differ from epithelial tight junctions, not only by distinct morphological and molecular properties, but also by the fact that endothelial tight junctions are more sensitive to microenvironmental than epithelial factors. 3. Many ubiquitous molecular tight junction components have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin and 7H6. Signaling pathways involved in tight junction regulation include G-proteins, serine-, threonine- and tyrosine-kinases, extra and intracellular calcium levels, cAMP levels, proteases and cytokines. Common to most of these pathways is the modulation of cytoskeletal elements and the connection of tight junction transmembrane molecules to the cytoskeleton. Additionally, crosstalk between components of the tight junction- and the cadherin-catenin system of the adherens junction suggests a close functional interdependence of the two cell-cell contact systems. 4. Important new molecular aspects of tight junction regulation were recently elucidated. This review provides an integration of these new results.

Expression of receptors for glial cell line-derived neurotrophic factor (GDNF) and neurturin in the inner blood-retinal barrier of rats

The retina is protected from somatic circulation by the blood-retinal barrrier (BRB) composed of tight junctions between retinal vascular endothelial cells (the inner BRB) and those between retinal pigment epithelial cells (the outer BRB). Our recent studies showed that glial cell line-derived neurotrophic factor (GDNF) secreted from astrocytes regulates the permeability of the BBB. In the present study, we immunohistochemically examined the expression of GDNF, neurturin (NTN) and their receptors, GFRalpha1 for GDNF and GFRalpha2 for NTN, because the capillaries of the inner BRB show specialization very similar to the blood-brain barrier (BBB). GDNF and NTN were detected in glial fibrillary acidic protein (GFAP)-positive cells, including Müller cells. GFRalpha1 and GFRalpha2 were localized in von Willebrand factor-positive cells. GDNF and NTN enhanced the barrier function of endothelial cells derived from porcine brain cortex. These results strongly suggest that the barrier function of the BRB is regulated by GDNF and NTN secreted from glial cells, like the BBB.