Transporters involved in regulation of intracellular pH in primary cultured rat brain endothelial cells

Alterations in brain fluid physiology during the early stages of development of ischaemic oedema

Oedema occurs when higher than normal amounts of solutes and water accumulate in tissues. In brain parenchymal tissue, vasogenic oedema arises from changes in blood-brain barrier permeability, e.g. in peritumoral oedema. Cytotoxic oedema arises from excess accumulation of solutes within cells, e.g. ischaemic oedema following stroke. This type of oedema is initiated when blood flow in the affected core region falls sufficiently to deprive brain cells of the ATP needed to maintain ion gradients. As a consequence, there is: depolarization of neurons; neural uptake of Na+ and Cl- and loss of K+; neuronal swelling; astrocytic uptake of Na+, K+ and anions; swelling of astrocytes; and reduction in ISF volume by fluid uptake into neurons and astrocytes. There is increased parenchymal solute content due to metabolic osmolyte production and solute influx from CSF and blood. The greatly increased [K+]isf triggers spreading depolarizations into the surrounding penumbra increasing metabolic load leading to increased size of the ischaemic core. Water enters the parenchyma primarily from blood, some passing into astrocyte endfeet via AQP4. In the medium term, e.g. after three hours, NaCl permeability and swelling rate increase with partial opening of tight junctions between blood-brain barrier endothelial cells and opening of SUR1-TPRM4 channels. Swelling is then driven by a Donnan-like effect. Longer term, there is gross failure of the blood-brain barrier. Oedema resolution is slower than its formation. Fluids without colloid, e.g. infused mock CSF, can be reabsorbed across the blood-brain barrier by a Starling-like mechanism whereas infused serum with its colloids must be removed by even slower extravascular means. Large scale oedema can increase intracranial pressure (ICP) sufficiently to cause fatal brain herniation. The potentially lethal increase in ICP can be avoided by craniectomy or by aspiration of the osmotically active infarcted region. However, the only satisfactory treatment resulting in retention of function is restoration of blood flow, providing this can be achieved relatively quickly. One important objective of current research is to find treatments that increase the time during which reperfusion is successful. Questions still to be resolved are discussed.

Blood-Brain Barrier Breakdown in Neuroinflammation: Current In Vitro Models

The blood-brain barrier, which is formed by tightly interconnected microvascular endothelial cells, separates the brain from the peripheral circulation. Together with other central nervous system-resident cell types, including pericytes and astrocytes, the blood-brain barrier forms the neurovascular unit. Upon neuroinflammation, this barrier becomes leaky, allowing molecules and cells to enter the brain and to potentially harm the tissue of the central nervous system. Despite the significance of animal models in research, they may not always adequately reflect human pathophysiology. Therefore, human models are needed. This review will provide an overview of the blood-brain barrier in terms of both health and disease. It will describe all key elements of the in vitro models and will explore how different compositions can be utilized to effectively model a variety of neuroinflammatory conditions. Furthermore, it will explore the existing types of models that are used in basic research to study the respective pathologies thus far.

Wnt Signaling in Inner Blood-Retinal Barrier Maintenance

The retina is a light-sensing ocular tissue that sends information to the brain to enable vision. The blood-retinal barrier (BRB) contributes to maintaining homeostasis in the retinal microenvironment by selectively regulating flux of molecules between systemic circulation and the retina. Maintaining such physiological balance is fundamental to visual function by facilitating the delivery of nutrients and oxygen and for protection from blood-borne toxins. The inner BRB (iBRB), composed mostly of inner retinal vasculature, controls substance exchange mainly via transportation processes between (paracellular) and through (transcellular) the retinal microvascular endothelium. Disruption of iBRB, characterized by retinal edema, is observed in many eye diseases and disturbs the physiological quiescence in the retina's extracellular space, resulting in vision loss. Consequently, understanding the mechanisms of iBRB formation, maintenance, and breakdown is pivotal to discovering potential targets to restore function to compromised physiological barriers. These unraveled targets can also inform potential drug delivery strategies across the BRB and the blood-brain barrier into retinas and brain tissues, respectively. This review summarizes mechanistic insights into the development and maintenance of iBRB in health and disease, with a specific focus on the Wnt signaling pathway and its regulatory role in both paracellular and transcellular transport across the retinal vascular endothelium.

Danhong injection enhances the therapeutic effect of mannitol on hemispheric ischemic stroke by ameliorating blood-brain barrier disruption

Mannitol, a representative of hyperosmolar therapy, is indispensable for the treatment of malignant cerebral infarction, but its therapeutic effect is limited by its exacerbation of blood-brain barrier (BBB) disruption. This study was to explore whether Danhong injection (DHI), a standardized product extracted from Salvia miltiorrhiza Bunge and Carthamus tinctorius L., inhibits the destructive effect of mannitol on BBB and thus enhancing the treatment of hemispheric ischemic stroke. SD rats were subjected to pMCAO followed by intravenous bolus injections of mannitol with/without DHI intervention. Neurological deficit score, brain edema, infarct volume at 24 h after MCAO and histopathology, microvascular ultrastructure, immunohistochemistry and immunofluorescence staining of endothelial cell junctions, energy metabolism in the ischemic penumbra were assessed. Intravenous mannitol after MCAO resulted in a decrease in 24 h mortality and cerebral edema, whereas no significant benefit on neurological deficits, infarct volume and microvascular ultrastructure. Moreover, mannitol led to the loss of endothelial integrity, manifested by the decreased expression of occludin, junctional adhesion molecule-1 (JAM-1) and zonula occluden-1 (ZO-1) and the discontinuity of occludin staining around the periphery of endothelial cells. Meanwhile, after mannitol treatment, energy-dependent vimentin and F-actin, ATP content, and ATP5D expression were down-regulated, while MMP2 and MMP9 expression increased in the ischemic penumbra. All the insults after mannitol treatment were attenuated by addition of intravenous DHI. The results suggest DHI as a potential remedy to attenuate mannitol-related BBB disruption, and the potential of DHI to upregulate energy metabolism and inhibit the activity of MMPs is likely attributable to its effects observed.

The role of brain barriers in the neurokinetics and pharmacodynamics of lithium

Lithium (Li) is the most widely used mood stabilizer in treating patients with bipolar disorder. However, more than half of the patients do not or partially respond to Li therapy, despite serum Li concentrations in the serum therapeutic range. The exact mechanisms underlying the pharmacokinetic-pharmacodynamic (PK-PD) relationships of lithium are still poorly understood and alteration in the brain pharmacokinetics of lithium may be one of the mechanisms explaining the variability in the clinical response to Li. Brain barriers such as the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) play a crucial role in controlling blood-to-brain and brain-to-blood exchanges of various molecules including central nervous system (CNS) drugs. Recent in vivo studies by nuclear resonance spectroscopy revealed heterogenous brain distribution of Li in human that were not always correlated with serum concentrations, suggesting regional and variable transport mechanisms of Li through the brain barriers. Moreover, alteration in the functionality and integrity of brain barriers is reported in various CNS diseases, as a cause or a consequence and in this regard, Li by itself is known to modulate BBB properties such as the expression and activity of various transporters, metabolizing enzymes, and the specialized tight junction proteins on BBB. In this review, we will focus on recent knowledge into the role of the brain barriers as key-element in the Li neuropharmacokinetics which might improve the understanding of PK-PD of Li and its interindividual variability in drug response.

Basic physiology of the blood-brain barrier in health and disease: a brief overview

The blood-brain barrier (BBB), a dynamic interface between blood and brain constituted mainly by endothelial cells of brain microvessels, robustly restricts the entry of potentially harmful blood-sourced substances and cells into the brain, however, many therapeutically active agents concurrently cannot gain access into the brain at effective doses in the presence of an intact barrier. On the other hand, breakdown of BBB integrity may involve in the pathogenesis of various neurodegenerative diseases. Besides, certain diseases/disorders such as Alzheimer's disease, hypertension, and epilepsy are associated with varying degrees of BBB disruption. In this review, we aim to highlight the current knowledge on the cellular and molecular composition of the BBB with special emphasis on the major transport pathways across the barrier type endothelial cells. We further provide a discussion on the innovative brain drug delivery strategies in which the obstacle formed by BBB interferes with effective pharmacological treatment of neurodegenerative diseases/disorders.

Mechanisms underlying the regulation of intracellular and luminal pH in vaginal epithelium

The vagina provides a characteristic low-Na⁺ and low-pH fluid microenvironment that is considered generally protective. Previous studies have shown that various types of epithelial cells harbor the capacity of intracellular pH (pHi) regulation. However, it remains elusive whether vaginal epithelium could actively regulate pHi by transporting acid-base ions. In this study, we verified that after transient exposure to NH₄ Cl, the pHi values could rapidly recover from acidification via Na⁺ -H⁺ exchanger (NHE), Na⁺ -HCO₃ ^- cotransporter (NBC), and carbonic anhydrase in human vaginal epithelial cell line VK2/E6E7. Positive expression of the main acid-base transporters including NHE1-2, NBCe1-2, and NBCn1 mRNA was also detected in VK2/E6E7 cells. Moreover, the in vivo study further showed that interfering with the function of V-type H⁺ -ATPase, NHE or NBC expressed in vagina impaired vaginal luminal pH homeostasis in rats. Taken together, our study reveals the property of pH regulation in vaginal epithelial cells, which might provide novel insights into the potential role of vaginal epithelium in the formation of the vaginal acidic microenvironment.

Blood-Brain Barrier: From Physiology to Disease and Back

The blood-brain barrier (BBB) prevents neurotoxic plasma components, blood cells, and pathogens from entering the brain. At the same time, the BBB regulates transport of molecules into and out of the central nervous system (CNS), which maintains tightly controlled chemical composition of the neuronal milieu that is required for proper neuronal functioning. In this review, we first examine molecular and cellular mechanisms underlying the establishment of the BBB. Then, we focus on BBB transport physiology, endothelial and pericyte transporters, and perivascular and paravascular transport. Next, we discuss rare human monogenic neurological disorders with the primary genetic defect in BBB-associated cells demonstrating the link between BBB breakdown and neurodegeneration. Then, we review the effects of genes underlying inheritance and/or increased susceptibility for Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and amyotrophic lateral sclerosis (ALS) on BBB in relation to other pathologies and neurological deficits. We next examine how BBB dysfunction relates to neurological deficits and other pathologies in the majority of sporadic AD, PD, and ALS cases, multiple sclerosis, other neurodegenerative disorders, and acute CNS disorders such as stroke, traumatic brain injury, spinal cord injury, and epilepsy. Lastly, we discuss BBB-based therapeutic opportunities. We conclude with lessons learned and future directions, with emphasis on technological advances to investigate the BBB functions in the living human brain, and at the molecular and cellular level, and address key unanswered questions.

Elimination of substances from the brain parenchyma: efflux via perivascular pathways and via the blood-brain barrier

This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g-1 min-1). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood-brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood-brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood-brain barrier, have higher total CL values. Substances crossing the blood-brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood-brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na+ and Cl- ions across the blood-brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood-brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood-brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.

Blood-brain barrier-supported neurogenesis in healthy and diseased brain

Adult neurogenesis is one of the most important mechanisms contributing to brain development, learning, and memory. Alterations in neurogenesis underlie a wide spectrum of brain diseases. Neurogenesis takes place in highly specialized neurogenic niches. The concept of neurogenic niches is becoming widely accepted due to growing evidence of the important role of the microenvironment established in the close vicinity to stem cells in order to provide adequate control of cell proliferation, differentiation, and apoptosis. Neurogenic niches represent the platform for tight integration of neurogenesis and angiogenesis supported by specific properties of cerebral microvessel endothelial cells contributing to establishment of partially compromised blood-brain barrier (BBB) for the adjustment of local conditions to the current metabolic needs of stem and progenitor cells. Here, we review up-to-date data on microvascular dynamics in activity-dependent neurogenesis, specific properties of BBB in neurogenic niches, endothelial-driven mechanisms of clonogenic activity, and future perspectives for reconstructing the neurogenic niches in vitro.

In vitro models and systems for evaluating the dynamics of drug delivery to the healthy and diseased brain

The blood-brain barrier (BBB) plays a crucial role in maintaining brain homeostasis and transport of drugs to the brain. The conventional animal and Transwell BBB models along with emerging microfluidic-based BBB-on-chip systems have provided fundamental functionalities of the BBB and facilitated the testing of drug delivery to the brain tissue. However, developing biomimetic and predictive BBB models capable of reasonably mimicking essential characteristics of the BBB functions is still a challenge. In addition, detailed analysis of the dynamics of drug delivery to the healthy or diseased brain requires not only biomimetic BBB tissue models but also new systems capable of monitoring the BBB microenvironment and dynamics of barrier function and delivery mechanisms. This review provides a comprehensive overview of recent advances in microengineering of BBB models with different functional complexity and mimicking capability of healthy and diseased states. It also discusses new technologies that can make the next generation of biomimetic human BBBs containing integrated biosensors for real-time monitoring the tissue microenvironment and barrier function and correlating it with the dynamics of drug delivery. Such integrated system addresses important brain drug delivery questions related to the treatment of brain diseases. We further discuss how the combination of in vitro BBB systems, computational models and nanotechnology supports for characterization of the dynamics of drug delivery to the brain.

Organic cation transporter 1 (OCT1) is involved in pentamidine transport at the human and mouse blood-brain barrier (BBB)

Pentamidine is an effective trypanocidal drug used against stage 1 Human African Trypanosomiasis (HAT). At the blood-brain barrier (BBB), it accumulates inside the endothelial cells but has limited entry into the brain. This study examined transporters involved in pentamidine transport at the human and mouse BBB using hCMEC/D3 and bEnd.3 cell lines, respectively. Results revealed that both cell lines expressed the organic cation transporters (OCT1, OCT2 and OCT3), however, P-gp was only expressed in hCMEC/D3 cells. Polarised expression of OCT1 was also observed. Functional assays found that ATP depletion significantly increased [³H]pentamidine accumulation in hCMEC/D3 cells (***p<0.001) but not in bEnd.3 cells. Incubation with unlabelled pentamidine significantly decreased accumulation in hCMEC/D3 and bEnd.3 cells after 120 minutes (***p<0.001). Treating both cell lines with haloperidol and amantadine also decreased [³H]pentamidine accumulation significantly (***p<0.001 and **p<0.01 respectively). However, prazosin treatment decreased [³H]pentamidine accumulation only in hCMEC/D3 cells (*p<0.05), and not bEnd.3 cells. Furthermore, the presence of OCTN, MATE, PMAT, ENT or CNT inhibitors/substrates had no significant effect on the accumulation of [³H]pentamidine in both cell lines. From the data, we conclude that pentamidine interacts with multiple transporters, is taken into brain endothelial cells by OCT1 transporter and is extruded into the blood by ATP-dependent mechanisms. These interactions along with the predominant presence of OCT1 in the luminal membrane of the BBB contribute to the limited entry of pentamidine into the brain. This information is of key importance to the development of pentamidine based combination therapies which could be used to treat CNS stage HAT by improving CNS delivery, efficacy against trypanosomes and safety profile of pentamidine.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

A Novel Mechanism of pH Buffering in C. elegans Glia: Bicarbonate Transport via the Voltage-Gated ClC Cl- Channel CLH-1

An important function of glia is the maintenance of the ionic composition and pH of the synaptic microenvironment. In terms of pH regulation, HCO3 (-) buffering has been shown to be important in both glia and neurons. Here, we used in vivo fluorescent pH imaging and RNA sequencing of the amphid sheath glia of Caenorhabditis elegans to reveal a novel mechanism of cellular HCO3 (-) uptake. While the classical mechanism of HCO3 (-) uptake involves Na(+)/HCO3 (-) cotransporters, here we demonstrate that the C. elegans ClC Cl(-) channel CLH-1 is highly permeable to HCO3 (-) and mediates HCO3 (-) uptake into amphid sheath glia. CLH-1 has homology and electrophysiological properties similar to the mammalian ClC-2 Cl(-) channel. Our data suggest that, in addition to maintaining synaptic Cl(-) concentration, these channels may also be involved in maintenance of synaptic pH via HCO3 (-) flux. These findings provide an exciting new facet of study regarding how pH is regulated in the brain.

SIGNIFICANCE STATEMENT

Maintenance of pH is essential for the physiological function of the nervous system. HCO3 (-) is crucial for pH regulation and is transported into the cell via ion transporters, including ion channels, the molecular identity of which remains unclear. In this manuscript, we describe our discovery that the C. elegans amphid sheath glia regulate intracellular pH via HCO3 (-) flux through the voltage-gated ClC channel CLH-1. This represents a novel function for ClC channels, which has implications for their possible role in mammalian glial pH regulation. This discovery may also provide a novel therapeutic target for pathologic conditions, such as ischemic stroke where acidosis leads to widespread death of glia and subsequently neurons.

A polyspecific drug/proton antiporter mediates diphenhydramine and clonidine transport at the mouse blood-retinal barrier

BACKGROUND AND PURPOSE

Transporters at the blood-retinal barrier (BRB), as at the blood-brain barrier (BBB), regulate the distribution of compounds into the neural parenchyma. However, the expression of BRB transporters and their quantitative impact in vivo are still poorly understood.

EXPERIMENTAL APPROACH

Clonidine and diphenhydramine are substrates of a novel BBB drug/proton-antiporter. We evaluated their transport at the BRB by in situ carotid perfusion in wild-type or knocked-out mice for Oct1-3 (Slc22a1-3).

KEY RESULTS

At pharmacological exposure levels, carrier-mediated BRB influx was 2 and 12 times greater than the passive diffusion rate for clonidine and diphenhydramine, respectively. Functional identification demonstrated the involvement of a high-capacity potassium- and sodium-independent proton-antiporter that shared the features of the previously characterized clonidine, diphenhydramine and cocaine BBB transporter. The functional characterization suggests that SLC transporters Oct1-3, Mate1 (Slc47a1) and Octn1-2 (Slc22a4-5) are not involved. Melanin/retinal toxic drugs like antimalarials (amodiaquine, quinine), quinidine and tricyclic antidepressants (imipramine) acted as inhibitors of this proton-antiporter. The endogenous indole derivative tryptamine inhibited the transporter, unlike 5-HT (serotonin), dopamine or L-DOPA. Trans-stimulation experiments with [(3) H]-clonidine at the BRB indicated that diphenhydramine, nicotine, oxycodone, naloxone, tramadol, 3,4-methylenedioxyamphetamine (MDMA, ecstasy), heroin, methadone and verapamil are common substrates.

CONCLUSIONS AND IMPLICATIONS

A proton-antiporter is physiologically involved in the transport of clonidine and diphenhydramine and is quantitatively more important than their passive diffusion flux at the mouse BRB. The features of this molecularly unidentified transporter highlight its importance in regulating drug delivery at the retina and suggest that it has the capacity to handle several drugs.

New insights into carbonic anhydrase inhibition, vasodilation, and treatment of hypertensive-related diseases

Carbonic anhydrase (CA) and its inhibitors are relevant to many physiological processes and diseases. The enzyme is differentially expressed throughout the body, in concentration and subcellular location, and as 13 catalytically active isoforms. Blood vessels contain small amounts of CA, but the enzyme's role in vascular physiology and blood pressure regulation is uncertain. However, considerable recent evidence points to vasodilation by CA inhibitors. CA inhibition in vascular smooth muscle, endothelium, heart, blood cells, and nervous system could all contribute. It is equally plausible that other targets besides CA for all known CA inhibitors may account for their vascular effects. I will review this knowledge and important remaining gaps relating to treatment of hypertensive-related diseases with potent sulfonamide inhibitors, such as acetazolamide; but also the possibility that CA inhibition by thiazides and loop diuretics, although generally weaker, may have antihypertensive effects beyond their inhibition of renal sodium transporters.

Ion transporters in brain endothelial cells that contribute to formation of brain interstitial fluid

Ions and water transported across the endothelium lining the blood–brain barrier contribute to the fluid secreted into the brain and are important in maintaining appropriate volume and ionic composition of brain interstitial fluid. Changes in this secretion process may occur after stroke. The present study identifies at transcript and protein level ion transporters involved in the movement of key ions and examines how levels of certain of these alter following oxidative stress. Immunohistochemistry provides evidence for Cl⁻/HCO₃⁻ exchanger, AE2, and Na⁺, HCO₃⁻ cotransporters, NBCe1 and NBCn1, on brain microvessels. mRNA analysis by RT-PCR reveals expression of these transporters in cultured rat brain microvascular endothelial cells (both primary and immortalized GPNT cells) and also Na⁺/H⁺ exchangers, NHE1 (primary and immortalized) and NHE2 (primary cells only). Knock-down using siRNA in immortalized GPNT cells identifies AE2 as responsible for much of the Cl⁻/HCO₃⁻ exchange following extracellular chloride removal and NHE1 as the transporter that accounts for most of the Na⁺/H⁺ exchange following intracellular acidification. Transcript levels of both AE2 and NHE1 are increased following hypoxia/reoxygenation. Further work is now required to determine the localization of the bicarbonate transporters to luminal or abluminal membranes of the endothelial cells as well as to identify and localize additional transport mechanisms that must exist for K⁺ and Cl⁻.

Carrier-mediated cocaine transport at the blood-brain barrier as a putative mechanism in addiction liability

BACKGROUND

The rate of entry of cocaine into the brain is a critical factor that influences neuronal plasticity and the development of cocaine addiction. Until now, passive diffusion has been considered the unique mechanism known by which cocaine crosses the blood-brain barrier.

METHODS

We reassessed mechanisms of transport of cocaine at the blood-brain barrier using a human cerebral capillary endothelial cell line (hCMEC/D3) and in situ mouse carotid perfusion.

RESULTS

Both in vivo and in vitro cocaine transport studies demonstrated the coexistence of a carrier-mediated process with passive diffusion. At pharmacological exposure level, passive diffusion of cocaine accounted for only 22.5% of the total cocaine influx in mice and 5.9% in hCMEC/D3 cells, whereas the carrier-mediated influx rate was 3.4 times greater than its passive diffusion rate in vivo. The functional identification of this carrier-mediated transport demonstrated the involvement of a proton antiporter that shared the properties of the previously characterized clonidine and nicotine transporter. The functionnal characterization suggests that the solute carrier (SLC) transporters Oct (Slc22a1-3), Mate (Slc47a1) and Octn (Slc22a4-5) are not involved in the cocaine transport in vivo and in vitro. Diphenhydramine, heroin, tramadol, cocaethylene, and norcocaine all strongly inhibited cocaine transport, unlike benzoylecgonine. Trans-stimulation studies indicated that diphenhydramine, nicotine, 3,4-methylenedioxyamphetamine (ecstasy) and the cathinone compound 3,4-methylenedioxypyrovalerone (MDPV) were also substrates of the cocaine transporter.

CONCLUSIONS

Cocaine transport at the BBB involves a proton-antiporter flux that is quantitatively much more important than its passive diffusion. The molecular identification and characterization of this transporter will provide new tools to understand its role in addictive mechanisms.

Blood-brain barrier Na transporters in ischemic stroke

Blood-brain barrier (BBB) endothelial cells form a barrier that is highly restrictive to passage of solutes between blood and brain. Many BBB transport mechanisms have been described that mediate transcellular movement of solutes across the barrier either into or out of the brain. One class of BBB transporters that is all too often overlooked is that of the ion transporters. The BBB has a rich array of ion transporters and channels that carry Na, K, Cl, HCO3, Ca, and other ions. Many of these are asymmetrically distributed between the luminal and abluminal membranes, giving BBB endothelial cells the ability to perform vectorial transport of ions across the barrier between blood and brain. In this manner, the BBB performs the important function of regulating the volume and composition of brain interstitial fluid. Through functional coupling of luminal and abluminal transporters and channels, the BBB carries Na, Cl, and other ions from blood into brain, producing up to 30% of brain interstitial fluid in healthy brain. During ischemic stroke cerebral edema forms by processes involving increased activity of BBB luminal Na transporters, resulting in "hypersecretion" of Na, Cl, and water into the brain interstitium. This review discusses the roles of luminal BBB Na transporters in edema formation in stroke, with an emphasis on Na-K-Cl cotransport and Na/H exchange. Evidence that these transporters provide effective therapeutic targets for reduction of edema in stroke is also discussed, as are recent findings regarding signaling pathways responsible for ischemia stimulation of the BBB Na transporters.

Na+/H+ exchangers induce autophagy in neurons and inhibit polyglutamine-induced aggregate formation

In polyglutamine diseases, an abnormally elongated polyglutamine results in protein misfolding and accumulation of intracellular aggregates. Autophagy is a major cellular degradative pathway responsible for eliminating unnecessary proteins, including polyglutamine aggregates. Basal autophagy constitutively occurs at low levels in cells for the performance of homeostatic function, but the regulatory mechanism for basal autophagy remains elusive. Here we show that the Na(+)/H(+) exchanger (NHE) family of ion transporters affect autophagy in a neuron-like cell line (Neuro-2a cells). We showed that expression of NHE1 and NHE5 is correlated to polyglutamine accumulation levels in a cellular model of Huntington's disease, a fatal neurodegenerative disorder characterized by accumulation of polyglutamine-containing aggregate formation in the brain. Furthermore, we showed that loss of NHE5 results in increased polyglutamine accumulation in an animal model of Huntington's disease. Our data suggest that cellular pH regulation by NHE1 and NHE5 plays a role in regulating basal autophagy and thereby promotes autophagy-mediated degradation of proteins including polyglutamine aggregates.

Bicarbonate disruption of the pulmonary endothelial barrier via activation of endogenous soluble adenylyl cyclase, isoform 10

It is becoming increasingly apparent that cAMP signals within the pulmonary endothelium are highly compartmentalized, and this compartmentalization is critical to maintaining endothelial barrier integrity. Studies demonstrate that the exogenous soluble bacterial toxin, ExoY, and heterologous expression of the forskolin-stimulated soluble mammalian adenylyl cyclase (AC) chimera, sACI/II, elevate cytosolic cAMP and disrupt the pulmonary microvascular endothelial barrier. The barrier-disruptive effects of cytosolic cAMP generated by exogenous soluble ACs are in contrast to the barrier-protective effects of subplasma membrane cAMP generated by transmembrane AC, which strengthens endothelial barrier integrity. Endogenous soluble AC isoform 10 (AC10 or commonly known as sAC) lacks transmembrane domains and localizes within the cytosolic compartment. AC10 is uniquely activated by bicarbonate to generate cytosolic cAMP, yet its role in regulation of endothelial barrier integrity has not been addressed. Here we demonstrate that, within the pulmonary circulation, AC10 is expressed in pulmonary microvascular endothelial cells (PMVECs) and pulmonary artery endothelial cells (PAECs), yet expression in PAECs is lower. Furthermore, pulmonary endothelial cells selectively express bicarbonate cotransporters. While extracellular bicarbonate generates a phosphodiesterase 4-sensitive cAMP pool in PMVECs, no such cAMP response is detected in PAECs. Finally, addition of extracellular bicarbonate decreases resistance across the PMVEC monolayer and increases the filtration coefficient in the isolated perfused lung above osmolality controls. Collectively, these findings suggest that PMVECs have a bicarbonate-sensitive cytosolic cAMP pool that disrupts endothelial barrier integrity. These studies could provide an alternative mechanism for the controversial effects of bicarbonate correction of acidosis of acute respiratory distress syndrome patients.

Peroxisome proliferator-activated receptors and Alzheimer's disease: hitting the blood-brain barrier

The blood-brain barrier (BBB) is often affected in several neurodegenerative disorders, such as Alzheimer's disease (AD). Integrity and proper functionality of the neurovascular unit are recognized to be critical for maintenance of the BBB. Research has traditionally focused on structural integrity more than functionality, and BBB alteration has usually been explained more as a consequence than a cause. However, ongoing evidence suggests that at the early stages, the BBB of a diseased brain often shows distinct expression patterns of specific carriers such as members of the ATP-binding cassette (ABC) transport protein family, which alter BBB traffic. In AD, amyloid-β (Aβ) deposits are a pathological hallmark and, as recently highlighted by Cramer et al. (2012), Aβ clearance is quite fundamental and is a less studied approach. Current knowledge suggests that BBB traffic plays a more important role than previously believed and that pharmacological modulation of the BBB may offer new therapeutic alternatives for AD. Recent investigations carried out in our laboratory indicate that peroxisome proliferator-activated receptor (PPAR) agonists are able to prevent Aβ-induced neurotoxicity in hippocampal neurons and cognitive impairment in a double transgenic mouse model of AD. However, even when enough literature about PPAR agonists and neurodegenerative disorders is available, the problem of how they exert their functions and help to prevent and rescue Aβ-induced neurotoxicity is poorly understood. In this review, along with highlighting the main features of the BBB and its role in AD, we will discuss information regarding the modulation of BBB components, including the possible role of PPAR agonists as BBB traffic modulators.

Endothelial alkalinisation inhibits gap junction communication and endothelium-derived hyperpolarisations in mouse mesenteric arteries

Abstract  Gap junctions mediate intercellular signalling in arteries and contribute to endothelium-dependent vasorelaxation, conducted vascular responses and vasomotion. Considering its putative role in vascular dysfunction, mechanistic insights regarding the control of gap junction conductivity are required. Here, we investigated the consequences of endothelial alkalinisation for gap junction communication and endothelium-dependent vasorelaxation in resistance arteries. We studied mesenteric arteries from NMRI mice by myography, confocal fluorescence microscopy and electrophysiological techniques. Removing CO2/HCO3(-), reducing extracellular [Cl(-)] or adding 4,4-diisothiocyanatostilbene-2,2-disulphonic acid inhibited or reversed Cl(-)/HCO3(-) exchange, alkalinised the endothelium by 0.2-0.3 pH units and inhibited acetylcholine-induced vasorelaxation. NO-synthase-dependent vasorelaxation was unaffected by endothelial alkalinisation whereas vasorelaxation dependent on small- and intermediate-conductance Ca(2+)-activated K(+) channels was attenuated by ∼75%. The difference in vasorelaxation between arteries with normal and elevated endothelial intracellular pH (pHi) was abolished by the gap junction inhibitors 18β-glycyrrhetinic acid and carbenoxolone while other putative modulators of endothelium-derived hyperpolarisations - Ba(2+), ouabain, iberiotoxin, 8Br-cAMP and polyethylene glycol catalase - had no effect. In the absence of CO2/HCO3(-), addition of the Na(+)/H(+)-exchange inhibitor cariporide normalised endothelial pHi and restored vasorelaxation to acetylcholine. Endothelial hyperpolarisations and Ca(2+) responses to acetylcholine were unaffected by omission of CO2/HCO3(-). By contrast, dye transfer between endothelial cells and endothelium-derived hyperpolarisations of vascular smooth muscle cells stimulated by acetylcholine or the proteinase-activated receptor 2 agonist SLIGRL-amide were inhibited in the absence of CO2/HCO3(-). We conclude that intracellular alkalinisation of endothelial cells attenuates endothelium-derived hyperpolarisations in resistance arteries due to inhibition of gap junction communication. These findings highlight the role of pHi in modulating vascular function.

Coexistence of passive and proton antiporter-mediated processes in nicotine transport at the mouse blood-brain barrier

Nicotine, the main tobacco alkaloid leading to smoking dependence, rapidly crosses the blood-brain barrier (BBB) to become concentrated in the brain. Recently, it has been shown that nicotine interacts with some organic cation transporters (OCT), but their influence at the BBB has not yet been assessed in vivo. In this study, we characterized the transport of nicotine at the mouse luminal BBB by in situ brain perfusion. Its influx was saturable and followed the Michaelis-Menten kinetics (K(m)=2.60 mM, V(max)=37.60 nmol/s/g at pH 7.40). At its usual micromolar concentrations in the plasma, most (79%) of the net transport of nicotine at the BBB was carrier-mediated, while passive diffusion accounted for 21%. Studies on knockout mice showed that the OCT Oct1-3, P-gp, and Bcrp did not alter [(3)H]-nicotine transport at the BBB. Neither did inhibiting the transporters Mate1, Octn, or Pmat. The in vivo manipulation of intracellular and/or extracellular pH, the chemical inhibition profile, and the trans-stimulation experiments demonstrated that the nicotine transporter at the BBB shared the properties of the clonidine/proton antiporter. The molecular features of this proton-coupled antiporter have not yet been identified, but it also transports diphenhydramine and tramadol and helps nicotine cross the BBB at a faster rate and to a greater extent. The pharmacological inhibition of this nicotine/proton antiporter could represent a new strategy to reduce nicotine uptake by the brain and thus help curb addiction to smoking.

Intracellular pH in the resistance vasculature: regulation and functional implications

Net acid extrusion from vascular smooth muscle (VSMCs) and endothelial cells (ECs) in the wall of resistance arteries is mediated by the Na(+),HCO(3)(-) cotransporter NBCn1 (SLC4A7) and the Na(+)/H(+) exchanger NHE1 (SLC9A1) and is essential for intracellular pH (pH(i)) control. Experimental evidence suggests that the pH(i) of VSMCs and ECs modulates both vasocontractile and vasodilatory functions in resistance arteries with implications for blood pressure regulation. The connection between disturbed pH(i) and altered cardiovascular function has been substantiated by a genome-wide association study showing a link between NBCn1 and human hypertension. On this basis, we here review the current evidence regarding (a) molecular mechanisms involved in pH(i) control in VSMCs and ECs of resistance arteries at rest and during contractions, (b) implications of disturbed pH(i) for resistance artery function, and (c) involvement of disturbed pH(i) in the pathogenesis of vascular disease. The current evidence clearly implies that pH(i) of VSMCs and ECs modulates vascular function and suggests that disturbed pH(i) either consequent to disturbed regulation or due to metabolic challenges needs to be taken into consideration as a mechanistic component of artery dysfunction and disturbed blood pressure regulation.

Theranostic liposomes loaded with quantum dots and apomorphine for brain targeting and bioimaging

Quantum dots (QDs) and apomorphine were incorporated into liposomes to eliminate uptake by the liver and enhance brain targeting. We describe the preparation, physicochemical characterization, in vivo bioimaging, and brain endothelial cell uptake of the theranostic liposomes. QDs and the drug were mainly located in the bilayer membrane and inner core of the liposomes, respectively. Spherical vesicles with a mean diameter of ~140 nm were formed. QDs were completely encapsulated by the vesicles. Nearly 80% encapsulation percentage was achieved for apomorphine. A greater fluorescence intensity was observed in mouse brains treated with liposomes compared to free QDs. This result was further confirmed by ex vivo imaging of the organs. QD uptake by the heart and liver was reduced by liposomal incorporation. Apomorphine accumulation in the brain increased by 2.4-fold after this incorporation. According to a hyperspectral imaging analysis, multifunctional liposomes but not the aqueous solution carried QDs into the brain. Liposomes were observed to have been efficiently endocytosed into bEND3 cells. The mechanisms involved in the cellular uptake were clathrin- and caveola-mediated endocytosis, which were energy-dependent. To the best of our knowledge, our group is the first to develop liposomes with a QD-drug hybrid for the aim of imaging and treating brain disorders.

Disruption of Na+,HCO₃⁻ cotransporter NBCn1 (slc4a7) inhibits NO-mediated vasorelaxation, smooth muscle Ca²⁺ sensitivity, and hypertension development in mice

BACKGROUND

Disturbances in pH affect artery function, but the mechanistic background remains controversial. We investigated whether Na(+), HCO₃- transporter NBCn1, by regulating intracellular pH(pH₁), influences artery function and blood pressure regulation.

METHODS AND RESULTS

Knockout of NBCn1 in mice eliminated Na+, HCO₃⁻ cotransport and caused a lower steady-state pH(i) in mesenteric artery smooth muscle and endothelial cells in situ evaluated by fluorescence microscopy. Using myography, arteries from NBCn1 knockout mice showed reduced acetylcholine-induced NO-mediated relaxations and lower rho-kinase-dependent norepinephrine-stimulated smooth muscle Ca²⁺ sensitivity. Acetylcholine-stimulated NO levels (electrode measurements) and N-nitro-l-arginine methyl ester-sensitive l-arginine conversion (radioisotope measurements) were reduced in arteries from NBCn1 knockout mice, whereas relaxation to NO-donor S-nitroso-N-acetylpenicillamine, acetylcholine-induced endothelial Ca²⁺ responses (fluorescence microscopy), and total and Ser-1177 phosphorylated endothelial NO-synthase expression (Western blot analyses) were unaffected. Reduced NO-mediated relaxations in arteries from NBCn1 knockout mice were not rescued by superoxide scavenging. Phosphorylation of myosin phosphatase targeting subunit at Thr-850 was reduced in arteries from NBCn1 knockout mice. Evaluated by an in vitro assay, rho-kinase activity was reduced at low pH. Without CO₂/HCO₃⁻, no differences in pH(i), contraction or relaxation were observed between arteries from NBCn1 knockout and wild-type mice. Based on radiotelemetry and tail-cuff measurements, NBCn1 knockout mice were mildly hypertensive at rest, displayed attenuated blood pressure responses to NO-synthase and rho-kinase inhibition and were resistant to developing hypertension during angiotensin-II infusion.

CONCLUSIONS

Intracellular acidification of smooth muscle and endothelial cells after knockout of NBCn1 inhibits NO-mediated and rho-kinase-dependent signaling in isolated arteries and perturbs blood pressure regulation.

Molecular biology of the blood-brain and the blood-cerebrospinal fluid barriers: similarities and differences

Efficient processing of information by the central nervous system (CNS) represents an important evolutionary advantage. Thus, homeostatic mechanisms have developed that provide appropriate circumstances for neuronal signaling, including a highly controlled and stable microenvironment. To provide such a milieu for neurons, extracellular fluids of the CNS are separated from the changeable environment of blood at three major interfaces: at the brain capillaries by the blood-brain barrier (BBB), which is localized at the level of the endothelial cells and separates brain interstitial fluid (ISF) from blood; at the epithelial layer of four choroid plexuses, the blood-cerebrospinal fluid (CSF) barrier (BCSFB), which separates CSF from the CP ISF, and at the arachnoid barrier. The two barriers that represent the largest interface between blood and brain extracellular fluids, the BBB and the BCSFB, prevent the free paracellular diffusion of polar molecules by complex morphological features, including tight junctions (TJs) that interconnect the endothelial and epithelial cells, respectively. The first part of this review focuses on the molecular biology of TJs and adherens junctions in the brain capillary endothelial cells and in the CP epithelial cells. However, normal function of the CNS depends on a constant supply of essential molecules, like glucose and amino acids from the blood, exchange of electrolytes between brain extracellular fluids and blood, as well as on efficient removal of metabolic waste products and excess neurotransmitters from the brain ISF. Therefore, a number of specific transport proteins are expressed in brain capillary endothelial cells and CP epithelial cells that provide transport of nutrients and ions into the CNS and removal of waste products and ions from the CSF. The second part of this review concentrates on the molecular biology of various solute carrier (SLC) transport proteins at those two barriers and underlines differences in their expression between the two barriers. Also, many blood-borne molecules and xenobiotics can diffuse into brain ISF and then into neuronal membranes due to their physicochemical properties. Entry of these compounds could be detrimental for neural transmission and signalling. Thus, BBB and BCSFB express transport proteins that actively restrict entry of lipophilic and amphipathic substances from blood and/or remove those molecules from the brain extracellular fluids. The third part of this review concentrates on the molecular biology of ATP-binding cassette (ABC)-transporters and those SLC transporters that are involved in efflux transport of xenobiotics, their expression at the BBB and BCSFB and differences in expression in the two major blood-brain interfaces. In addition, transport and diffusion of ions by the BBB and CP epithelium are involved in the formation of fluid, the ISF and CSF, respectively, so the last part of this review discusses molecular biology of ion transporters/exchangers and ion channels in the brain endothelial and CP epithelial cells.

Morphology and properties of brain endothelial cells

The molecular advances in various aspects of brain endothelial cell function in steady states are considerable and difficult to summarize in one chapter. Therefore, this chapter focuses on endothelial permeability mechanisms in steady states and disease namely vasogenic edema. The morphology and properties of caveolae and tight junctions that are involved in endothelial permeability to macromolecules are reviewed. Endothelial transport functions are briefly reviewed. Diseases with alterations of endothelial permeability are mentioned and details are provided of the molecular alterations in caveolae and tight junctions in vasogenic edema. Other factors involved in increased endothelial permeability such as the matrix metalloproteinases are briefly discussed. Of the modulators of endothelial permeability, angioneurins such as the vascular endothelial growth factors and angiopoietins are discussed. The chapter concludes with a brief discussion on delivery of therapeutic substances across endothelium.

Intracellular acidification is associated with changes in free cytosolic calcium and inhibition of action potentials in rat trigeminal ganglion

The effect of intracellular acidification and subsequent pH recovery in sensory neurons has not been well characterized. We have studied the mechanisms underlying Ca(2+)-induced acidification and subsequent recovery of intracellular pH (pH(i)) in rat trigeminal ganglion neurons and report their effects on neuronal excitability. Glutamate (500 μM) and capsaicin (1 μM) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) with a following decrease in pH(i). The recovery of [Ca(2+)](i) to the prestimulus level was inhibited by LaCl(3) (1 mM) and o-vanadate (10 mM), a plasma membrane Ca(2+)/ATPase (PMCA) inhibitor. Removal of extracellular Ca(2+) also completely inhibited the acidification induced by capsaicin. TRPV1 was expressed only in small and medium sized trigeminal ganglion neurons. mRNAs for Na(+)/H(+) exchanger type 1 (NHE1), pancreatic Na(+)-HCO(3)(-) cotransporter type 1 (pNBC1), NBC3, NBC4, and PMCA types 1-3 were detected by RT-PCR. pH(i) recovery was significantly inhibited by pretreatment with NHE1 or pNBC1 siRNA. We found that the frequency of action potentials (APs) was dependent on pH(i). Application of the NHE1 inhibitor 5'-(N-ethyl-N-isopropyl) amiloride (5 μM) or the pNBC1 inhibitor 4',4'-di-isothiocyanostilbene-2',2'-sulfonic acid (500 μM) delayed pH(i) recovery and decreased AP frequency. Simultaneous application of 5'-(N-ethyl-N-isopropyl) amiloride and 4',4'-di-isothiocyanostilbene-2',2'-sulfonic acid almost completely inhibited APs. In summary, our results demonstrate that the rise in [Ca(2+)](i) in sensory neurons by glutamate and capsaicin causes intracellular acidification by activation of PMCA type 3, that the pH(i) recovery from acidification is mediated by membrane transporters NHE1 and pNBC1 specifically, and that the activity of these transporters has direct consequences for neuronal excitability.

SLC4A7 sodium bicarbonate co-transporter controls mitochondrial apoptosis in ischaemic coronary endothelial cells

AIMS

Bicarbonate transport has been shown to participate in apoptosis under ischaemic stress. However, the precise transporting mechanisms involved in ischaemic apoptosis are unknown and were thus the aim of the present study.

METHODS AND RESULTS

Rat coronary endothelial cells (EC) were exposed to simulated in vitro ischaemia for 2 h, and apoptosis was subsequently determined by chromatin staining and caspase-3 activity analysis. By examining the expression of bicarbonate transporters (BT) in EC by reverse transcriptase polymerase chain reaction and western blotting, a marked expression of the electroneutral sodium bicarbonate co-transporter (SLC4A7) was defined. To analyse the potential role of this transporter during apoptosis, a selective inhibitor (S0859, Sanofi-Aventis) was applied. Treatment with S0859 significantly increased caspase-3 activity and elevated the number of apoptotic EC. These results were comparable with an unselective inhibition of all BT due to withdrawal of bicarbonate in the anoxic medium. Knockdown of SLC4A7 in EC by transfecting appropriate siRNA similarly increased apoptosis of EC under simulated ischaemia. The initial characterization of the participating mechanisms of SLC4A7-dependent apoptosis revealed an activation of the mitochondrial pathway of apoptosis, i.e. cleavage of caspase-9 and binding of Bax to mitochondria. In contrast, no activation of the endoplasmic reticulum-dependent pathway (caspase-12 cleavage) or the extrinsic apoptotic pathway (caspase-8 cleavage) was found. Finally, a mitochondrial localization of SLC4A7 was demonstrated.

CONCLUSION

The electroneutral sodium bicarbonate co-transporter SLC4A7 localizes in mitochondria and suppresses the ischaemia-induced activation of the mitochondrial pathway of apoptosis in coronary EC.

Matriptase activation, an early cellular response to acidosis

Extracellular acidosis often rapidly causes intracellular acidification, alters ion channel activities, and activates G protein-coupled receptors. In this report, we demonstrated a novel cellular response to acidosis: induction of the zymogen activation of matriptase. Acid-induced matriptase activation is ubiquitous among epithelial and carcinoma cells and is characterized by rapid onset, fast kinetics, and the magnitude of activation seen. Trace amounts of activated matriptase can be detected 1 min after cells are exposed to pH 6.0 buffer, and the vast majority of latent matriptase within the cells is converted to activated matriptase within 20 min. Matriptase activation may be a direct response to proton exposure because acid-induced matriptase activation also occurs in an in vitro, cell-free setting in which intracellular signaling molecules and ion channel activities are largely absent. Acid-induced matriptase activation takes place both on the cell surface and inside the cells, likely due to the parallel intracellular acidification that activates intracellular matriptase. Following matriptase activation, the active enzyme is immediately inhibited by binding to hepatocyte growth factor activator inhibitor 1, resulting in stable matriptase-hepatocyte growth factor activator inhibitor 1 complexes that are rapidly secreted. As an early response to acidosis, matriptase activation can also be induced by perturbation of intracellular pH homeostasis by 5-(N-methyl-N-isobutyl)-amiloride and 5-(N-ethyl-N-isopropyl)-amiloride, both of which inhibit Na(+)/H(+) exchangers, and diisothiocyanostilbene-2,2'-disulfonic acid, which can inhibit other acid-base ion channels. This study uncovers a novel mechanism regulating proteolysis in epithelial and carcinoma cells, and also demonstrates that a likely function of matriptase is as an early response to acidosis.

Clonidine transport at the mouse blood-brain barrier by a new H+ antiporter that interacts with addictive drugs

Identifying drug transporters and their in vivo significance will help to explain why some central nervous system (CNS) drugs cross the blood-brain barrier (BBB) and reach the brain parenchyma. We characterized the transport of the drug clonidine at the luminal BBB by in situ mouse brain perfusion. Clonidine influx was saturable, followed by Michaelis-Menten kinetics (K(m)=0.62 mmol/L, V(max)=1.76 nmol/sec per g at pH 7.40), and was insensitive to both sodium and trans-membrane potential. In vivo manipulation of intracellular and/or extracellular pH and trans-stimulation showed that clonidine was transported by an H+-coupled antiporter regulated by both proton and clonidine gradients, and that diphenhydramine was also a substrate. Organic cation transporters (Oct1-3), P-gp, and Bcrp did not alter clonidine transport at the BBB in knockout mice. Secondary or tertiary amine CNS compounds such as oxycodone, morphine, diacetylmorphine, methylenedioxyamphetamine (MDMA), cocaine, and nicotine inhibited clonidine transport. However, cationic compounds that interact with choline, Mate, Octn, and Pmat transporters did not. This suggests that clonidine is transported at the luminal mouse BBB by a new H+-coupled reversible antiporter.

Role of the sodium hydrogen exchanger in maitotoxin-induced cell death in cultured rat cortical neurons

Maitotoxin (MTX) is one of the most potent toxins known to date. It causes massive calcium (Ca(2+)) influx and necrotic cell death in various tissues. However, the exact mechanism(s) underlying its cellular toxicity is not fully understood. In the present study, the role of the sodium hydrogen exchanger (NHE) in MTX-induced increases in intracellular Ca(2+) and subsequent cell death were investigated in cultured rat cortical neurons. Intracellular Ca(2+) concentrations ([Ca(2+)](i)) were measured fluorimetrically using FURA-2 as the fluorescence indicator. Cell death was measured with the alamarBlue cell viability assay and the vital dye ethidium bromide (EB) uptake assay. Results showed that MTX increased, in a concentration dependent manner, both [Ca(2+)](i) and cell death in cortical neurons. Decreasing the pH of the treatment medium from 7.5 to 6.0 diminished MTX-induced cell death. The protection offered by lowering extracellular pH was not due to MTX degradation, because it was still effective even if the cells were treated with MTX in normal pH and then switched to a lower pH. Pretreatment of cells with the specific NHE inhibitor, 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), prevented MTX-induced increases in [Ca(2+)](i), as well as cell death in a concentration dependent manner. Furthermore, knockdown of NHE1 by SiRNA transfection suppressed MTX-induced cell death in human embryonic kidney (HEK) cells. Together, these results suggest that NHE1 plays a major role in MTX-induced neurotoxicity.

CFTR functions as a bicarbonate channel in pancreatic duct cells

Pancreatic duct epithelium secretes a HCO(3)(-)-rich fluid by a mechanism dependent on cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. However, the exact role of CFTR remains unclear. One possibility is that the HCO(3)(-) permeability of CFTR provides a pathway for apical HCO(3)(-) efflux during maximal secretion. We have therefore attempted to measure electrodiffusive fluxes of HCO(3)(-) induced by changes in membrane potential across the apical membrane of interlobular ducts isolated from the guinea pig pancreas. This was done by recording the changes in intracellular pH (pH(i)) that occurred in luminally perfused ducts when membrane potential was altered by manipulation of bath K(+) concentration. Apical HCO(3)(-) fluxes activated by cyclic AMP were independent of Cl(-) and luminal Na(+), and substantially inhibited by the CFTR blocker, CFTR(inh)-172. Furthermore, comparable HCO(3)(-) fluxes observed in ducts isolated from wild-type mice were absent in ducts from cystic fibrosis (Delta F) mice. To estimate the HCO(3)(-) permeability of the apical membrane under physiological conditions, guinea pig ducts were luminally perfused with a solution containing 125 mM HCO(3)(-) and 24 mM Cl(-) in the presence of 5% CO(2). From the changes in pH(i), membrane potential, and buffering capacity, the flux and electrochemical gradient of HCO(3)(-) across the apical membrane were determined and used to calculate the HCO(3)(-) permeability. Our estimate of approximately 0.1 microm sec(-1) for the apical HCO(3)(-) permeability of guinea pig duct cells under these conditions is close to the value required to account for observed rates of HCO(3)(-) secretion. This suggests that CFTR functions as a HCO(3)(-) channel in pancreatic duct cells, and that it provides a significant pathway for HCO(3)(-) transport across the apical membrane.

Management of diabetic ketoacidosis in children and adolescents

Diabetic ketoacidosis (DKA) is a life-threatening complication of diabetes mellitus. While it can occur in all types of diabetes mellitus, it is seen most often in patients with type 1 diabetes, either at presentation or as a result of non-compliance with medical therapy. DKA is characterized by hyperglycemia, acidosis, dehydration, and electrolyte abnormalities, which result from a deficiency of insulin and an excess of counter-regulatory hormones. Therapy is aimed at repleting fluids, and correcting acidosis and electrolyte disturbances by administration of intravenous fluid and intravenous insulin. Rapid correction should be avoided as it may result in untoward effects, including cerebral edema. Frequent monitoring of neurologic status and metabolic parameters aids in avoidance or early detection of complications. While much is still not understood about the most serious complication, cerebral edema, recent studies suggest that its development may be tied to a loss of cerebral autoregulation and a vasogenic mechanism of edema formation. Treatment of cerebral edema includes fluid restriction and administration of mannitol. Once DKA has resolved, subcutaneous insulin is initiated with careful consideration of its pharmacokinetics to avoid a period of insulin deficiency and metabolic decompensation.

Involvement of the pyrilamine transporter, a putative organic cation transporter, in blood-brain barrier transport of oxycodone

The purpose of this study was to characterize blood-brain barrier (BBB) transport of oxycodone, a cationic opioid agonist, via the pyrilamine transporter, a putative organic cation transporter, using conditionally immortalized rat brain capillary endothelial cells (TR-BBB13). Oxycodone and [3H]pyrilamine were both transported into TR-BBB13 cells in a temperature- and concentration-dependent manner with Km values of 89 and 28 microM, respectively. The initial uptake of oxycodone was significantly enhanced by preloading with pyrilamine and vice versa. Furthermore, mutual uptake inhibition by oxycodone and pyrilamine suggests that a common mechanism is involved in their transport. Transport of both substrates was inhibited by type II cations (quinidine, verapamil, and amantadine), but not by classic organic cation transporter (OCT) substrates and/or inhibitors (tetraethylammonium, 1-methyl-4-phenylpyridinium, and corticosterone), substrates of OCTN1 (ergothioneine) and OCTN2 (L-carnitine), or organic anions. The transport was inhibited by metabolic inhibitors (rotenone and sodium azide) but was insensitive to extracellular sodium and membrane potential for both substrates. Furthermore, the transport of both substrates was increased at alkaline extracellular pH and decreased in the presence of a protonophore (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone). Intracellular acidification induced with ammonium chloride enhanced the uptakes, suggesting that the transport is driven by an oppositely directed proton gradient. The brain uptake of oxycodone measured by in situ rat brain perfusion was increased in alkaline perfusate and was significantly inhibited by pyrilamine. These results suggest that blood-brain barrier transport of oxycodone is at least partly mediated by a common transporter with pyrilamine, and this transporter is an energy-dependent, proton-coupled antiporter.

The blood-brain barrier in health and chronic neurodegenerative disorders

The blood-brain barrier (BBB) is a highly specialized brain endothelial structure of the fully differentiated neurovascular system. In concert with pericytes, astrocytes, and microglia, the BBB separates components of the circulating blood from neurons. Moreover, the BBB maintains the chemical composition of the neuronal "milieu," which is required for proper functioning of neuronal circuits, synaptic transmission, synaptic remodeling, angiogenesis, and neurogenesis in the adult brain. BBB breakdown, due to disruption of the tight junctions, altered transport of molecules between blood and brain and brain and blood, aberrant angiogenesis, vessel regression, brain hypoperfusion, and inflammatory responses, may initiate and/or contribute to a "vicious circle" of the disease process, resulting in progressive synaptic and neuronal dysfunction and loss in disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, and others. These findings support developments of new therapeutic approaches for chronic neurodegenerative disorders directed at the BBB and other nonneuronal cells of the neurovascular unit.

Expression and localization of Na-driven Cl-HCO(3)(-) exchanger (SLC4A8) in rodent CNS

The Na(+)-driven Cl-HCO(3) exchanger (NDCBE or SLC4A8) is a member of the solute carrier 4 (SLC4) family of HCO(3)(-) transporters, which includes products of 10 genes with similar sequences. Most SLC4 members play important roles in regulating intracellular pH (pH(i)). Physiological studies suggest that NDCBE is a major pH(i) regulator in at least hippocampal (HC) pyramidal neurons. We generated a polyclonal rabbit antibody directed against the first 18 residues of the cytoplasmic N terminus (Nt) of human NDCBE. By Western blotting, the antibody distinguishes NDCBE-as a purified Nt peptide or a full-length transporter (expressed in Xenopus oocytes)-from other Na(+)-coupled HCO(3)(-) transporters. By Western blotting, the antiserum recognizes an approximately 135-kDa band in several brain regions of adult mice: the cerebral cortex (CX), subcortex (SCX), cerebellum (CB), and HC. In CX, PNGase F treatment reduces the molecular weight to approximately 116 kDa. By immunocytochemistry, affinity-purified (AP) NDCBE antibody stains the plasma membrane of neuron cell bodies and processes of rat HC neurons in primary culture as well as freshly dissociated mouse HC neurons. The AP antibody does not detect substantial NDCBE levels in freshly dissociated HC astrocytes, or astrocytes in HC or CB sections. By immunohistochemistry, the AP antibody recognizes high levels of NDCBE in neurons of CX, HC (including pyramidal neurons in Cornu Ammonis (CA)1-3 and dentate gyrus), substantial nigra, medulla, cerebellum (especially Purkinje and granular cells), and the basolateral membrane of fetal choroid plexus. Thus, NDCBE is in a position to contribute substantially to pH(i) regulation in multiple CNS neurons.

Transport activities involved in intracellular pH recovery following acid and alkali challenges in rat brain microvascular endothelial cells

Transport activities involved in intracellular pH (pH(i)) recovery after acid or alkali challenge were investigated in cultured rat brain microvascular endothelial cells by monitoring pH(i) using a pH-sensitive dye. Following relatively small acid loads with pH(i) approximately 6.5, HCO(-)(3) influx accounted for most of the acid extrusion from the cell with both Cl(-)-independent and Cl(-)-dependent, Na(+)-dependent transporters involved. The Cl(-)-independent component has the same properties as the NBC-like transporter previously shown to account for most of the acid extrusion near the resting pH(i). Following large acid loads with pH(i) < 6.5, most of the acid extrusion was mediated by Na(+)/H(+) exchange, the rate of which was steeply dependent on pH(i). Concanamycin A, an inhibitor of V-type ATPase, had no effect on the rates of acid extrusion. Following an alkali challenge, the major component of the acid loading leading to recovery of pH(i) occurred by Cl(-)/HCO(-)(3) exchange. This exchange had the same properties as the AE-like transporter previously identified as a major acid loader near resting pH(i). These acid-loading and acid-extruding transport mechanisms together with the Na(+), K(+), ATPase may be sufficient to account not only for pH(i) regulation in brain endothelial cells but also for the net secretion of HCO(-)(3) across the blood-brain barrier.

Cytoplasmic condensation is both necessary and sufficient to induce apoptotic cell death

Programmed cell death (apoptosis) is important in tissue maintenance. Hallmarks of apoptosis include caspase activation, DNA fragmentation and an overall reduction in cell volume. Whether this apoptotic volume decrease (AVD) is a mere response to initiators of apoptosis or whether it is functionally significant is not clear. In this study, we sought to answer this question using human malignant glioma cells as a model system. In vivo, high grade gliomas demonstrate an increased percentage of apoptotic cells as well as upregulation of death ligand receptors. By dynamically monitoring cell volume, we show that the induction of apoptosis, via activation of either the intrinsic or extrinsic pathways with staurosporine or TRAIL, respectively, resulted in a rapid AVD in D54-MG human glioma cells. This decrease in cell volume could be prevented by inhibiting the efflux of Cl(-) through channels. Such suppression of AVD also reduced the activation of caspases 3, 8 and 9 and suppressed DNA fragmentation. Importantly, experimental manipulations that reduce the cell volume to 70% of the original volume for periods of at least 3 hours were sufficient to initiate apoptosis even in the absence of death ligands. Hence, this data suggests that cell condensation is both necessary and sufficient for the induction of apoptosis.

Antibody-independent localization of the electroneutral Na+-HCO3- cotransporter NBCn1 (slc4a7) in mice

The expression pattern of the electroneutral Na(+)-HCO(3)(-)cotransporter NBCn1 (slc4a7) was investigated by beta-galactosidase staining of mice with a LacZ insertion into the NBCn1 gene. This method is of particular value because it is independent of immunoreactivity. We find that the NBCn1 promoter is active in a number of tissues where NBCn1 has previously been functionally or immunohistochemically identified, including a broad range of blood vessels (vascular smooth muscle cells and endothelial cells), kidney thick ascending limb and medullary collecting duct epithelial cells, the epithelial lining of the kidney pelvis, duodenal enterocytes, choroid plexus epithelial cells, hippocampus, and retina. Kidney corpuscles, colonic mucosa, and nonvascular smooth muscle cells (from the urinary bladder, trachea, gastrointestinal wall, and uterus) were novel areas of promoter activity. Atrial but not ventricular cardiomyocytes were stained. In the brain, distinct layers of the cerebral cortex and cerebellar Purkinje cells were stained as was the dentate nucleus. No staining of skeletal muscle or cortical collecting ducts was observed. RT-PCR analyses confirmed the expression of NBCn1 and beta-galactosidase in selected tissues. Disruption of the NBCn1 gene resulted in reduced NBCn1 expression, and in bladder smooth muscle cells, reduced amiloride-insensitive Na(+)-dependent HCO(3)(-) influx was observed. Furthermore, disruption of the NBCn1 gene resulted in a lower intracellular steady-state pH of bladder smooth muscle cells in the presence of CO(2)/HCO(3)(-) but not in its nominal absence. We conclude that NBCn1 has a broad expression profile, supporting previous findings based on immunoreactivity, and suggest several new tissues where NBCn1 may be important.

Importance of bicarbonate transport for ischaemia-induced apoptosis of coronary endothelial cells

Bicarbonate transport (BT) has been previously shown to participate in apoptosis induced by various stress factors. However, the precise role of BT in ischaemia-induced apoptosis is still unknown. To investigate this subject, rat coronary endothelial cells (EC) were exposed to simulated ischaemia (glucose free anoxia at Ph 6.4) for 2 hrs and cells undergoing apoptosis were visualized by nuclear staining or by determination of cas-pase- 3 activity. To inhibit BT, EC were either treated with the inhibitor of BT 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 300 mumol/l) or exposed to ischaemia in bicarbonate free, 4-(2-hydroxyethyl)-I-piperazi-neethanesulphonic acid (HEPES)-buffered medium. Simulated ischaemia in bicarbonate-buffered medium (Bic) increased caspase-3 activity and the number of apoptotic cell (23.7 + 1.4%versus 5.1 + 1.2% in control). Omission of bicarbonate during ischaemia further significantly increased caspase-3 activity and the number of apoptotic cells (36.7 1.7%). Similar proapoptotic effect was produced by DIDS treatment during ischaemia in Bic, whereas DIDS had no effect when applied in bicarbonate-free, HEPES-buffered medium (Hep). Inhibition of BT was without influence on cytosolic acidification during ischaemia and slightly reduced cytosolic Ca(2+) accumulation. Initial characterization of the underlying mechanism leading to apoptosis induced by BT inhibition revealed activation of the mitochondrial pathway of apoptosis, i.e., increase of cytochrome C release, depolarization of mitochondria and translocation of Bax protein to mitochondria. In contrast, no activation of death receptor-dependent pathway (caspase-8 cleavage) and endoplasmic reticulum- dependent pathway (caspase-12 cleavage) was detected. In conclusion, BT plays an important role in ischaemia-induced apoptosis of coronary EC by suppression of mitochondria-dependent apoptotic pathway.

Chronic continuous hypoxia decreases the expression of SLC4A7 (NBCn1) and SLC4A10 (NCBE) in mouse brain

In the mammalian CNS, hypoxia causes a wide range of physiological effects, and these effects often depend on the stage of development. Among the effects are alterations in pH homeostasis. Na+-coupled HCO3(-) transporters can play critical roles in intracellular pH regulation and several, such as NCBE and NBCn1, are expressed abundantly in the central nervous system. In the present study, we examined the effect of chronic continuous hypoxia on the expression of two electroneutral Na-coupled HCO3(-) transporters, SLC4a7 (NBCn1) and SLC4a10 (NCBE), in mouse brain, the first such study on any acid-base transporter. We placed the mice in normobaric chambers and either maintained normoxia (21% inspired O2) or imposed continuous chronic hypoxia (11% O2) for a duration of either 14 days or 28 days, starting from ages of either postnatal age 2 days (P2) or P90. We assessed protein abundance by Western blot analysis, loading equal amounts of total protein for each condition. In most cases, hypoxia reduced NBCn1 levels by 20-50%, and NCBE levels by 15-40% in cerebral cortex, subcortex, cerebellum, and hippocampus, both after 14 and 28 days, and in both pups and adults. We hypothesize that these decreases, which are out of proportion to the expected overall decreases in brain protein levels, may especially be important for reducing energy consumption.