TNF activates P-glycoprotein in cerebral microvascular endothelial cells

Hopping the Hurdle: Strategies to Enhance the Molecular Delivery to the Brain through the Blood-Brain Barrier

Modern medicine has allowed for many advances in neurological and neurodegenerative disease (ND). However, the number of patients suffering from brain diseases is ever increasing and the treatment of brain diseases remains an issue, as drug efficacy is dramatically reduced due to the existence of the unique vascular structure, namely the blood-brain barrier (BBB). Several approaches to enhance drug delivery to the brain have been investigated but many have proven to be unsuccessful due to limited transport or damage induced in the BBB. Alternative approaches to enhance molecular delivery to the brain have been revealed in recent studies through the existence of molecular delivery pathways that regulate the passage of peripheral molecules. In this review, we present recent advancements of the basic research for these delivery pathways as well as examples of promising ventures to overcome the molecular hurdles that will enhance therapeutic interventions in the brain and potentially save the lives of millions of patients.

Regulation of P-Glycoprotein in the Brain

Maintenance of the tightly regulated homeostatic environment of the brain is facilitated by the blood-brain barrier (BBB). P-glycoprotein (P-gp), an ATP-binding cassette transporter, is expressed on the luminal surface of the endothelial cells in the BBB, and actively exports a wide variety of substrates to limit exposure of the vulnerable brain environment to waste buildup and neurotoxic compounds. Downregulation of P-gp expression and activity at the BBB have been reported with ageing and in neurodegenerative diseases. Upregulation of P-gp at the BBB contributes to poor therapeutic outcomes due to altered pharmacokinetics of CNS-acting drugs. The regulation of P-gp is highly complex, but unravelling the mechanisms involved may help the development of novel and nuanced strategies to modulate P-gp expression for therapeutic benefit. This review summarises the current understanding of P-gp regulation in the brain, encompassing the transcriptional, post-transcriptional and post-translational mechanisms that have been identified to affect P-gp expression and transport activity.

From the Molecular Mechanism to Pre-clinical Results: Anti-epileptic Effects of Fingolimod

Epilepsy is a devastating neurological condition characterized by long-term tendency to generate unprovoked seizures, affecting around 1-2% of the population worldwide. Epilepsy is a serious health concern which often associates with other neurobehavioral comorbidities that further worsen disease conditions. Despite tremendous research, the mainstream anti-epileptic drugs (AEDs) exert only symptomatic relief leading to 30% of untreatable patients. This reflects the complexity of the disease pathogenesis and urges the precise understanding of underlying mechanisms in order to explore novel therapeutic strategies that might alter the disease progression as well as minimize the epilepsy-associated comorbidities. Unfortunately, the development of novel AEDs might be a difficult process engaging huge funds, tremendous scientific efforts and stringent regulatory compliance with a possible chance of end-stage drug failure. Hence, an alternate strategy is drug repurposing, where anti-epileptic effects are elicited from drugs that are already used to treat non-epileptic disorders.

Herein, we provide evidence of the anti-epileptic effects of Fingolimod (FTY720), a modulator of sphingosine-1-phosphate (S1P) receptor, USFDA approved already for Relapsing-Remitting Multiple Sclerosis (RRMS). Emerging experimental findings suggest that Fingolimod treatment exerts disease-modifying anti-epileptic effects based on its anti-neuroinflammatory properties, potent neuroprotection, anti-gliotic effects, myelin protection, reduction of mTOR signaling pathway and activation of microglia and astrocytes. We further discuss the underlying molecular crosstalk associated with the anti-epileptic effects of Fingolimod and provide evidence for repurposing Fingolimod to overcome the limitations of current AEDs.

Increased penetration of diphenhydramine in brain via proton-coupled organic cation antiporter in rats with lipopolysaccharide-induced inflammation

Uptake transporters in brain microvascular endothelial cells (BMECs) are involved in the penetration of basic (cationic) drugs such as diphenhydramine (DPHM) into the brain. Lipopolysaccharide (LPS)-induced inflammation alters the expression levels and activities of uptake transporters, which change the penetration of DPHM into the brain. A brain microdialysis study showed that the unbound brain-to-plasma partition coefficient (K_p,uu,brain) for DPHM in LPS rats was approximately two times higher than that in control rats. The transcellular transport of DPHM to BMECs was increased when BMECs were cultured with serum from LPS rats. Compared with control rats or BMECs, the brain uptake of DPHM in LPS rats was increased and the intracellular accumulation of DPHM was increased under a high intracellular pH in BMECs from LPS rats, respectively. Treatment of BMECs with transporter inhibitors or inflammatory cytokines had little impact on the intracellular accumulation of DPHM in BMECs. This study suggests that LPS-induced inflammation promotes unidentified proton-coupled organic cation (H⁺/OC) antiporters that improve the penetration of DPHM into rat brain via the blood-brain barrier.

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The unbound brain-to-plasma partition coefficient for diphenhydramine (DPHM) was increased in lipopolysaccharide-induced inflammation in rats.

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The uptake of DPHM to brain microvascular endothelial cells (BMECs) was promoted by treatments of serum from rats with inflammation.

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Treatment of BMECs with transporter inhibitors or inflammatory cytokines had little impact on the intracellular accumulation of DPHM in BMECs.

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LPS-induced inflammation promotes unidentified proton-coupled organic cation antiporters that improve the brain penetration of DPHM.

A Spectrum of Topics for 2019: Advances in Neuroinflammation, Oxidative Stress, Obesity, Diabetes Mellitus, Cardiovascular Disease, Autism, Exosomes, and Central Nervous System Diseases

Advances in various fields were discussed in the reviews and original research articles published in 2019 in Current Pharmaceutical Design. Here, I review some of the major highlights for selected areas. A better understanding of disease mechanisms was a prominent recurrent theme and new therapeutic targets based on those mechanisms are highlighted here. Inflammation and oxidative stress are major features of many diseases, therefore, interventions to address these processes are reviewed. Although repurposing of old drugs occurred in several fields, drug targeting and drug delivery, especially of nanoparticles, also continues to be a major area of interest.

In vitro modeling of blood-brain barrier and interface functions in neuroimmune communication

Neuroimmune communication contributes to both baseline and adaptive physiological functions, as well as disease states. The vascular blood-brain barrier (BBB) and associated cells of the neurovascular unit (NVU) serve as an important interface for immune communication between the brain and periphery through the blood. Immune functions and interactions of the BBB and NVU in this context can be categorized into at least five neuroimmune axes, which include (1) immune modulation of BBB impermeability, (2) immune regulation of BBB transporters, secretions, and other functions, (3) BBB uptake and transport of immunoactive substances, (4) immune cell trafficking, and (5) BBB secretions of immunoactive substances. These axes may act separately or in concert to mediate various aspects of immune signaling at the BBB. Much of what we understand about immune axes has been from work conducted using in vitro BBB models, and recent advances in BBB and NVU modeling highlight the potential of these newer models for improving our understanding of how the brain and immune system communicate. In this review, we discuss how conventional in vitro models of the BBB have improved our understanding of the 5 neuroimmune axes. We further evaluate the existing literature on neuroimmune functions of novel in vitro BBB models, such as those derived from human induced pluripotent stem cells (iPSCs) and discuss their utility in evaluating aspects of neuroimmune communication.

The blood-brain barrier as an endocrine tissue

The blood-brain barrier (BBB) was first noted for its ability to prevent the unregulated exchange of substances between the blood and the central nervous system (CNS). Over time, its characterization as an interface that enables regulated exchanges between the CNS and substances that are carried in the blood in a hormone-like fashion have emerged. Therefore, communication between the CNS, BBB and peripheral tissues has many endocrine-like properties. In this Review, I examine the various ways in which the BBB exhibits endocrine-related properties. The BBB is a target for hormones, such as leptin and insulin, that affect many of its functions. The BBB is also a secretory body, releasing substances either into the blood or the interstitial fluid of the brain. The BBB selectively allows classical and non-classical hormones entry to and exit from the CNS, thus allowing the CNS to be both an endocrine target and a secretory tissue. The BBB is affected by endocrine diseases such as diabetes mellitus and can cause or participate in endocrine diseases, including those related to thyroid hormones and obesity. The endocrine-like mechanisms of the BBB can extend the definition of endocrine disease to include neurodegenerative conditions, including Alzheimer disease, and of hormones to include cytokines, triglycerides and fatty acids.

P-glycoprotein Inhibitor Tariquidar Potentiates Efficacy of Astragaloside IV in Experimental Autoimmune Encephalomyelitis Mice

ATP-binding cassette (ABC) transporters, such as P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), often reduce drug efficacy and are the major cause of drug resistance. Astragaloside IV (ASIV), one of the bioactive saponins isolated from Astragalus membranaceus, has been demonstrated to alleviate the progression of experimental autoimmune encephalomyelitis (EAE) in mice, an animal model for multiple sclerosis (MS). In the present study, we found for the first time that ASIV induced the upregulation of P-gp and BCRP in the central nervous system (CNS) microvascular endothelial cells of EAE mice. Further study disclosed that tariquidar, a P-gp inhibitor, could facilitate the penetration of ASIV into CNS. On bEnd.3 cells, a mouse brain microvascular endothelial cell line, tariquidar benefited the net uptake and transport of ASIV. Additional molecular docking experiment suggested that ASIV might be a potential substrate of P-gp. In EAE mice, tariquidar was demonstrated to enhance the efficacy of ASIV, as shown by attenuated clinical symptom and reduced incidence rate as well as mitigated inflammatory infiltration and decreased demyelination in the CNS. Collectively, our findings implicate that P-gp inhibitor can promote the therapeutic efficacy of ASIV on EAE mice, which may boost its clinical usage together with ASIV in the therapy of MS.

Verapamil and riluzole cocktail liposomes overcome pharmacoresistance by inhibiting P-glycoprotein in brain endothelial and astrocyte cells: A potent approach to treat amyotrophic lateral sclerosis

Riluzole is currently one of two approved medications for the treatment of amyotrophic lateral sclerosis (ALS). However, brain disposition of riluzole, as a substrate of P-glycoprotein (P-gp), is limited by the efflux transporters at the blood-brain barrier (BBB). We propose to develop a liposomal co-delivery system that could effectively transport riluzole to brain cells by reducing efflux pumps with a P-gp inhibitor, verapamil. Riluzole and verapamil cocktail liposomes were prepared by lipid film hydration. The average particle size of cocktail liposomes was 194.3 ± 6.0 nm and their polydispersity index (PDI) was 0.272 ± 0.017. The encapsulation efficiencies of verapamil and riluzole in the cocktail liposomes were 86.0 ± 1.4% and 85.6 ± 1.1%, respectively. The drug release from cocktail liposomes after 8 h in PBS at 37 °C was 78.4 ± 6.2% of riluzole and 76.7 ± 3.8% of verapamil. The average particle size of liposomes did not show significant changes at 4 °C after three months. Verapamil cocktail liposomes inhibited P-gp levels measured by western blotting in dose and time-dependent manners in brain endothelial bEND.3 cells. Increased drug efflux transporters were detected in bEND.3 and astrocytes C8D1A cells, promoted by tumor necrosis factor (TNF-α) or hydrogen peroxide (H₂O₂). Restored accumulations of riluzole and fluorescent dye rhodamine 123 were observed in bEND.3 cells after treatments with cocktail liposomes. It indicated that inhibitory potential of co-delivery liposome system towards P-gp could mediate the transport of both P-gp substrates. Verapamil and riluzole co-loaded liposomes may be used to overcome pharmacoresistance of riluzole for improving ALS therapy.

Neuroimmune Axes of the Blood-Brain Barriers and Blood-Brain Interfaces: Bases for Physiological Regulation, Disease States, and Pharmacological Interventions

Central nervous system (CNS) barriers predominantly mediate the immune-privileged status of the brain, and are also important regulators of neuroimmune communication. It is increasingly appreciated that communication between the brain and immune system contributes to physiologic processes, adaptive responses, and disease states. In this review, we discuss the highly specialized features of brain barriers that regulate neuroimmune communication in health and disease. In section I, we discuss the concept of immune privilege, provide working definitions of brain barriers, and outline the historical work that contributed to the understanding of CNS barrier functions. In section II, we discuss the unique anatomic, cellular, and molecular characteristics of the vascular blood-brain barrier (BBB), blood-cerebrospinal fluid barrier, and tanycytic barriers that confer their functions as neuroimmune interfaces. In section III, we consider BBB-mediated neuroimmune functions and interactions categorized as five neuroimmune axes: disruption, responses to immune stimuli, uptake and transport of immunoactive substances, immune cell trafficking, and secretions of immunoactive substances. In section IV, we discuss neuroimmune functions of CNS barriers in physiologic and disease states, as well as pharmacological interventions for CNS diseases. Throughout this review, we highlight many recent advances that have contributed to the modern understanding of CNS barriers and their interface functions.

Inflammatory cytokine production in tumor cells upon chemotherapy drug exposure or upon selection for drug resistance

Tumor Necrosis Factor alpha (TNF-α) has been shown to be released by tumor cells in response to docetaxel, and lipopolysaccharides (LPS), the latter through activation of toll-like receptor 4 (TLR4). However, it is unclear whether the former involves TLR4 receptor activation through direct binding of the drug to TLR4 at the cell surface. The current study was intended to better understand drug-induced TNF-α production in tumor cells, whether from short-term drug exposure or in cells selected for drug resistance. ELISAs were employed to measure cytokine release from breast and ovarian tumor cells in response to several structurally distinct chemotherapy agents and/or TLR4 agonists or antagonists. Drug uptake and drug sensitivity studies were also performed. We observed that several drugs induced TNF-αrelease from multiple tumor cell lines. Docetaxel-induced cytokine production was distinct from that of LPS in both MyD88-positive (MCF-7) and MyD88-deficient (A2780) cells. The acquisition of docetaxel resistance was accompanied by increased constitutive production of TNF-αand CXCL1, which waned at higher levels of resistance. In docetaxel-resistant MCF-7 and A2780 cell lines, the production of TNF-α could not be significantly augmented by docetaxel without the inhibition of P-gp, a transporter protein that promotes drug efflux from tumor cells. Pretreatment of tumor cells with LPS sensitized MyD88-positive cells (but not MyD88-deficient) to docetaxel cytotoxicity in both drug-naive and drug-resistant cells. Our findings suggest that taxane-induced inflammatory cytokine production from tumor cells depends on the duration of exposure, requires cellular drug-accumulation, and is distinct from the LPS response seen in breast tumor cells. Also, stimulation of the LPS-induced pathway may be an attractive target for treatment of drug-resistant disease.

An observational study examining the effects of a surgically induced inflammatory response on the distribution of morphine and its metabolites into cerebrospinal fluid

PURPOSE

Morphine is administered intravenously for pain management in the perioperative period. The effect of the inflammatory response to surgery on morphine distribution across the blood-brain barrier (BBB) in humans was investigated. We hypothesized that a graded surgically induced, systemic inflammatory response alters cerebrospinal fluid (CSF) levels of morphine, morphine-3-glucuronide (M3G), and morphine-6-glucuronide (M6G) through a temporary reduction in BBB drug efflux transporter function.

METHODS

We conducted a prospective pharmacokinetic study of the plasma and CSF distribution of the P-glycoprotein (PGP) substrate morphine in 33 patients undergoing open thoracic (n = 18) or endovascular (n = 15) aortic aneurysm repair. Morphine was administered with induction of anesthesia and in the intensive care unit. Plasma and CSF concentrations of interleukin (IL)-6, morphine, M3G, M6G, and albumin were measured prior to surgery (baseline), during surgery, and postoperatively every six hours until removal of the CSF drain. The area under the curve (AUC) was determined for plasma and CSF IL-6, morphine, M3G, and M6G concentrations vs time. The primary endpoint measures were the correlations between the morphine, M6G, and M3G AUC CSF/plasma ratios and systemic inflammation as quantified by the time-normalized IL-6 exposure, which was calculated for each individual by dividing the total exposure (AUC) by time (t). A Bonferroni corrected P < 0.017 indicated a significant correlation.

RESULTS

Plasma and CSF IL-6 concentrations increased postoperatively. The median [interquartile range] IL-6 exposures were significantly higher in the open vs endovascular surgical group for plasma (105 [40-256] pg·mL^-1 vs 29 [16-70] pg·mL^-1, respectively; P = 0.013) and CSF (79 [26-133] pg·mL^-1 vs 16 [9-80] pg·mL^-1, respectively; P = 0.013). For the primary endpoint, the plasma IL-6 AUC/t did not correlate with the CSF accumulation of morphine (r = -0.009; P = 0.96) or M3G (r = 0.37; P = 0.04) when corrected for surgical procedure, age, and sex. There were insufficient data on CSF concentration to complete the primary analysis for M6G.

CONCLUSION

Morphine distribution into the CSF was not significantly altered in patients undergoing thoracic aortic aneurysm repair. This suggests that BBB PGP function may not be affected by the perioperative inflammatory response.

TRIAL REGISTRATION

www.clinicaltrials.gov , NCT 00878371. Registered 7 April 2009.

Physical insights into the blood-brain barrier translocation mechanisms

The number of individuals suffering from diseases of the central nervous system (CNS) is growing with an aging population. While candidate drugs for many of these diseases are available, most of these pharmaceutical agents cannot reach the brain rendering most of the drug therapies that target the CNS inefficient. The reason is the blood-brain barrier (BBB), a complex and dynamic interface that controls the influx and efflux of substances through a number of different translocation mechanisms. Here, we present these mechanisms providing, also, the necessary background related to the morphology and various characteristics of the BBB. Moreover, we discuss various numerical and simulation approaches used to study the BBB, and possible future directions based on multi-scale methods. We anticipate that this review will motivate multi-disciplinary research on the BBB aiming at the design of effective drug therapies.

Inflammation and Immune Regulation as Potential Drug Targets in Antidepressant Treatment

Growing evidence supports a mutual relationship between inflammation and major depression. A variety of mechanisms are outlined, indicating how inflammation may be involved in the pathogenesis, course and treatment of major depression. In particular, this review addresses 1) inflammatory cytokines as markers of depression and potential predictors of treatment response, 2) findings that cytokines interact with antidepressants and non-pharmacological antidepressive therapies, such as electroconvulsive therapy, deep brain stimulation and physical activity, 3) the influence of cytokines on the cytochrome (CYP) p450-system and drug efflux transporters, and 4) how cascades of inflammation might serve as antidepressant drug targets. A number of clinical trials have focused on agents with immunmodulatory properties in the treatment of depression, of which this review covers nonsteroidal anti-inflammatory drugs (NSAIDs), cytokine inhibitors, ketamine, polyunsaturated fatty acids, statins and curcumin. A perspective is also provided on possible future immune targets for antidepressant therapy, such as toll-like receptor-inhibitors, glycogen synthase kinase-3 inhibitors, oleanolic acid analogs and minocycline. Concluding from the available data, markers of inflammation may become relevant factors for more personalised planning and prediction of response of antidepressant treatment strategies. Agents with anti-inflammatory properties have the potential to serve as clinically relevant antidepressants. Further studies are required to better define and identify subgroups of patients responsive to inflammatory agents as well as to define optimal time points for treatment onset and duration.

Age-Dependent Regulation of the Blood-Brain Barrier Influx/Efflux Equilibrium of Amyloid-β 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 Aβ 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 Aβ 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]Aβ1 - 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 Aβ 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 Aβ is maintained in 3 and 6-month-old 3xTg-AD mice, suggesting that Abcg4 and Abca1 control the efflux of Aβ through the BBB by a direct (Abcg4) or indirect (Abca1) mechanism. At 18 months, the BBB Aβ 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 Aβ and cholesterol transporters at the BBB could ultimately limit the brain accumulation of Aβ.

From blood-brain barrier to blood-brain interface: new opportunities for CNS drug delivery

One of the biggest challenges in the development of therapeutics for central nervous system (CNS) disorders is achieving sufficient blood-brain barrier (BBB) penetration. Research in the past few decades has revealed that the BBB is not only a substantial barrier for drug delivery to the CNS but also a complex, dynamic interface that adapts to the needs of the CNS, responds to physiological changes, and is affected by and can even promote disease. This complexity confounds simple strategies for drug delivery to the CNS, but provides a wealth of opportunities and approaches for drug development. Here, I review some of the most important areas that have recently redefined the BBB and discuss how they can be applied to the development of CNS therapeutics.

Sleep fragmentation and sepsis differentially impact blood-brain barrier integrity and transport of tumor necrosis factor-α in aging

The factors by which aging predisposes to critical illness are varied, complex, and not well understood. Sepsis is considered a quintessential disease of old age because the incidence and mortality of severe sepsis increases in old and the oldest old individuals. Aging is associated with dramatic changes in sleep quality and quantity and sleep increasingly becomes fragmented with age. In healthy adults, sleep disruption induces inflammation. Multiple aspects of aging and of sleep dysregulation interact via neuroimmune mechanisms. Tumor necrosis factor-α (TNF), a cytokine involved in sleep regulation and neuroimmune processes, exerts some of its effects on the CNS by crossing the blood-brain barrier (BBB). In this study we examined the impact of sepsis, sleep fragmentation, and aging on BBB disruption and TNF transport into brain. We used the cecal ligation and puncture (CLP) model of sepsis in young and aged mice that were either undisturbed or had their sleep disrupted. There was a dichotomous effect of sepsis and sleep disruption with age: sepsis disrupted the BBB and increased TNF transport in young mice but not in aged mice, whereas sleep fragmentation disrupted the BBB and increased TNF transport in aged mice, but not in young mice. Combining sleep fragmentation and CLP did not produce a greater effect on either of these BBB parameters than did either of these manipulations alone. These results suggest that the mechanisms by which sleep fragmentation and sepsis alter BBB functions are fundamentally different from one another and that a major change in the organism's responses to those insults occurs with aging.

Peptides at the blood brain barrier: Knowing me knowing you

When the Davis Lab was first asked to contribute to this special edition of Peptides to celebrate the career and influence of Abba Kastin on peptide research, it felt like a daunting task. It is difficult to really understand and appreciate the influence that Abba has had, not only on a generation of peptide researchers, but also on the field of blood brain barrier (BBB) research, unless you lived it as we did. When we look back at our careers and those of our former students, one can truly see that several of Abba's papers played an influential role in the development of our personal research programs.

The blood-brain barrier in neuroimmunology: Tales of separation and assimilation

Neuroimmunology is concerned with the relations between the central nervous and immune systems and with the mechanisms that drive those relations. The blood-brain barrier (BBB) employs mechanisms that both separate and connect these two systems. In fact, the relative immune privilege of the central nervous system (CNS) is largely attributable to the BBB's ability to prevent the unregulated exchange of immune cells and their secretions between the CNS and blood. Having separated the two systems, the BBB then participates in mechanisms that allow them to influence, communicate, and interact with one another. Likewise, the BBB itself is influenced by immune events that are occurring in the periphery and in the CNS so that these three components (the BBB, the immune system, and the CNS) form neuroimmune axes that adapt to physiological and pathological conditions. To date, four major themes have emerged by which the BBB participates in these neuroimmune axes. The first of these four, the formation of the barrier, acts to separate the immune and central nervous systems. The other three themes provide mechanisms for re-establishing communication: response of the BBB to immunomodulatory molecules (e.g., prostaglandins, cytokines, chemokines, nitric oxide) secreted by immune and CNS cells; the controlled, regulated exchange of chemokines, cytokines, and immune cells between the CNS and the blood (i.e., transport across the BBB); the secretion of immunomodulatory molecules by the BBB, often in a polarized fashion. Taken together, these mechanisms reveal the BBB to be a dynamic, interactive, and adaptable interface between the immune system and the CNS, separating them on the one hand and fostering their interactions on the other hand, adjusting to physiological changes, while being a target for disease processes. This review examines specific examples by which the BBB plays an interactive, defining role in neuroimmunology.

Alterations in function and expression of ABC transporters at blood-brain barrier under diabetes and the clinical significances

Diabetes is a systematic metabolic disease, which often develops a number of well-recognized vascular complications including brain complications which may partly result from the dysfunction of blood-brain barrier (BBB). BBB is generally considered as a mechanism for protecting the brain from unwanted actions resulting from substances in the blood and maintaining brain homeostasis via monitoring the entry or efflux of compounds. ATP-binding cassette (ABC) family of transporters including P-glycoprotein (P-GP) and breast cancer-related protein (BCRP), widely expressed in the luminal membrane of the microvessel endothelium and in the apical membrane of the choroids plexus epithelium, play important roles in the function of BBB. However, these transporters are easily altered by some diseases. The present article was focused on the alteration in expression and function of both P-GP and BCRP at BBB by diabetes and the clinical significances.

The Changes of P-glycoprotein Activity by Interferon-γ and Tumor Necrosis Factor-α in Primary and Immortalized Human Brain Microvascular Endothelial Cells

The purpose of this study was to investigate the modification of expression and functionality of the drug transporter P-glycoprotein (P-gp) by tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ) at the blood-brain barrier (BBB). We used immortalized human brain microvessel endothelial cells (iHBMEC) and primary human brain microvessel endothelial cells (pHBMEC) as in vitro BBB model. To investigate the change of p-gp expression, we carried out real time PCR analysis and Western blotting. To test the change of p-gp activity, we performed rhodamin123 (Rh123) accumulation study in the cells. In results of real time PCR analysis, the P-gp mRNA expression was increased by TNF-α or IFN-γ treatment for 24 hr in both cell types. However, 48 hr treatment of TNF-α or IFN-γ did not affect P-gp mRNA expression. In addition, co-treatment of TNF-α and IFN-γ markedly increased the P-gp mRNA expression in both cells. TNF-α or IFN-γ did not influence P-gp protein expression whatever the concentration of cytokines or duration of treatment in both cells. However, P-gp expression was increased after treatments of both cytokines together in iHBMEC cells only compared with untreated control. Furthermore, in both cell lines, TNF-α or IFN-γ induced significant decrease of P-gp activity for 24 hr treatment. And, both cytokines combination treatment also decreased significantly P-gp activity. These results suggest that P-gp expression and function at the BBB is modulated by TNF-α or/and IFN-γ. Therefore, the distribution of P-gp depending drugs in the central nervous system can be modulated by neurological inflammatory diseases.

Cytokine signaling modulates blood-brain barrier function

The blood-brain barrier (BBB) provides a vast interface for cytokines to affect CNS function. The BBB is a target for therapeutic intervention. It is essential, therefore, to understand how cytokines interact with each other at the level of the BBB and how secondary signals modulate CNS functions beyond the BBB. The interactions between cytokines and lipids, however, have not been fully addressed at the level of the BBB. Here, we summarize current understanding of the localization of cytokine receptors and transporters in specific membrane microdomains, particularly lipid rafts, on the luminal (apical) surface of the microvascular endothelial cells composing the BBB. We then illustrate the clinical context of cytokine effects on the BBB by neuroendocrine regulation and amplification of inflammatory signals. Two unusual aspects discussed are signaling crosstalk by different classes of cytokines and genetic regulation of drug efflux transporters. We also introduce a novel area of focus on how cytokines may act through nuclear hormone receptors to modulate efflux transporters and other targets. A specific example discussed is the ATP-binding cassette transporter-1 (ABCA-1) that regulates lipid metabolism. Overall, cytokine signaling at the level of the BBB is a crucial feature of the dynamic regulation that can rapidly change BBB function and affect brain health and disease.

The androgen metabolite, 5α-androstane-3β,17β-diol, decreases cytokine-induced cyclooxygenase-2, vascular cell adhesion molecule-1 expression, and P-glycoprotein expression in male human brain microvascular endothelial cells

P-glycoprotein (Pgp), a multiple drug resistance transporter expressed by vascular endothelial cells, is a key component of the blood-brain barrier and has been shown to increase after inflammation. The nonaromatizable androgen, dihydrotestosterone (DHT), decreases inflammatory markers in vascular smooth muscle cells, independent of androgen receptor (AR) stimulation. The principal metabolite of DHT, 5α-androstane-3β,17β-diol (3β-diol), activates estrogen receptor (ER)β and similarly decreases inflammatory markers in vascular cells. Therefore, we tested the hypothesis that either DHT or 3β-diol decrease cytokine-induced proinflammatory mediators, vascular cell adhesion molecule-1 (VCAM-1) and cyclooxygenase-2 (COX-2), to regulate Pgp expression in male primary human brain microvascular endothelial cells (HBMECs). Using RT-qPCR, the mRNAs for AR, ERα, and ERβ and steroid metabolizing enzymes necessary for DHT conversion to 3β-diol were detected in male HBMECs demonstrating that the enzymes and receptors for production of and responsiveness to 3β-diol are present. Western analysis showed that 3β-diol reduced COX-2 and Pgp expression; the effect on Pgp was inhibited by the ER antagonist, ICI-182,780. IL-1β-caused an increase in COX-2 and VCAM-1 that was reduced by either DHT or 3β-diol. 3β-diol also decreased cytokine-induced Pgp expression. ICI-182,780 blocked the effect of 3β-diol on COX-2 and VCAM-1, but not Pgp expression. Therefore, in cytokine-stimulated male HBMECs, the effect of 3β-diol on proinflammatory mediator expression is ER dependent, whereas its effect on Pgp expression is ER independent. These studies suggest a novel role of 3β-diol in regulating blood-brain barrier function and support the concept that 3β-diol can be protective against proinflammatory mediator stimulation.

Morphine affects HIV-induced inflammatory response without influencing viral replication in human monocyte-derived macrophages

Opiate-abusing individuals are in the top three risk-factor groups for HIV infection. In fact, almost 30% of HIV-infected individuals in the USA are reported to abuse opiates, highlighting the intersection of drugs of abuse with HIV/AIDS. Opiate-abusers are cognitively impaired and suffer from neurological dysfunctions that may lead to high-risk sexual behavior, poor adherence to antiretroviral regimens, and hepatitis-C virus infection. Collectively, these factors may contribute to accelerated HIV central nervous system (CNS) disease progression. To understand the role of morphine in disease progression, we sought to determine whether morphine influences HIV-induced inflammation or viral replication in human monocyte-derived macrophages (h-mdms) and MAGI cells infected with HIV and exposed to morphine. Chronic morphine exposure of HIV-infected h-mdms led to significant alterations in the secretion of IL-6 and monocyte chemoattractant protein 2 (MCP-2). Morphine enhanced IL-6 secretion and blunted MCP-2 secretion from HIV-infected h-mdms. However, exposure of HIV-infected h-mdms to morphine had no effect on tumor necrosis factor alpha secretion. Morphine had no effect on later stages of viral replication in HIV-infected h-mdms. Morphine had a potentially additive effect on the HIV-induced production of IL-6 and delayed HIV-induced MCP-2 production. These results suggest that in HIV-infected opiate-abusers, enhanced CNS inflammation might result even when HIV disease is controlled.

Drug delivery to the brain in Alzheimer's disease: consideration of the blood-brain barrier

The successful treatment of Alzheimer's disease (AD) will require drugs that can negotiate the blood-brain barrier (BBB). However, the BBB is not simply a physical barrier, but a complex interface that is in intimate communication with the rest of the central nervous system (CNS) and influenced by peripheral tissues. This review examines three aspects of the BBB in AD. First, it considers how the BBB may be contributing to the onset and progression of AD. In this regard, the BBB itself is a therapeutic target in the treatment of AD. Second, it examines how the BBB restricts drugs that might otherwise be useful in the treatment of AD and examines strategies being developed to deliver drugs to the CNS for the treatment of AD. Third, it considers how drug penetration across the AD BBB may differ from the BBB of normal aging. In this case, those differences can complicate the treatment of CNS diseases such as depression, delirium, psychoses, and pain control in the AD population.

Interactions between antidepressants and P-glycoprotein at the blood-brain barrier: clinical significance of in vitro and in vivo findings

The drug efflux pump P-glycoprotein (P-gp) plays an important role in the function of the blood-brain barrier by selectively extruding certain endogenous and exogenous molecules, thus limiting the ability of its substrates to reach the brain. Emerging evidence suggests that P-gp may restrict the uptake of several antidepressants into the brain, thus contributing to the poor success rate of current antidepressant therapies. Despite some inconsistency in the literature, clinical investigations of potential associations between functional single nucleotide polymorphisms in ABCB1, the gene which encodes P-gp, and antidepressant response have highlighted a potential link between P-gp function and treatment-resistant depression (TRD). Therefore, co-administration of P-gp inhibitors with antidepressants to patients who are refractory to antidepressant therapy may represent a novel therapeutic approach in the management of TRD. Furthermore, certain antidepressants inhibit P-gp in vitro, and it has been hypothesized that inhibition of P-gp by such antidepressant drugs may play a role in their therapeutic action. The present review summarizes the available in vitro, in vivo and clinical data pertaining to interactions between antidepressant drugs and P-gp, and discusses the potential relevance of these interactions in the treatment of depression.

Neuroinflammation: a common pathway in CNS diseases as mediated at the blood-brain barrier

The blood-brain barrier (BBB) is not simply a physical barrier but a regulatory interface between the central nervous system (CNS) and immune system. The BBB both affects and is affected by the immune system and connects at many levels with the CNS, including the following: (1) the BBB transports cytokines and secretes various substances with neuroinflammatory properties; (2) transporters are altered in disease states including traumatic injury, Alzheimer's disease and inflammatory processes; (3) cytokines and other immune secretions from the cells comprising the BBB are both constitutive and inducible; (4) immune cells are transported across the BBB by the highly regulated process termed diapedesis, which involves communication and interactions between the brain endothelial cells and the immune cells; (5) the neuroimmune system has various effects on the BBB, including modulation of important transport systems and in extreme pathological conditions even disruption of the BBB, and (6) the brain-to-blood efflux transporter P-glycoprotein is altered in inflammatory conditions, thus affecting drug delivery to the brain. In summary, the BBB is an interactive interface that regulates and defines many of the ways that the CNS and the immune system communicate with one another.

Concepts for biologically active peptides

Here we review a unique aspect of CNS research on biologically active peptides that started against a background of prevalent dogmas but ended by exerting considerable influence on the field. During the course of refuting some doctrines, we introduced several concepts that were unconventional and paradigm-shifting at the time. We showed that (1) hypothalamic peptides can act 'up' on the brain as well as 'down' on the pituitary, (2) peripheral peptides can affect the brain, (3) peptides can cross the blood-brain barrier, (4) the actions of peptides can persist longer than their half-lives in blood, (5) perinatal administration of peptides can exert actions persisting into adulthood, (6) a single peptide can have more than one action, (7) dose-response relationships of peptides need not be linear, (8) the brain produces antiopiate as well as opiate peptides, (9) there is a selective high affinity endogenous peptide ligand for the mu-opiate receptor, (10) a peptide's name does not restrict its effects, and (11) astrocytes assume an active role in response to metabolic disturbance and hyperleptinemia. The evolving questions in our laboratories reflect the diligent effort of the neuropeptide community to identify the roles of peptides in the CNS. The next decade is expected to see greater progress in the following areas: (a) interactions of peptides with other molecules in the CNS; (b) peptide involvement in cell-cell interactions; and (c) peptides in neuropsychiatric, autoimmune, and neurodegenerative diseases. The development of peptidomics and gene silencing approaches will expedite the formation of many new concepts in a new era.

Morphine treatment of human monocyte-derived macrophages induces differential miRNA and protein expression: impact on inflammation and oxidative stress in the central nervous system

HIV-1-infected opiate abusers often exhibit an accelerated form of HIV-1-associated dementia and enhanced neurological dysfunction. Productive HIV-1 infection of microglia and perivascular macrophages and the resultant secretion of neurotoxic molecules by these cells contribute to this phenomenon. In order to understand the role of morphine in this process, we performed a genome-wide association study at the micro RNA (miRNA) and protein levels in human monocyte-derived macrophages (h-mdms). A total of 26 differentially expressed miRNA were identified (P < 0.01), of which hsa-miR-15b and hsa-miR-181b had the greatest increase and decrease in expression levels, respectively. Computational analysis predicted fibroblast growth factor-2 (FGF-2) as the strongest target gene for hsa-miR15b. Of note, we observed a decrease in FGF-2 protein expression in response to morphine. Both hsa-miR-15b and hsa-miR-181b have several predicted gene targets involved in inflammation and T-cell activation pathways. In this context, we observed induction of MCP-2 and IL-6 by morphine. Moreover, proteomic analysis revealed the induction of mitochondrial superoxide dismutase in response to morphine treatment. HIV-1 infection did not induce mitochondrial superoxide dismutase. Collectively, these observations demonstrate that morphine induces inflammation and oxidative stress in h-mdms thereby contributing to expansion of HIV-1 CNS reservoir expansion and disease progression. Of note, differentially expressed miRNAs (hsa-miR-15b and 181-b) may have a potential role in regulating these processes.

The role of cerebral vascular NFkappaB in LPS-induced inflammation: differential regulation of efflux transporter and transporting cytokine receptors

BACKGROUND/AIMS

The transcription factor NFkappaB is a major mediator of lipopolysaccharide (LPS) signaling. We determined the role of NFkappaB activation in regulatory changes of the P-glycoprotein (Pgp) drug efflux transporter at the blood-brain barrier (BBB) and proinflammatory cytokine receptors.

METHODS

We treated NFkappaB knockout and wildtype mice with LPS or vehicle, obtained enriched cerebral microvessels, and determined target mRNA by qPCR for MDR1a/b, IL15Ralpha, IL2 Ralpha, IL2Rgamma, LIFR, gp130, and TNFR1/2, and protein expression by western blotting for P-gp, IL15Ralpha, IL2Rgamma, LIFR, and gp130.

RESULTS

The effects of LPS on the transporters and cytokine receptors showed differences between wildtype and NFkappaB knockout mice, and between mRNA and protein changes. NFkappaB not only mediated the LPS-induced increase of MDR1b, IL2Rgamma, and TNFR2 mRNA in the wildtype mice, but it showed opposite effects by elevating IL15Ralpha and TNFR1 mRNA and decreasing IL2Ralpha in the knockout mice. Although basal vinblastine uptake was unchanged in the NFkappaB knockout mice, LPS induced an increase of the uptake (depressed efflux transport) greater than that seen in the wildtype mice, indicating that NFkappaB helps to maintain Pgp efflux transporter function.

CONCLUSION

The results show differential involvement of NFkappaB signaling in response to LPS at the BBB.

Cessation of blood-to-brain influx of interleukin-15 during development of EAE

Regulatory changes in cytokine permeation across the blood-brain barrier (BBB) may have crucial roles in central nervous system (CNS) autoimmune disease. Accordingly, we examined the interactions of interleukin (IL)-15 with the cerebral vasculature after induction of experimental autoimmune encephalomyelitis (EAE). In contrast to the influx of (125)I-IL15 from blood to the CNS in normal mice and the persistence of IL15 influx in the spinal cord of EAE mice, influx was reduced in the EAE brain. Analyses of disappearance kinetics, FITC (fluorescein isothiocyanate)-albumin space, and delivery of IL15 by in situ perfusion, all indicate that the changes were not caused by BBB disruption but by the rapid availability (high volume of distribution) of IL15 and albumin. Although there was no significant change in the BBB permeation of IL15 in either direction in EAE mice, there was an upregulation of its specific receptor, IL15Ralpha, and an increased in situ production of IL15 mRNA that showed regional variation in both basal and EAE states. Overall, for IL15, its increased cerebral vascular space in the brain was equally as important as its persistent influx across the blood-spinal cord barrier, indicating that it is fully capable of activating the upregulated IL15Ralpha in the brain along with the intrinsic CNS source of IL15 in EAE mice.

Cerebral microvascular IL15 is a novel mediator of TNF action

The blood-brain barrier is a gatekeeper and modulatory interface for the CNS. Cerebral endothelial cells are the major component of the blood-brain barrier, and they modify inflammatory signals from the circulation to the CNS by production and secretion of secondary substances. The inflammatory mediators induced by tumor necrosis factor alpha (TNF) were determined by microarray analysis of RBE4 cerebral endothelial cells, at 0, 6, 12, or 24 h after TNF treatment. Interleukin (IL)-15 and its receptors were among the most robustly up-regulated genes. This was confirmed by real-time RT-PCR and western blotting. The three subunits of the IL15 receptor complex (IL15Ralpha, IL2Rbeta, and IL2Rgamma) showed differential regulation by TNF in their time course and amplitude of increased expression. Consistent with increased expression of the specific high affinity receptor IL15Ralpha, TNF increased cellular uptake of (125)I-IL15 and enhanced the fluorescent intensity of Alexa568-IL15 in RBE4 cells. TNF treatment in mice also increased the level of expression of IL15 receptors in enriched cerebral microvessels. We conclude that the cerebral microvascular IL15 system is a novel inflammatory mediator that transduces the actions of TNF.

The blood-brain barrier and immune function and dysfunction

The blood-brain barrier (BBB) is the monocellular interface that divides the peripheral circulation from direct contact with the central nervous system (CNS). This interface consists of several parallel barriers that include most notably the capillary bed of the CNS and the choroid plexus. These barriers at one level create the dichotomy between the circulating factors of the immune system and the components of the CNS only to regulate interactions between the immune and central nervous systems at other levels. The BBB is thus an integral part of the neuroimmune axis. Here, we will consider four aspects of BBB-neuroimmune interactions: BBB disruption as mediated by LPS and cytokines, cytokine transport across the BBB, immune cell trafficking, and effects of lipopolysaccharide (LPS) on various functions of the BBB.

Regulation of major efflux transporters under inflammatory conditions at the blood-brain barrier in vitro

ATP-driven efflux transport proteins at the blood-brain barrier protect the healthy brain but impede pharmacotherapy of the disordered CNS. To investigate the question how ATP-binding cassette (ABC)-transporters are regulated during inflammation or infection we analysed the effects of the cytokines tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) on the expression of brain multidrug resistance proteins in primary cultures of porcine brain capillary endothelial cells. We found that TNF-alpha and IL-1beta rapidly decrease Abcg2 (BMDP/BCRP) mRNA expression within 6 h. After 24 and 48 h the mRNA level came back to control values. The mRNA reduction at 6 h was counter-regulated by the anti-inflammatory glucocorticoid hydrocortisone. Abcg2 protein levels were suppressed at prolonged stimulations but not after 6 h of stimulation which correlates with Abcg2 specific substrate uptake measurements. Abcb1 (p-glycoprotein) protein expression was transiently increased after TNF-alpha addition within 6 h of incubation followed by a reduction after 24 and 48 h whereas the Abcb1 mRNA levels were not changed. IL-1beta caused a continuous decrease in protein expression of both ABC-transporters. Long-term treatment with an assumed TNF-alpha-downstream agent, the vasoconstrictor endothelin-1, induced Abcg2 protein expression but suppressed Abcb1. Other efflux pumps like multidrug resistance-associated proteins/Abcc were rarely affected. The present results imply a complex regulation of the two most abundant ABC-transporters at the blood-brain barrier during early inflammation stages suggesting that Abcb1 (p-glycoprotein) is an early target of TNF-alpha-signalling counterbalanced by Abcg2.

Neuroinflammation activates Mdr1b efflux transport through NFkappaB: promoter analysis in BBB endothelia

BACKGROUND/AIMS

Although it is known that drug delivery across the blood-brain barrier (BBB) may be hampered by efflux transport activity of the multidrug resistance (mdr) gene product P-glycoprotein, it is not clear how inflammation regulates efflux transporters. In rat brain endothelial (RBE4) cells of BBB origin, the proinflammatory cytokine TNF mainly induced transcriptional upregulation of mdr1b, and to a lesser extent mdr1a, resulting in greater efflux of the substrates. This study further determines the mechanisms by which TNF activates mdr1b promoter activity.

METHODS/RESULTS

Luciferase reporter assays and DNA binding studies show that (1) maximal basal promoter activity was conferred by a 476 bp sequence upstream to the mdr1b transcriptional initiation site; (2) TNF induced upregulation of promoter activity by NFkappaB nuclear translocation; and (3) the NFkappaB binding site of the mdr1b promoter was solely responsible for basal and TNF-activated gene transcription, whereas the p53 binding site was not involved. Binding of the p65 subunit of NFkappaB to nuclear DNA from RBE4 cells was shown by electrophoretic mobility shift assay and chromatin immunoprecipitation assays.

CONCLUSION

NFkappaB mediates TNF-induced upregulation of mdr1b promoter activity, illustrating how inflammation activates BBB efflux transport.

Itraconazole, a potent inhibitor of P-glycoprotein, moderately increases plasma concentrations of oral morphine

BACKGROUND

Individual variation in opioid response is considerable, partly due to pharmacokinetic factors. Transporter proteins are becoming increasingly interesting also in the pharmacokinetics of opioids. The efflux transporter P-glycoprotein can affect gastrointestinal absorption and tissue distribution, particularly brain access of many opioids. The aim of this study was to evaluate whether itraconazole, which is a potent inhibitor of P-glycoprotein and CYP3A4, would change the pharmacokinetics or the pharmacodynamics of oral morphine.

METHODS

Twelve healthy male volunteers ingested, in a randomized crossover study, once daily 200 mg itraconazole or placebo for 4 days. On day 4, 1 h after the last pre-treatment dose, the subjects ingested 0.3 mg/kg morphine. Blood samples for the determination of plasma morphine, morphine-3-glucuronide (M3G), morphine-6-glucuronide (M6G) and itraconazole concentrations were drawn up to 48 h after morphine ingestion. Pharmacodynamic effects were evaluated using a questionnaire, visual analogue scales, a reaction time test, the Digit Symbol Substitution Test and the Critical Flicker Fusion Test.

RESULTS

Itraconazole increased the mean area under the plasma concentration-time curve [AUC (0-9)] of morphine by 29% (P=0.002), its AUC (0-48) by 22% (P=0.013) and its peak plasma concentration by 28% (P=0.035). Itraconazole did not significantly affect the pharmacokinetic variables of M3G or M6G or the pharmacodynamic effects of morphine.

CONCLUSIONS

Itraconazole moderately increases plasma concentrations of oral morphine, probably by enhancing its absorption by inhibiting intestinal wall P-glycoprotein. A possible improvement of morphine penetration to the brain could not be observed.

Tight junction regulation by morphine and HIV-1 tat modulates blood-brain barrier permeability

Human immunodeficiency virus (HIV)-1 patients who abuse opiates are at a greater risk of developing neurological complications of AIDS. Alterations in blood-brain barrier (BBB) integrity are associated with cytoskeletal disorganization and disruption of tight junction (TJ) integrity. We hypothesize that opiates in combination with HIV-1 viral proteins can modulate TJ expression in primary brain microvascular endothelial cells (BMVEC), thereby compromising BBB integrity and exacerbating HIV-1 neuropathogenesis. We investigated the effect of morphine and/or tat on the expression of TJ proteins ZO-1, JAM-2, Occludin and P-glycoprotein and the functional effects of TJ modulation in BMVEC. Morphine and/or tat, via the activation of pro-inflammatory cytokines, intracellular Ca(2+) release, and activation of myosin light chain kinase, modulated TJ expression resulting in decreased transendothelial electric resistance and enhanced transendothelial migration across the BBB. These studies may lead to the development of novel anti-HIV-1 therapeutics that target specific TJ proteins, thus preventing TJ disruption in opiate using HIV-1 patients.

Neuroinflammation facilitates LIF entry into brain: role of TNF

Leukemia inhibitory factor (LIF) is a proinflammatory cytokine mediating a variety of central nervous system (CNS) responses to inflammatory stimuli. During lipopolysaccharide (LPS)-induced inflammation, blood concentrations of LIF increase, correlating with lethality of sepsis. Circulating LIF crosses the blood-brain barrier (BBB) by a saturable transport system. Here we determine how this transport system is regulated in neuroinflammation. Using transport assays that quantify the influx rate and volume of distribution of LIF in mice, we show that LPS facilitated the permeation of LIF from the blood to the brain without compromising the paracellular permeability of the BBB as determined by coadministration of fluorescein. Concurrently, gp130 (shared by the interleukin-6 family of cytokines), but not gp190 (the specific receptor for LIF) or cilliary neutrophic factor (CNTF-Ralpha, a unique receptor for cilliary neurotrophic factor that also uses gp130 and gp190), showed increased levels of mRNA and protein expression in cerebral microvessels from the LPS-treated mice. The upregulation of gp130 by LPS was at least partially mediated by vascular tumor necrosis factor receptor (TNFR)1 and TNFR2. This was shown by elevated TNFR1 and TNFR2 mRNA and protein in cerebral microvessels after LPS and by the absence of the LPS effect on gp130 in knockout mice lacking these receptors. The results show that neuroinflammation by LPS induces endothelial signaling and enhances cytokine transport across the BBB.

Developmental changes of leptin receptors in cerebral microvessels: unexpected relation to leptin transport

The adipokine leptin participates not only in the regulation of feeding and obesity in adults but also in neonatal development. It crosses the blood-brain barrier (BBB) by receptor-mediated transport. Leptin concentrations in blood differ between neonates and adults. We determined the developmental changes of leptin receptor subtypes in the cerebral microvessels composing the BBB and examined their expected correlation with leptin transport across the BBB. Total RNA was extracted from enriched cerebral microvessels of mice 1, 7, 14, and 60 d of age for real-time RT-PCR analysis of leptin receptor subtypes. In cerebral microvessels from neonates, ObRa, ObRb, ObRc, and ObRe mRNA were all higher than in adults, but ObRd was not detectable. Hypothalamus showed similar age-related changes except for ObRb, which was higher in adults. The homologous receptor gp130 did not show significant age-related changes in either region. Despite the increase of leptin receptors, leptin permeation across the BBB after iv injection was less in the neonates. In situ brain perfusion with blood-free buffer showed no significant difference in the brain uptake of leptin between neonates and adults, indicating an antagonistic role of leptin-binding proteins in the circulation, especially the soluble receptor ObRe. The results are consistent with our previous finding that ObRe antagonizes leptin endocytosis in cultured endothelia and transport from blood to brain in mice. Overall, the developmental changes observed for leptin receptors unexpectedly failed to correlate with the entry of leptin into brain, and this may indicate different functions of the receptors in neonates and adults.