Primary culture of choroidal epithelial cells: characterization of an in vitro model of blood-CSF barrier

Pb induces ferroptosis in choroid plexus epithelial cells via Fe metabolism

Pb can enhance blood-cerebrospinal fluid barrier (BCSFB) permeability and accumulate in brain tissue, leading to central nervous system (CNS) dysfunction. Choroid plexus (CP) epithelial cells are the main components of the BCSFB with crucial functions in BCSFB maintenance. However, the mechanism by which Pb exposure affects CP epithelial cells remains unclear. Here, ferroptosis was identified as the major programmed cell death modality by sophisticated high-throughput sequencing and biochemical investigations in primary cultured CP epithelial cells following Pb exposure. Bioinformatics analysis using the ferroptosis database revealed that 16 ferroptosis-related genes were differentially expressed in primary cultured CP epithelial cells following Pb exposure. Among them, Gpx4, Slc7a11, Tfrc, and Slc40a1 were hub ferroptosis-related genes. In addition, CP epithelial cells can be impaired when the concentration of the Pb²⁺ reached 2050 μg/L (10 μM PbAc), which included the decrease of cell viability, Gpx4 and Slc7a11 proteins expression, etc. Moreover, inhibition of ferroptosis enhanced CP epithelial cell viability and reduced BCSFB permeability in vitro following Pb exposure. In summary, ferroptosis of CP epithelial cells is involved in BCSFB dysfunction following Pb exposure. Gpx4, Slc7a11, Tfrc, and Slc40a1 are hub ferroptosis-related genes in CP epithelial cells.

In Vitro Models of the Blood–Cerebrospinal Fluid Barrier and Their Applications in the Development and Research of (Neuro)Pharmaceuticals

The pharmaceutical research sector has been facing the challenge of neurotherapeutics development and its inherited high-risk and high-failure-rate nature for decades. This hurdle is partly attributable to the presence of brain barriers, considered both as obstacles and opportunities for the entry of drug substances. The blood–cerebrospinal fluid (CSF) barrier (BCSFB), an under-studied brain barrier site compared to the blood–brain barrier (BBB), can be considered a potential therapeutic target to improve the delivery of CNS therapeutics and provide brain protection measures. Therefore, leveraging robust and authentic in vitro models of the BCSFB can diminish the time and effort spent on unproductive or redundant development activities by a preliminary assessment of the desired physiochemical behavior of an agent toward this barrier. To this end, the current review summarizes the efforts and progresses made to this research area with a notable focus on the attribution of these models and applied techniques to the pharmaceutical sector and the development of neuropharmacological therapeutics and diagnostics. A survey of available in vitro models, with their advantages and limitations and cell lines in hand will be provided, followed by highlighting the potential applications of such models in the (neuro)therapeutics discovery and development pipelines.

Tracking Calcium Dynamics and Immune Surveillance at the Choroid Plexus Blood-Cerebrospinal Fluid Interface

The choroid plexus (ChP) epithelium is a source of secreted signaling factors in cerebrospinal fluid (CSF) and a key barrier between blood and brain. Here, we develop imaging tools to interrogate these functions in adult lateral ventricle ChP in whole-mount explants and in awake mice. By imaging epithelial cells in intact ChP explants, we observed calcium activity and secretory events that increased in frequency following delivery of serotonergic agonists. Using chronic two-photon imaging in awake mice, we observed spontaneous subcellular calcium events as well as strong agonist-evoked calcium activation and cytoplasmic secretion into CSF. Three-dimensional imaging of motility and mobility of multiple types of ChP immune cells at baseline and following immune challenge or focal injury revealed a range of surveillance and defensive behaviors. Together, these tools should help illuminate the diverse functions of this understudied body-brain interface.

Localization of ZIP14 and ZIP8 in HIBCPP Cells

The blood-cerebrospinal fluid barrier (BCB) is important in maintaining brain manganese (Mn) homeostasis. This barrier consists of a single layer of epithelial cells, connected by tight junctions, that restrict the passage of nutrients to only allow molecules to be carried through the membrane by a transporter. These epithelial cells are polarized with asymmetrical blood-facing and cerebrospinal fluid-facing sides. Here, we have established a polarized model of a human choroid plexus papilloma cell line, HIBCPP. For the first time, Mn importers ZIP14 and ZIP8 were identified in HIBCPP cells and were found to be enriched at the basolateral and apical sides of the cell monolayer, respectively. The localization of each ZIP protein adds to the understanding of Mn transport across the HIBCPP BCB model to help understand the mechanism of Mn homeostasis within the brain.

Melanocortins, Melanocortin Receptors and Multiple Sclerosis

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

Modeling immune functions of the mouse blood-cerebrospinal fluid barrier in vitro: primary rather than immortalized mouse choroid plexus epithelial cells are suited to study immune cell migration across this brain barrier

Background

The blood–cerebrospinal fluid barrier (BCSFB) established by the choroid plexus (CP) epithelium has been recognized as a potential entry site of immune cells into the central nervous system during immunosurveillance and neuroinflammation. The location of the choroid plexus impedes in vivo analysis of immune cell trafficking across the BCSFB. Thus, research on cellular and molecular mechanisms of immune cell migration across the BCSFB is largely limited to in vitro models. In addition to forming contact-inhibited epithelial monolayers that express adhesion molecules, the optimal in vitro model must establish a tight permeability barrier as this influences immune cell diapedesis.

Methods

We compared cell line models of the mouse BCSFB derived from the Immortomouse^® and the ECPC4 line to primary mouse choroid plexus epithelial cell (pmCPEC) cultures for their ability to establish differentiated and tight in vitro models of the BCSFB.

Results

We found that inducible cell line models established from the Immortomouse^® or the ECPC4 tumor cell line did not express characteristic epithelial proteins such as cytokeratin and E-cadherin and failed to reproducibly establish contact-inhibited epithelial monolayers that formed a tight permeability barrier. In contrast, cultures of highly-purified pmCPECs expressed cytokeratin and displayed mature BCSFB characteristic junctional complexes as visualized by the junctional localization of E-cadherin, β-catenin and claudins-1, -2, -3 and -11. pmCPECs formed a tight barrier with low permeability and high electrical resistance. When grown in inverted filter cultures, pmCPECs were suitable to study T cell migration from the basolateral to the apical side of the BCSFB, thus correctly modelling in vivo migration of immune cells from the blood to the CSF.

Conclusions

Our study excludes inducible and tumor cell line mouse models as suitable to study immune functions of the BCSFB in vitro. Rather, we introduce here an in vitro inverted filter model of the primary mouse BCSFB suited to study the cellular and molecular mechanisms mediating immune cell migration across the BCSFB during immunosurveillance and neuroinflammation.

Effects of titanium dioxide nanoparticles on lead bioconcentration and toxicity on thyroid endocrine system and neuronal development in zebrafish larvae

Nanoparticles (NPs) have attracted considerable attention because of their wide range of applications. Interactions between heavy metals (e.g., Pb) and NPs in aquatic environments may modify the bioavailability and toxicity of heavy metals. Therefore, this study investigated the influence of NPs (e.g., nano-TiO2) on the bioavailability and toxicity of Pb and its effects in the thyroid endocrine and nervous systems of zebrafish (Danio rerio) larvae. Zebrafish embryos (2-h post-fertilization) were exposed to five concentrations of Pb alone (0, 5, 10, 20, and 30μg/L) or in combination with nano-TiO2 (0.1mg/L) until 6 days post-fertilization. Results showed that the bioconcentration of Pb was significantly enhanced when combined with nano-TiO2 than when used alone. Zebrafish exposure to Pb alone at 30μg/L significantly decreased the thyroid hormone levels (T4 and T3), whereas nano-TiO2 treatment alone did not produce detectable changes. The levels of T4 and T3 were further decreased when Pb was combined with nano-TiO2 than when used alone. The transcription of the thyroid hormone-related factor tg gene was remarkably down-regulated by Pb treatment alone but up-regulated when Pb was combined with nano-TiO2. The significant up-regulation of tshβ gene and the down-regulation of TTR gene expression in the hypothalamic-pituitary-thyroid were observed in Pb with or without nano-TiO2 treatment groups. In addition, the transcription of genes involved in central nervous system (CNS) development (α-tubulin, mbp, gfap and shha) were significantly down-regulated by Pb and nano-TiO2 co-exposure as compared with Pb exposure alone. The locomotion activity analyzes confirmed that nano-TiO2 might enhance the toxicity of Pb to CNS development. These results suggest that nano-TiO2 increase bioconcentration of lead, which lead to the disruption of thyroid endocrine and neuronal system in zebrafish larvae.

Expression and Transport of α-Synuclein at the Blood-Cerebrospinal Fluid Barrier and Effects of Manganese Exposure

The choroid plexus maintains the homeostasis of critical molecules in the brain by regulating their transport between the blood and cerebrospinal fluid (CSF). The current study was designed to investigate the potential role of the blood-CSF barrier (BCSFB) in α-synuclein (a-Syn) transport in the brain as affected by exposure to manganese (Mn), the toxic metal implicated in Parkinsonian disorders. Immunohistochemistry was used to identify intracellular a-Syn expression at the BCSFB. Quantitative real-time PCR was used to quantify the change in a-Syn mRNA expression following Mn treatments at the BCSFB in vitro. ELISA was used to quantify a-Syn levels following in vivo and in vitro treatments of Mn, copper (Cu), and/or external a-Syn. Thioflavin-T assay was used to investigate a-Syn aggregation after incubating with Mn and/or Cu in vitro. A two-chamber Transwell system was used to study a-Syn transport by BCSFB monolayer. Data revealed the expression of endogenous a-Syn in rat choroid plexus tissue and immortalized choroidal epithelial Z310 cells. The cultured primary choroidal epithelia from rats showed the ability to take up a-Syn from extracellular medium and transport a-Syn across the cellular monolayer from the donor to receiver chamber. Exposure of cells with Mn induced intracellular a-Syn accumulation without causing any significant changes in a-Syn mRNA expression. A significant increase in a-Syn aggregation in a cell-free system was observed with the presence of Mn. Moreover, Mn exposure resulted in a significant uptake of a-Syn by primary cells. These data indicate that the BCSFB expresses a-Syn endogenously and is capable of transporting a-Syn across the BCSFB monolayer; Mn exposure apparently increases a-Syn accumulation in the BCSFB by facilitating its uptake and intracellular aggregation.

Proliferation of cultured mouse choroid plexus epithelial cells

The choroid plexus (ChP) epithelium is a multifunctional tissue found in the ventricles of the brain. The major function of the ChP epithelium is to produce cerebrospinal fluid (CSF) that bathes and nourishes the central nervous system (CNS). In addition to the CSF, ChP epithelial cells (CPECs) produce and secrete numerous neurotrophic factors that support brain homeostasis, such as adult hippocampal neurogenesis. Accordingly, damage and dysfunction to CPECs are thought to accelerate and intensify multiple disease phenotypes, and CPEC regeneration would represent a potential therapeutic approach for these diseases. However, previous reports suggest that CPECs rarely divide, although this has not been extensively studied in response to extrinsic factors. Utilizing a cell-cycle reporter mouse line and live cell imaging, we identified scratch injury and the growth factors insulin-like growth factor 1 (IGF-1) and epidermal growth factor (EGF) as extrinsic cues that promote increased CPEC expansion in vitro. Furthermore, we found that IGF-1 and EGF treatment enhances scratch injury-induced proliferation. Finally, we established whole tissue explant cultures and observed that IGF-1 and EGF promote CPEC division within the intact ChP epithelium. We conclude that although CPECs normally have a slow turnover rate, they expand in response to external stimuli such as injury and/or growth factors, which provides a potential avenue for enhancing ChP function after brain injury or neurodegeneration.

Curcumin downregulates aquaporin-1 expression in cultured rat choroid plexus cells

Aquaporin-1 (AQP1) is a water channel that is highly expressed on the apical side of the choroid plexus epithelium (CP) and thought to be one of the major pathways for the high water permeability of this structure. Blockade of AQP1 in the CP reduce the production of cerebrospinal fluid (CSF). Downregulation of AQP1 might be protective against some neurological disorders correlated with increased intracranial pressure and/or poor drainage of CSF. Curcumin, the major constituent of the rhizome of Curcuma longa, has been shown to inhibit potassium channels, Na⁺-K⁺ ATPase, as well as AQP3 in some cells. We therefore speculated that curcumin might be a useful tool to inhibit and/or decrease AQP1, and thus might be useful in the regulation of CSF production in pathophysiological conditions, including traumatic brain injury, hydrocephalus, stroke, systemic hyponatremia, acute cerebral edema, and hypertension. Choroidal epithelial cells of the lateral ventricle of Wistar rats were isolated and grown in in-vitro cultures for 24 h. Curcumin was then added to the medium at different concentrations, and the cell viability tested by the (3,4,5-dimethylthiazol-2-yl)-2-5-diphenyltetrazolium bromide assay. Additional wells of cells were tested for AQP1 protein expression using immunocytochemistry, immunoblotting, and flow cytometry. Our results showed that curcumin treatment decreases AQP1 expression in rat choroid epithelium cells in a dose-dependent manner. We conclude that curcumin may be a useful tool to regulate CSF production in pathophysiological conditions such as hydrocephalus, systemic hyponatremia, hypertension, and other neurological conditions.

Culture of choroid plexus epithelial cells and in vitro model of blood-CSF barrier

Chemical homeostasis in the extracellular fluid of the central nervous system (CNS) is maintained by two brain barrier systems, i.e., the blood-brain barrier (BBB) that separates the blood circulation from brain interstitial fluid and the blood-cerebrospinal fluid barrier (BCB) that separates the blood from the cerebrospinal fluid (CSF). The choroid plexus, where the BCB is located, is a polarized tissue, with the basolateral side of the choroidal epithelium facing the blood and the apical microvilli in direct contact with the CSF. The tissue plays a wide range of roles in brain development, aging, nutrient transport, endocrine regulation, and pathogenesis of certain neurodegenerative disorders. This chapter describes two in vitro cultures that have been well established to allow for study of the BCB structure and function. The primary choroidal epithelial cell culture can be established from rat choroid plexus tissue, and a similar immortalized murine choroidal epithelial cell culture known as Z310 cells has also been established. Both cultures display a dominant polygonal morphology, and immunochemical studies demonstrate the presence of transthyretin, a thyroxine transport protein known to be exclusively produced by the choroidal epithelia in the CNS. These cultures have been adapted for use on freely permeable Transwell(®) membranes sandwiched between two culture chambers, facilitating transport studies of various compounds across this barrier in vitro. These choroidal epithelia cultures with the Transwell system will perceivably assist blood-CSF barrier research.

Transmigration of macrophages across the choroid plexus epithelium in response to the feline immunodeficiency virus

Although lentiviruses such as human, feline and simian immunodeficiency viruses (HIV, FIV, SIV) rapidly gain access to cerebrospinal fluid (CSF), the mechanisms that control this entry are not well understood. One possibility is that the virus may be carried into the brain by immune cells that traffic across the blood-CSF barrier in the choroid plexus. Since few studies have directly examined macrophage trafficking across the blood-CSF barrier, we established transwell and explant cultures of feline choroid plexus epithelium and measured trafficking in the presence or absence of FIV. Macrophages in co-culture with the epithelium showed significant proliferation and robust trafficking that was dependent on the presence of epithelium. Macrophage migration to the apical surface of the epithelium was particularly robust in the choroid plexus explants where 3-fold increases were seen over the first 24 h. Addition of FIV to the cultures greatly increased the number of surface macrophages without influencing replication. The epithelium in the transwell cultures was also permissive to PBMC trafficking, which increased from 17 to 26% of total cells after exposure to FIV. Thus, the choroid plexus epithelium supports trafficking of both macrophages and PBMCs. FIV significantly enhanced translocation of macrophages and T cells indicating that the choroid plexus epithelium is likely to be an active site of immune cell trafficking in response to infection.

Regulation of brain copper homeostasis by the brain barrier systems: effects of Fe-overload and Fe-deficiency

Maintaining brain Cu homeostasis is vital for normal brain function. The role of systemic Fe deficiency (FeD) or overload (FeO) due to metabolic diseases or environmental insults in Cu homeostasis in the cerebrospinal fluid (CSF) and brain tissues remains unknown. This study was designed to investigate how blood-brain barrier (BBB) and blood-SCF barrier (BCB) regulated Cu transport and how FeO or FeD altered brain Cu homeostasis. Rats received an Fe-enriched or Fe-depleted diet for 4 weeks. FeD and FeO treatment resulted in a significant increase (+55%) and decrease (-56%) in CSF Cu levels (p<0.05), respectively; however, neither treatment had any effect on CSF Fe levels. The FeD, but not FeO, led to significant increases in Cu levels in brain parenchyma and the choroid plexus. In situ brain perfusion studies demonstrated that the rate of Cu transport into the brain parenchyma was significantly faster in FeD rats (+92%) and significantly slower (-53%) in FeO rats than in controls. In vitro two chamber Transwell transepithelial transport studies using primary choroidal epithelial cells revealed a predominant efflux of Cu from the CSF to blood compartment by the BCB. Further ventriculo-cisternal perfusion studies showed that Cu clearance by the choroid plexus in FeD animals was significantly greater than control (p<0.05). Taken together, our results demonstrate that both the BBB and BCB contribute to maintain a stable Cu homeostasis in the brain and CSF. Cu appears to enter the brain primarily via the BBB and is subsequently removed from the CSF by the BCB. FeD has a more profound effect on brain Cu levels than FeO. FeD increases Cu transport at the brain barriers and prompts Cu overload in the CNS. The BCB plays a key role in removing the excess Cu from the CSF.

Characterization of immortalized choroid plexus epithelial cell lines for studies of transport processes across the blood-cerebrospinal fluid barrier

BACKGROUND

Two rodent choroid plexus (CP) epithelial cell lines, Z310 and TR-CSFB, were compared with primary rat CP epithelial cells and intact CP tissue with respect to transport protein expression, function and tight junction (TJ) formation.

METHODS

For expression profiles of transporters and TJ proteins, qPCR and western blot analysis were used. Uptake assays were performed to study the functional activity of transporters and TJ formation was measured by trans-epithelial electrical resistance (TEER) and visualized by electron microscopy.

RESULTS

The expression of known ATP-binding cassette (Abc) transporter and solute carrier (Slc) genes in CP was confirmed by qPCR. Primary cells and cell lines showed similar, but overall lower expression of Abc transporters and absent Slc expression when compared to intact tissue. Consistent with this Mrp1, Mrp4 and P-gp protein levels were higher in intact CP compared to cell lines. Functionality of P-gp and Mrp1 was confirmed by Calcein-AM and CMFDA uptake assays and studies using [3H]bis-POM-PMEA as a substrate indicated Mrp4 function. Cell lines showed low or absent TJ protein expression. After treatment of cell lines with corticosteroids, RNA expression of claudin1, 2 and 11 and occludin was elevated, as well as claudin1 and occludin protein expression. TJ formation was further investigated by freeze-fracture electron microscopy and only rarely observed. Increases in TJ particles with steroid treatment were not accompanied by an increase in transepithelial electrical resistance (TEER).

CONCLUSION

Taken together, immortalized cell lines may be a tool to study transport processes mediated by P-gp, Mrp1 or Mrp4, but overall expression of transport proteins and TJ formation do not reflect the situation in intact CP tissue.

Transplantation of cultured choroid plexus epithelial cells via cerebrospinal fluid shows prominent neuroprotective effects against acute ischemic brain injury in the rat

Choroid plexus (CP) epithelial cells (CPECs) produce cerebrospinal fluid (CSF) to provide the CNS with a specialized microenvironment. Our previous study showed that the conditioned medium of cultured CPECs enhanced the survival and neurite extension of hippocampal neurons. The present study examined the ability of cultured CPECs to protect against ischemic brain injury when transplanted into the CSF. Rats were subjected to a transient occlusion of the middle cerebral artery, followed by an injection of cultured CPECs into the fourth ventricle. The injection markedly reduced neurological deficits and infarction volume within 24h. Other beneficial effects were (1) a reduction in number of apoptotic and inflammatory cells, (2) an up-regulation of the mRNA expression of an anti-apoptotic effecter, cAMP-response element binding protein, and (3) a down-regulation of the production of pro-inflammatory factors such as interleukin-1 beta and inducible nitric oxide synthase. The injected CPECs were located within the ventricles and on the brain's surface, not in the ischemic foci, suggesting that they exert their effects by releasing diffusible neuroprotective factors into the CSF. The transplantation of CPECs via CSF is a potential new strategy for protecting against ischemic brain injury.

Efflux of iron from the cerebrospinal fluid to the blood at the blood-CSF barrier: effect of manganese exposure

The blood-cerebrospinal fluid (CSF) barrier (BCB) resides within the choroid plexus, with the apical side facing the CSF and the basolateral side towards the blood. Previous studies demonstrate that manganese (Mn) exposure in rats disrupts iron (Fe) homeostasis in the blood and CSF. The present study used a primary culture of rat choroidal epithelial cells grown in the two-chamber Transwell system to investigate the transepithelial transport of Fe across the BCB. Free, unbound Fe as [(59)Fe] was added to the donor chamber and the radioactivity in the acceptor chamber was quantified to determine the direction of Fe fluxes. Under the normal condition, the [(59)Fe] efflux (from the CSF to the blood) was 128% higher than that of the influx (P < 0.01). Mn exposure significantly increased the efflux rate of [(59)Fe] (P < 0.01) and the effect was inhibited when the cells were pre-incubated with the antibody against divalent metal transport 1 (DMT1). Moreover, when the siRNA knocked down the cellular DMT1 expression, the elevated Fe uptake caused by Mn exposure in the choroidal epithelial Z310 cells was completely abolished, indicating that Mn may facilitate Fe efflux via a DMT1-mediated transport mechanism. In vivo subchronic exposure to Mn in rats reduced Fe clearance from the CSF, as demonstrated by the ventriculo-cisternal brain perfusion, along with up-regulated mRNAs encoding DMT1 and transferrin receptor (TfR) in the same animals. Taken together, these data suggest that free Fe appears to be favorably transported from the CSF toward the blood by DMT1 and this process can be facilitated by Mn exposure. Enhanced TfR-mediated influx of Fe from the blood and ferroportin-mediated expelling Fe toward the CSF may compromise DMT1-mediated efflux, leading to an increased Fe concentration in the CSF as seen in Mn-exposed animals.

5Alpha-dihydrotestosterone up-regulates transthyretin levels in mice and rat choroid plexus via an androgen receptor independent pathway

Transthyretin (TTR) is a 55 kDa plasma homotetrameric protein mainly synthesized in the liver and choroid plexuses (CPs) of the brain that, functions as a carrier for thyroxin and retinol binding protein. It sequesters amyloid beta (Abeta) peptide, and TTR levels in the cerebrospinal fluid (CSF) appear to be inversely correlated with Alzheimer's disease (AD) onset and progression. Androgen deprivation increases plasma Abeta levels, which indicate that androgens may reduce the levels of soluble Abeta, the peptide widely implicated in the initiation of AD pathogenesis; however, the underlying mechanisms are still poorly understood. In this study we examined the effects of 5alpha-dihydrotestosterone (DHT) on TTR protein and mRNA levels, in primary cultures of rat CPs epithelial cells (CPEC) by Western blot, and real time PCR, respectively. Moreover, TTR concentrations were measured in the CSF of castrated wild-type, and transgenic mice expressing human TTR subjected to DHT treatment, by radioimmunoassay and ELISA, respectively. TTR mRNA expression was also compared in the CPs, of the animals from each experimental group by real time PCR. DHT treatment increased TTR protein levels in CPEC, and induced TTR transcription in these cells. The combination of flutamide with DHT in the treatment of CPEC did not abrogate DHT-induced TTR levels, suggesting that TTR is up-regulated via an androgen receptor independent pathway. In the CPs of both mice strains, DHT also increased TTR mRNA levels, but no significant differences in TTR protein levels were detected in the CSF of these animals. These findings open a wide range of possibilities for future studies on Abeta deposition and cognitive function, in response to androgen induction of TTR in animal models of AD.

Use of Z310 cells as an in vitro blood-cerebrospinal fluid barrier model: tight junction proteins and transport properties

Immortalized rat choroidal epithelial Z310 cells have the potential to become an in vitro model for studying transport of materials at blood-cerebrospinal fluid barrier (BCB) (Shi and Zheng, 2005) [Shi, L.Z., Zheng, W., 2005. Establishment of an in vitro brain barrier epithelial transport system for pharmacological and toxicological study. Brain Research 1057, 37-48]. This study was designed to demonstrate the presence of tight junction properties in Z310 cells and the functionality of Z310 monolayer in transport of selected model compounds. Western blot analyses revealed the presence of claudin-1, ZO-1, and occludin in Z310 cells. Transmission electron microscopy showed a "tight junction" type of structure in the sub-apical lateral membranes between adjacent Z310 cells. Real-time RT-PCR revealed that Z310 cells expressed representative transporters such as DMT1, MTP1, TfR, p-glycoprotein, ATP7A, ZnT1, ABCC1, Oat3, OCT1 and OB-Ra. Moreover, Z310 cells cultured in a two-chamber Transwell device possessed the ability to transport zidovudine (anionic drug), thyroxine (hormone), thymidine (nucleoside), and leptin (large polypeptide) with kinetic properties similar to those obtained from the in vitro model based on primary culture of choroidal epithelial cells. Taken together, these data indicate that the Z310 BCB model expresses major tight junction proteins and forms a tight barrier in vitro. The model also exhibits the ability to transport substances of various categories across the barrier.

Ion channel function of aquaporin-1 natively expressed in choroid plexus

Aquaporins are known as water channels; however, an additional ion channel function has been observed for several including aquaporin-1 (AQP1). Using primary cultures of rat choroid plexus, a brain tissue that secretes CSF and abundantly expresses AQP1, we confirmed the ion channel function of AQP1 and assessed its functional relevance. The cGMP-gated cationic conductance associated with AQP1 is activated by an endogenous receptor guanylate cyclase for atrial natriuretic peptide (ANP). Fluid transport assays with confluent polarized choroid plexus cultures showed that AQP1 current activation by 4.5 mum ANP decreases the normal basal-to-apical fluid transport in the choroid plexus; conversely, AQP1 block with 500 mum Cd2+ restores fluid transport. The cGMP-gated conductance in the choroid plexus is lost with targeted knockdown of AQP1 by small interfering RNA (siRNA), as confirmed by immunocytochemistry and whole-cell patch electrophysiology of transiently transfected cells identified by enhanced green fluorescent protein. The properties of the current (permeability to Na+, K+, TEA+, and Cs+; voltage insensitivity; and dependence on cGMP) matched properties characterized previously in AQP1-expressing oocytes. Background K+ and Cl- currents in the choroid plexus were dissected from AQP1 currents using Cs-methanesulfonate recording salines; the background currents recorded in physiological salines were not affected by AQP1-siRNA treatment. These results confirm that AQP1 can function as both a water channel and a gated ion channel. The conclusion that the AQP1-associated cation current contributes to modulating CSF production resolves a lingering concern as to whether an aquaporin ionic conductance can have a physiologically relevant function.

Establishment of an in vitro brain barrier epithelial transport system for pharmacological and toxicological study

An immortalized Z310 murine choroidal epithelial cell line was recently established in this laboratory. The purposes of this study were (1) to investigate the presence of tight junction (TJ) proteins in Z310 cells and (2) to develop a Z310 cell-based in vitro brain barrier transport model. Real-time RT-PCR studies revealed that Z310 cells possess mRNAs encoding ZO-1, -2, and -3, claudin-1, -2, -4, and -8, occludin, and connexin-32. Confocal microscopic analyses confirmed the presence of claudin-1 and ZO-1 in Z310 cells at cell-cell contact sites. When Z310 cells were grown on a two-chamber Transwell device, the [14C]sucrose permeability coefficient and transepithelial electrical resistance (TEER) across the cell monolayer were 6 x 10(-4) cm/min and 61 omega-cm2, respectively. To improve the tightness of Z310 barrier, the cells were cultured in astrocyte-conditioned medium (ACM), or in the presence of eicosapentaenoic acids (EPA, 10 microM), epidermal growth factor (EGF, 100 ng/mL), or dexamethasone (1 microM) in the growth medium. Treatment with ACM, EPA, EGF and dexamethasone significantly increased the TEER by 33%, 38%, 40%, and 50% above controls, respectively. However, only dexamethasone significantly reduced [14C]sucrose paracellular permeability (-231% of controls). These data suggest that Z310 cells possess the TJ proteins. The presence of dexamethasone in the growth medium improves the tightness of Z310 cell monolayer to the level better than that of the primary culture of choroidal epithelial cells. The Z310 cell-based in vitro model appears to be suitable for transepithelial transport study of drugs and toxicants.

Alteration at translational but not transcriptional level of transferrin receptor expression following manganese exposure at the blood-CSF barrier in vitro

Manganese exposure alters iron homeostasis in blood and cerebrospinal fluid (CSF), possibly by acting on iron transport mechanisms localized at the blood-brain barrier and/or blood-CSF barrier. This study was designed to test the hypothesis that manganese exposure may change the binding affinity of iron regulatory proteins (IRPs) to mRNAs encoding transferrin receptor (TfR), thereby influencing iron transport at the blood-CSF barrier. A primary culture of choroidal epithelial cells was adapted to grow on a permeable membrane sandwiched between two culture chambers to mimic blood-CSF barrier. Trace (59)Fe was used to determine the transepithelial transport of iron. Following manganese treatment (100 microM for 24 h), the initial flux rate constant (K(i)) of iron was increased by 34%, whereas the storage of iron in cells was reduced by 58%, as compared to controls. A gel shift assay demonstrated that manganese exposure increased the binding of IRP1 and IRP2 to the stem loop-containing mRNAs. Consequently, the cellular concentrations of TfR proteins were increased by 84% in comparison to controls. Assays utilizing RT-PCR, quantitative real-time reverse transcriptase-PCR, and nuclear run off techniques showed that manganese treatment did not affect the level of heterogeneous nuclear RNA (hnRNA) encoding TfR, nor did it affect the level of nascent TfR mRNA. However, manganese exposure resulted in a significantly increased level of TfR mRNA and reduced levels of ferritin mRNA. Taken together, these results suggest that manganese exposure increases iron transport at the blood-CSF barrier; the effect is likely due to manganese action on translational events relevant to the production of TfR, but not due to its action on transcriptional, gene expression of TfR. The disrupted protein-TfR mRNA interaction in the choroidal epithelial cells may explain the toxicity of manganese at the blood-CSF barrier.

Establishment and characterization of an immortalized Z310 choroidal epithelial cell line from murine choroid plexus

The choroid plexus plays a wide range of roles in brain development, maturation, aging process, endocrine regulation, and pathogenesis of certain neurodegenerative diseases. To facilitate in vitro study, we have used a gene transfection technique to immortalize murine choroidal epithelial cells. A viral plasmid (pSV3neo) was inserted into the host genome of primary choroidal epithelia by calcium phosphate precipitation. The transfected epithelial cells, i.e., Z310 cells, that survived from cytotoxic selection expressed SV40 large-T antigen throughout the life span, suggesting a successful gene transfection. The cells displayed the same polygonal epithelial morphology as the starting cells by light microscopy. Immunocytochemical studies demonstrate the presence of transthyretin (TTR), a thyroxine transport protein known to be exclusively produced by the choroidal epithelia in the CNS, in both transfected and starting cells. Western blot analyses further confirm the production and secretion of TTR by these cells. The mRNAs encoding transferrin receptor (TfR) were identified by Northern blot analyses. The cells grow at a steady rate, currently in the 110th passage with a population doubling time of 20-22 h in the established culture. When Z310 cells were cultured onto a Trans-well apparatus, the cells formed an epithelial monolayer similar to primary choroidal cells, possessing features such as an uneven fluid level between inner and outer chambers and an electrical resistance approximately 150-200 omega-cm(2). These results indicate that immortalized Z310 cells possess the characteristics of choroidal epithelia and may have the potential for application in blood-CSF barrier (BCB) research.

Variant transthyretin in blood circulation can transverse the blood-cerebrospinal barrier: qualitative analyses of transthyretin metabolism in sequential liver transplantation

BACKGROUND

Although the choroid6 plexus of the brain is one of the most important production sites of transthyretin (TTR), the metabolism of TTR secreted in cerebrospinal fluid (CSF) remains to be elucidated.

METHODS

To perform qualitative analysis of variant TTR in CSF of patients who underwent a sequential liver transplantation using an explanted familial amyloidotic polyneuropathy (FAP) ATTR Val30 Met patient's liver, levels and forms of TTR of the two patients were analyzed by means of enzyme linked immunosorbent assay (ELISA) and matrix-assisted laser desorption/time-of-flight mass spectrometer (MALDI/TOF-MS), respectively.

RESULTS

After the operation, variant TTR levels in serum increased, and in CSF, a significant peak of free form of ATTR Val30 Met was detected in the transplanted patients whose CSF had shown no variant TTR before the operation.

CONCLUSIONS

These findings suggest that the variant TTR can cross-the blood-CSF barrier and migrate into CSF from blood circulation. Because leptomeningeal amyloidosis occurs in FAP ATTR Val30 Met as the progression of the disease, this information suggests that in addition to peripheral neuropathy, disorders of the central nervous system (CNS) should be given an attention in patients who underwent sequential liver transplantation using an explanted FAP ATTR Val30 Met patient's liver.

Transthyretin, thyroxine, and retinol-binding protein in human cerebrospinal fluid: effect of lead exposure

Transthyretin (TTR), synthesized by the choroid plexus, is proposed to have a role in transport of thyroid hormones in the brain. Our previous studies in animals suggest that sequestration of lead (Pb) in the choroid plexus may lead to a marked decrease in TTR levels in the cerebrospinal fluid (CSF). The objectives of this study were to establish in humans whether TTR and thyroxine (T(4)) are correlated in the CSF, and whether CSF levels of Pb are associated with those of TTR, T(4), and/or retinol-binding protein (RBP). Eighty-two paired CSF and blood/serum samples were collected from patients undergoing clinical diagnosis of CSF chemistry. Results showed that the mean value of CSF concentrations for TTR was 3.33 +/- 1.60 microg/mg of CSF proteins (mean +/- SD, n = 82), for total T(4) (TT(4)) was 1.56 +/- 1.68 ng/mg (n = 82), for RBP was 0.34 +/- 0.19 microg/mg (n = 82), and for Pb was 0.53 +/- 0.69 microg/dl (n = 61 for those above the detection limit). Linear regression analyses revealed that CSF TTR levels were positively associated with those of CSF TT(4) (r = 0.33, p < 0.005). CSF TTR concentrations, however, were inversely associated with CSF Pb concentrations (r = -0.29, p < 0.05). There was an inverse, albeit weak, correlation between CSF TT(4) and CSF Pb concentrations (r = -0.22, p = 0.09). The concentrations of TTR, TT(4), and Pb in the CSF did not vary as the function of their levels in blood or serum, but RBP concentrations in the CSF did correlate to those of serum (r = 0.39, p < 0.0005). Unlike TTR, CSF RBP concentrations were not influenced by PB: These human data are consistent with our earlier observations in animals, which suggest that TTR is required for thyroxine transport in the CSF and that Pb exposure is likely associated with diminished TTR levels in the CSF.

Transthyretin, thyroxine, and retinol-binding protein in human cerebrospinal fluid: effect of lead exposure

Transthyretin (TTR), synthesized by the choroid plexus, is proposed to have a role in transport of thyroid hormones in the brain. Our previous studies in animals suggest that sequestration of lead (Pb) in the choroid plexus may lead to a marked decrease in TTR levels in the cerebrospinal fluid (CSF). The objectives of this study were to establish in humans whether TTR and thyroxine (T(4)) are correlated in the CSF, and whether CSF levels of Pb are associated with those of TTR, T(4), and/or retinol-binding protein (RBP). Eighty-two paired CSF and blood/serum samples were collected from patients undergoing clinical diagnosis of CSF chemistry. Results showed that the mean value of CSF concentrations for TTR was 3.33 +/- 1.60 microg/mg of CSF proteins (mean +/- SD, n = 82), for total T(4) (TT(4)) was 1.56 +/- 1.68 ng/mg (n = 82), for RBP was 0.34 +/- 0.19 microg/mg (n = 82), and for Pb was 0.53 +/- 0.69 microg/dl (n = 61 for those above the detection limit). Linear regression analyses revealed that CSF TTR levels were positively associated with those of CSF TT(4) (r = 0.33, p < 0.005). CSF TTR concentrations, however, were inversely associated with CSF Pb concentrations (r = -0.29, p < 0.05). There was an inverse, albeit weak, correlation between CSF TT(4) and CSF Pb concentrations (r = -0.22, p = 0.09). The concentrations of TTR, TT(4), and Pb in the CSF did not vary as the function of their levels in blood or serum, but RBP concentrations in the CSF did correlate to those of serum (r = 0.39, p < 0.0005). Unlike TTR, CSF RBP concentrations were not influenced by PB: These human data are consistent with our earlier observations in animals, which suggest that TTR is required for thyroxine transport in the CSF and that Pb exposure is likely associated with diminished TTR levels in the CSF.

Characterization of the amino acid transport of new immortalized choroid plexus epithelial cell lines: a novel in vitro system for investigating transport functions at the blood-cerebrospinal fluid barrier

PURPOSE

To establish and characterize a choroid plexus epithelial cell line (TR-CSFB) from a new type of transgenic rat harboring the temperature-sensitive simian virus 40 (ts SV 40) large T-antigen gene (Tg rat).

METHODS

Choroid plexus epithelial cells were isolated from the Tg rat and cultured on a collagen-coated dish at 37 degrees C during the first period of 3 days. Cells were subsequently cultured at 33 degrees C to activate large T-antigen. At the third passage, cells were cloned by colony formation and isolated from other cells using a penicillin cup.

RESULTS

Five immortalized cell lines of choroid plexus epithelial cells (TR-CSFB 1 approximately 5) were obtained from two Tg rats. These cell lines had a polygonal cell morphology, expressed the typical choroid plexus epithelial cell marker, transthyretin, and possessed Na+, K+-ATPase on their apical side. TR-CSFBs cells expressed a large T-antigen and grew well at 33 degrees C with a doubling-time of 35 approximately 40 hr. [3H]-L-Proline uptake by TR-CSFB cells took place in an Na+-dependent, ouabain-sensitive, energy-dependent, and concentration-dependent manner. It was also inhibited by alpha-methylaminoisobutylic acid, suggesting that system A for amino acids operates in TR-CSFB cells. When [3H]-L-proline uptake was measured using the Transwell device, the L-proline uptake rate following application to the apical side was five-fold greater than that following application to the basal side. In addition, both Na+-dependent and Na+-independent L-glutamic acid (L-Glu) uptake processes were present in TR-CSFB cells.

CONCLUSIONS

Immortalized choroid plexus epithelial cell lines were successfully established from Tg rats and have the properties of choroid plexus epithelial cells, and amino acid transport activity was observed in vivo.

Toxicology of choroid plexus: special reference to metal-induced neurotoxicities

The chemical stability in the brain underlies normal human thinking, learning, and behavior. Compelling evidence demonstrates a definite capacity of the choroid plexus in sequestering toxic heavy metal and metalloid ions. As the integrity of blood-brain and blood-CSF barriers, both structurally and functionally, is essential to brain chemical stability, the role of the choroid plexus in metal-induced neurotoxicities has become an important, yet under-investigated research area in neurotoxicology. Metals acting on the choroid plexus can be categorized into three major groups. A general choroid plexus toxicant can directly damage the choroid plexus structure such as mercury and cadmium. A selective choroid plexus toxicant may impair specific plexus regulatory pathways that are critical to brain development and function, rather than induce massive pathological alteration. The typical examples in this category include lead-induced alteration in transthyretin production and secretion as well as manganese interaction with iron in the choroid plexus. Furthermore, a sequestered choroid plexus toxicant, such as iron, silver, or gold, may be sequestered by the choroid plexus as an essential CNS defense mechanism. Our current knowledge on the toxicological aspect of choroid plexus research is still incomplete. Thus, the future research needs have been suggested to focus on the role of choroid plexus in early CNS development as affected by metal sequestration in this tissue, to explore how metal accumulation alters the capacity of the choroid plexus in regulation of certain essential elements involved in the etiology of neurodegenerative diseases, and to better understand the blood-CSF barrier as a defense mechanism in overall CNS function.

Demonstration of a coupled metabolism-efflux process at the choroid plexus as a mechanism of brain protection toward xenobiotics

Brain homeostasis depends on the composition of both brain interstitial fluid and CSF. Whereas the former is largely controlled by the blood-brain barrier, the latter is regulated by a highly specialized blood-CSF interface, the choroid plexus epithelium, which acts either by controlling the influx of blood-borne compounds, or by clearing deleterious molecules and metabolites from CSF. To investigate mechanisms of brain protection at the choroid plexus, the blood-CSF barrier was reconstituted in vitro by culturing epithelial cells isolated from newborn rat choroid plexuses of either the fourth or the lateral ventricle. The cells grown in primary culture on semipermeable membranes established a pure polarized monolayer displaying structural and functional barrier features, (tight junctions, high electric resistance, low permeability to paracellular markers) and maintaining tissue-specific markers (transthyretin) and specific transporters for micronutriments (amino acids, nucleosides). In particular, the high enzymatic drug metabolism capacity of choroid plexus was preserved in the in vitro blood-CSF interface. Using this model, we demonstrated that choroid plexuses can act as an absolute blood-CSF barrier toward 1-naphthol, a cytotoxic, lipophilic model compound, by a coupled metabolism-efflux mechanism. This compound was metabolized in situ via uridine diphosphate glururonosyltransferase-catalyzed conjugation, and the cellular efflux of the glucurono-conjugate was mediated by a transporter predominantly located at the basolateral, i.e., blood-facing membrane. The transport process was temperature-dependent, probenecid-sensitive, and recognized other glucuronides. Efflux of 1-naphthol metabolite was inhibited by intracellular glutathione S-conjugates. This metabolism-polarized efflux process adds a new facet to the understanding of the protective functions of choroid plexuses.

Demonstration of a coupled metabolism-efflux process at the choroid plexus as a mechanism of brain protection toward xenobiotics

Brain homeostasis depends on the composition of both brain interstitial fluid and CSF. Whereas the former is largely controlled by the blood-brain barrier, the latter is regulated by a highly specialized blood-CSF interface, the choroid plexus epithelium, which acts either by controlling the influx of blood-borne compounds, or by clearing deleterious molecules and metabolites from CSF. To investigate mechanisms of brain protection at the choroid plexus, the blood-CSF barrier was reconstituted in vitro by culturing epithelial cells isolated from newborn rat choroid plexuses of either the fourth or the lateral ventricle. The cells grown in primary culture on semipermeable membranes established a pure polarized monolayer displaying structural and functional barrier features, (tight junctions, high electric resistance, low permeability to paracellular markers) and maintaining tissue-specific markers (transthyretin) and specific transporters for micronutriments (amino acids, nucleosides). In particular, the high enzymatic drug metabolism capacity of choroid plexus was preserved in the in vitro blood-CSF interface. Using this model, we demonstrated that choroid plexuses can act as an absolute blood-CSF barrier toward 1-naphthol, a cytotoxic, lipophilic model compound, by a coupled metabolism-efflux mechanism. This compound was metabolized in situ via uridine diphosphate glururonosyltransferase-catalyzed conjugation, and the cellular efflux of the glucurono-conjugate was mediated by a transporter predominantly located at the basolateral, i.e., blood-facing membrane. The transport process was temperature-dependent, probenecid-sensitive, and recognized other glucuronides. Efflux of 1-naphthol metabolite was inhibited by intracellular glutathione S-conjugates. This metabolism-polarized efflux process adds a new facet to the understanding of the protective functions of choroid plexuses.

Alteration of iron homeostasis following chronic exposure to manganese in rats

Recent studies suggest that manganese-induced neurodegenerative toxicity may be partly due to its action on aconitase, which participates in cellular iron regulation and mitochondrial energy production. This study was performed to investigate whether chronic manganese exposure in rats influenced the homeostasis of iron in blood and cerebrospinal fluid (CSF). Groups of 8-10 rats received intraperitoneal injections of MnCl2 at the dose of 6 mg Mn/kg/day or equal volume of saline for 30 days. Concentrations of manganese and iron in plasma and CSF were determined by atomic absorption spectrophotometry. Rats exposed to manganese showed a greatly elevated manganese concentration in both plasma and CSF. The magnitude of increase in CSF manganese (11-fold) was equivalent to that of plasma (10-fold). Chronic manganese exposure resulted in a 32% decrease in plasma iron (p<0.01) and no changes in plasma total iron binding capacity (TIBC). However, it increased CSF iron by 3-fold as compared to the controls (p<0.01). Northern blot analyses of whole brain homogenates revealed a 34% increase in the expression of glutamine synthetase (p<0.05) with unchanged metallothionein-I in manganese-intoxicated rats. When the cultured choroidal epithelial cells derived from rat choroid plexus were incubated with MnCl2 (100 microM) for four days, the expression of transferrin receptor mRNA appeared to exceed by 50% that of control (p<0.002). The results indicate that chronic manganese exposure alters iron homeostasis possibly by expediting unidirectional influx of iron from the systemic circulation to cerebral compartment. The action appears likely to be mediated by manganese-facilitated iron transport at brain barrier systems.

Inhibition by lead of production and secretion of transthyretin in the choroid plexus: its relation to thyroxine transport at blood-CSF barrier

Long-term, low-dose Pb exposure in rats is associated with a significant decrease in transthyretin (TTR) concentrations in the CSF. Since CSF TTR, a primary carrier of thyroxine in brain, is produced and secreted by the choroid plexus, in vitro studies were conducted to test whether Pb exposure interferes with TTR production and/or secretion by the choroid plexus, leading to an impaired thyroxine transport at the blood-CSF barrier. Newly synthesized TTR molecules in the cultured choroidal epithelial cells were pulse-labeled with [35S]methionine. [35S]TTR in the cell lysates and culture media was immunoprecipitated and separated by SDS-PAGE, and quantitated by autoradiography and liquid scintillation counting. Pb treatment did not significantly alter the protein concentrations in the culture, but inhibited the synthesis of total [35S]TTR (cells + media), particularly during the later chase phase. Two-way ANOVA of the chase phase revealed that Pb exposure (30 microM) significantly suppressed the rate of secretion of [35S]TTR compared to the controls (p < 0.05). Accordingly, Pb treatment caused a retention of [35S]TTR by the cells. In a two-chamber transport system with a monolayer of epithelial barrier, Pb exposure (30 microM) reduced the initial release rate constant (kr) of [125I]T4 from the cell monolayer to the culture media and impeded the transepithelial transport of [125I]T4 from the basal to apical side of epithelial cells by 27%. Taken together, these in vitro data suggest that sequestration of Pb in the choroid plexus hinders the production and secretion of TTR by this tissue. Consequently, this may alter the transport of thyroxine across this blood-CSF barrier.

Lead exposure promotes translocation of protein kinase C activities in rat choroid plexus in vitro, but not in vivo

Lead (Pb) exposure reportedly modulates PKC activity in brain endothelial preparations, which may underlie Pb-induced damage at the blood-brain barrier. Our previous work indicates that Pb accumulates in the choroid plexus and causes dysfunction of this blood-cerebrospinal fluid (CSF) barrier. The present studies were undertaken to test the hypothesis that Pb in the choroid plexus may alter PKC activity and thus affect the functions of the blood-CSF barrier. When choroidal epithelial cells in a primary culture were exposed to Pb (10 microM in culture medium), the membrane-bound PKC activity increased by 5.2-fold, while the cytosolic PKC activities decreased, an indication of the induction of PKC translocation by Pb. The effect of Pb on cellular PKC was concentration dependent in the range of 0.1-10 microM. We further evaluated PKC activity of the choroid plexus in rats chronically exposed to Pb in the drinking water (control, 50 or 250 micrograms Pb/ml) for 30, 60, or 90 days. Two-way analysis of variance revealed a significant age-related decline of PKC activities in both cytosol and membrane of the choroid plexus. However, Pb treatment did not alter plexus PKC activities. In addition, we found that short-term, acute Pb exposure in rats did not significantly change PKC activities nor did it affect the expression of PKC isoenzymes in the choroid plexus. Our results suggest that Pb exposure may promote the translocation of PKC from cytosol to membrane in rat blood-CSF barrier in vitro, but not in vivo.